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Title: The Petrol Engine - A Text-book dealing with the Principles of Design and - Construction, with a Special Chapter on the Two-stroke - Engine
Author: Kean, Francis John
Language: English
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*** Start of this LibraryBlog Digital Book "The Petrol Engine - A Text-book dealing with the Principles of Design and - Construction, with a Special Chapter on the Two-stroke - Engine" ***
THE PETROL ENGINE
_BOOKS FOR MOTOR ENGINEERS._
=Electrical Ignition for Internal Combustion Engines.= By =M. A.
Codd=. 109 illus., 163 pp., cr. 8vo. 3_s._ net.
Introduction—Principle of Electric
Flow—Batteries—Switches—Coils—Auto-Tremblers—Lodge
Ignition—Distributors—Magneto Ignition—High Tension Magnetos—Faults
and Remedies—Magneto Repairs—Induction Coil Design—Index.
=Dynamo Lighting for Motor Cars.= By =M. A. Codd=, Author of
“Electrical Ignition for Internal Combustion Engines.” 128 illus., vi
+ 96 pp. 8vo. 2_s._ 6_d._ net.
Introduction and General Principles—Fitting the System—Wiring
the Car—Permanent Magnet System—Permanent and Electro-Magnet
System—Electro-Magnetically Governed System—Electro-Magnetically
Controlled System—Mechanically Controlled System—Hot Wire Controlled
System—Some useful Accessories—Upkeep, Maintenance, and Location of
Faults—Index.
=English-French and French-English Dictionary of the Motor Car, Cycle
and Boat.= By =Frederick Lucas=. 171 pp., cr. 8vo. 2_s._ net.
=Motor Cycles, Side Cars and Cycle Cars=, Construction, Management and
Repair. By =V. W. Page=, M.E. A comprehensive non-technical treatise,
defining all forms of the lighter self-propelled vehicles, principles
of operation, construction, and practical application of component
parts. 8vo, 344 illus., 550 pp. (_New York._) 6_s._ 6_d._ net.
=The Modern Gasolene Automobile=, its Design, Construction,
Maintenance and Repair. By =Victor W. Page=, M.E., late Technical
Editor of the “Automobile Journal.” 500 illus., 693 pp., 8vo. (_New
York._) 12_s._ net.
=Drawings for Medium Sized Repetition Work=, with Examples of Drawings
for Motor-Car Parts. By =R. D. Spinney=, A.M.I.Mech.E. 47 illus., 130
pp., 8vo. 3_s._ 6_d._ net.
=Motor Body Building= in all its branches. By =C. W. Terry=,
Organizer and Inspector of the City and Guilds of London Institute.
With additional matter by =Arthur Hall=, Graduate member of “The
Institute of British Carriage Manufacturers,” 1st class certificate in
honours of the City and Guilds of London Institute, and other awards;
Instructor in Motor Body Building, Municipal Technical College,
Brighton, etc., etc. Medium 8vo, 256 pp., 15 illus., 5 plates. 10_s._
6_d._ net.
E. & F. N. SPON, LTD., 57 HAYMARKET, LONDON, S.W.
The Petrol Engine
A Text-book dealing with the Principles
of Design and Construction, with
a Special Chapter on the
Two-stroke Engine
By
FRANCIS JOHN KEAN
B.SC. (LOND.); M.I.M.E.
First-Class Honourman in Engineering; Head of the Motor Car Engineering
Department of the Polytechnic School of Engineering, Regent Street,
London, W.; Formerly Lecturer on Experimental Engineering
at McGill University, Montreal, Canada
71 ILLUSTRATIONS
[Colophon]
London
E. & F. N. SPON, LIMITED, 57 HAYMARKET
New York
SPON & CHAMBERLAIN, 123 LIBERTY STREET
1915
CONTENTS
PAGE
LIST OF ILLUSTRATIONS ix
PREFACE xiii
CHAPTER I
GENERAL PRINCIPLES—
Explosive Mixtures 1
The Meaning of Suction 2
The Meaning of Compression 3
The Meaning of a Stroke 3
The Otto Cycle 5
CHAPTER II
DESCRIPTION OF A TYPICAL PETROL ENGINE—
The Cylinder 8
CHAPTER III
ENGINE DETAILS—
The Piston 17
The Connecting Rod 21
The Crankshaft 23
The Flywheel 25
CHAPTER IV
THE VALVES—
Poppet Valves 29
Sleeve Valves 31
The Camshafts and Eccentric Shafts 33
The Timing Wheels 37
The Crankchamber 38
CHAPTER V
THE CARBURETTOR AND CARBURATION—
The Float Chamber 44
The Petrol Jet and Choke Tube 46
The Mixing Chamber and Throttle Valve 47
Recent Improvements in Carburettors 47
Pressure Feed and Gravity Feed 50
CHAPTER VI
IGNITION AND IGNITION DEVICES—
The Sparking Plug 51
The High Tension Magneto 52
The Ignition Coil 57
Wiring Diagram for Magneto Ignition System 60
Wiring Diagram for a Coil Ignition System 60
Timing the Ignition 62
CHAPTER VII
LUBRICATION—
Properties of Oils 63
Splash System of Lubrication 63
Improved System of Splash Lubrication 64
Forced Lubrication 65
CHAPTER VIII
COOLING—
Natural or Thermo-Syphon Circulation 69
Forced or Pump Circulation 71
CHAPTER IX
THE POINTS OF A GOOD ENGINE—
Choosing the Number of Cylinders 75
The Question of the Valves 77
Economy and Durability 79
CHAPTER X
TWO-STROKE ENGINES—
The Two-port Two-stroke Engine 80
The “Kean” Duplex Air Scavenging Engine 85
The Twin-cylinder Two-stroke Engine 96
CHAPTER XI
HORSE-POWER AND THE INDICATOR DIAGRAM—
Work 98
Power 98
Brake Horse-power 99
Rated Horse-power 100
Indicated Horse-power 101
The Indicator Diagram 102
CHAPTER XII
LIQUID FUELS—
Petrol 108
Benzol 108
Alcohol 109
Paraffin 109
Thermal Efficiency 110
APPENDIX
ENGINE TROUBLES 113
TIMING THE IGNITION 115
INDEX 117
LIST OF ILLUSTRATIONS
FIG. DESCRIPTION. PAGE
1. Diagram to explain the meaning of _Suction_ 1
2. Diagram to explain the meaning of _Compression_ 2
3. Otto Cycle. The Suction Stroke 3
4. Otto Cycle. The Compression Stroke 4
5. Otto Cycle. The Power Stroke 5
6. Otto Cycle. The Exhaust Stroke 6
7. General arrangement of a Modern Petrol Engine 9
8. Sectional Drawing of a =T=-headed Cylinder 12
9. Outside View of a Water-jacketed Cylinder 13
10. Stud 14
11. Bolt 14
12. Setscrew 14
13. Motor-cycle Engine with air-cooled Cylinder 14
14. Aeroplane Engine Cylinder 15
15. Cast-iron Piston 18
16. Method of fixing Gudgeon Pin 19
17. Three forms of Piston-head 19
18. Connecting Rod in the form of a Stamping 20
19. Connecting Rod turned from a solid Bar of Steel 21
20. Crankpin and Crankwebs 22
21. Four-throw Crankshaft 23
22. Motor-cycle Crankpin 24
23. Balanced Crank 25
24. Sketch showing the unbalanced portion of a Crank 25
25. Balanced Two-throw Crankshaft 26
26. Force acting on a Flywheel Rim 26
27. Built-up Steel Flywheel 27
28. Flywheel turned from a Steel Stamping 28
29. General arrangement of a Poppet Valve 30
30. Sectional Drawing of the Cylinder of a Sleeve-valve
Engine 31
31. Sectional Drawing of the Cylinder of a Sleeve-valve
Engine 32
32. Poppet Valve-head, showing Slot for Grinding-in purposes 34
33. Inlet and Exhaust Valve Cams 34
34. Eccentric Sheave and Rod for a Sleeve Valve 36
35. A Pair of Timing Wheels 37
36. A Crank Chamber, outside end view 39
37. A Crank Chamber, sectional view 39
38. General arrangement of the Carburetting Plant 43
39. Sectional Drawing of a Carburettor of the _Jet_ Type 44
40. Plain Form of the Choke Tube 47
41. Petrol Jet for _atomising_ the Petrol 48
42. Compensated Petrol Jet 48
43. Automatic Spring-controlled Extra-air Valve 49
44. Plan View of Automatic Extra-air Valve 49
45. Sectional Drawing of a Sparking Plug 51
46. A Sparking Plug 52
47. Outside View of a High-tension Magneto 52
48. View of High Tension Magneto showing Distributor and
Contact Breaker 53
49. End View of High Tension Magneto 54
50. An Ignition Coil 56
51. An Ignition Coil Case 57
52. Low Tension Contact Breaker for Coil Ignition (Wipe
Form) 58
53. Wiring Diagram for Four Cylinder Engine with Magneto
Ignition (High Tension) 60
54. Wiring Diagram for Four Cylinder Engine with Trembler
Coil Ignition 61
55. Improved System of Splash Lubrication 64
56. Sectional View of Connecting Rod end, showing Scoop
and Oil Trough 65
57. Forced Lubrication System 66
58. Sectional View of Rotary Oil Pump 67
59. A Rotary Oil Pump 67
60. Thermo-syphon Water Cooling System 69
61. Forced Water Circulation by means of a Pump 70
62. Forms of Water Piping 74
63. Two-port Two-stroke Engine with Crankchamber Compression 81
64. Diagrammatic Sketch of a Duplex Two-stroke Air
Scavenging Engine 87
65. General Arrangement of the “Kean” Two-stroke
Engine 91
66. Twin-cylinder Two-stroke Engine with Crankchamber
Compression 97
67. Petrol Engine Brake 100
68. Force-space or “Work” Diagram 103
69. Petrol Engine Indicator Diagram Four-stroke Cycle 105
70. Petrol Indicator Diagram for a Two-stroke Engine 106
71. Diagram of Valve-setting 116
PREFACE
This book deals with _principles_. There are many books which give a
descriptive account of existing types of engines, but my object in
writing this volume has been to assist the reader to obtain thoroughly
sound notions of the _principles_ of design and construction which
underlie all current practice. If a man understands, for example, the
construction of the _elements_ of a carburettor and how they ought
to perform their several functions, he should have no difficulty in
understanding any special type of carburettor placed upon the market.
In dealing with the subject of ignition I have purposely avoided any
detailed explanation of the manner in which the spark discharge is
produced, because I felt that it introduces new ideas and probably
causes the reader to lose sight of the fact that the magneto is only,
after all, an _accessory_, although of course a most important one. I
hope that the accounts of my experiments with the _two-stroke_ will
be of some service to inventors and others; the many extraordinary
breakdowns, defects and adventures encountered during this period of my
career have not been inserted because they would undoubtedly cause the
reader to forget, for the time being, his fundamental _principles_.
My colleague, Mr. Oliver Mitchell, who lectures at the Polytechnic on
“Motor Car Management and Inspection,” has read through the proofs
for me and very kindly suggested several small additions to the
text, which I have incorporated; he also suggested the insertion of
the valve-setting diagram in the Appendix. My thanks are due to Mr.
Mitchell for his services and also to my wife for her assistance in the
preparation of the Index.
FRANCIS JOHN KEAN.
THE POLYTECHNIC SCHOOL OF ENGINEERING,
REGENT STREET, LONDON, W.
_July, 1915._
THE PETROL ENGINE
CHAPTER I
GENERAL PRINCIPLES
[Illustration: FIG. 1.—DIAGRAM TO EXPLAIN THE MEANING OF “SUCTION.”]
=Explosive Mixtures.=—If a small quantity of liquid petrol or benzol
be placed in an open vessel and exposed to a current of air it will
quickly disappear or _evaporate_. We say that the liquid petrol has
been vaporized or turned into _petrol vapour_. A mixture of air and
petrol vapour can be ignited and burnt, the rate of burning being
affected by the _strength_ of the mixture. The strength of the mixture
is determined by measuring the respective volumes of air and petrol
vapour present in a known volume of the mixture. It is possible to
form a mixture of air and petrol vapour in such proportions that when
ignited by an electric spark it will be completely burnt at such
a rate that the combustion is almost instantaneous, i.e., it will
_explode_. This mixture of air and petrol vapour would then be referred
to as an _explosive mixture_ and would be suitable for supplying to the
cylinder of a petrol engine.
[Illustration: FIG. 2.—DIAGRAM TO EXPLAIN THE MEANING OF “COMPRESSION.”]
=The Meaning of Suction.=—Imagine an iron cylinder A (Fig. 1) held
down on a rigid base C and fitted with a gas-tight piston B. If we
pull the piston down sharply to the position shown in Fig. 2 we will
realize that there is apparently some force inside the cylinder which
is trying to _suck_ the piston up again. The fact that the piston is
being withdrawn and no more air or gas admitted above it to fill up the
volume it has displaced on its descent causes a partial vacuum in the
cylinder. Now if by means of a tap or valve of some kind we could put
the cylinder in communication with the atmosphere, air would rush in
and fill up the cylinder until the pressure of the gases in it became
equal to atmospheric pressure, when no more air could enter, because
there would be no _excess of pressure_ to force it in. In technical
language we would say, “the piston has =sucked in a charge= of air”
through the tap or valve.
[Illustration: FIG. 3.—OTTO CYCLE. THE SUCTION STROKE.]
=The Meaning of Compression.=—Close the tap or valve and push the
piston up again sharply to its original position of Fig. 1. You will
now encounter considerable resistance and experience a force pushing
down against you because you are reducing the volume of the gas and
thereby increasing its pressure; that is to say, you are _compressing_
the gas, because you are now making an amount of gas that recently
occupied the whole cylinder fit itself into the small space between the
top of the cylinder and the crown of the piston. In technical language
you would say, “the piston has now =compressed the charge=” of gas
within the cylinder.
[Illustration: FIG. 4.—OTTO CYCLE. THE COMPRESSION STROKE.]
=The Meaning, of a Stroke.=—In an engine such as is shown
diagrammatically in Figs. 3 and 4, when the piston P moves from its
topmost position in the cylinder down to its very lowest position we
say it has completed a _downstroke_, and when it moves upwards from
its lowest to its highest position we say the piston has completed an
_upstroke_. The length of the piston’s stroke is equal to twice the
length of the crank radius R, and is measured by observing the distance
moved by the piston in travelling from its highest position in the
cylinder to its lowest or vice versa. The space existing above the
piston between it and the cylinder head when the piston has reached its
highest position in the cylinder is called the _clearance space_. It is
also referred to as the _combustion chamber_, or chamber in which the
petrol gas is exploded. When the piston is either at the top or bottom
of its stroke the crank radius R and connecting rod T are in one and
the same straight line; under these conditions we say the crank is on
its inner or outer _dead-centre_.
=The Otto Cycle.=—Most petrol engines operate on what is known as
the “Otto” cycle, in which the cycle of events is completed once in
every four strokes (or _two_ revolutions) made by the engine. The
“Otto” cycle is therefore usually referred to as the _four-stroke_
cycle. In the accompanying diagrams (Figs. 3, 4, 5, and 6) we show in
diagrammatic form the interior of a petrol engine cylinder fitted with
mushroom type valves.
[Illustration: FIG. 5.—OTTO CYCLE. THE POWER STROKE.]
In studying the figures we assume the engine is being cranked round by
hand in the direction of the arrow while we view it from the “flywheel”
end (i.e. the end adjacent to the driver’s seat), then A is the pipe
which leads the mixture of air and petrol vapour from the _carburettor_
to the cylinder and is called the _induction pipe_. C is the cylinder,
P the piston, I the inlet valve, E the exhaust valve, T the connecting
rod, R the crank, and S the sparking plug. The pipe B which leads the
burnt gases from the exhaust valve to the _silencer_ is called the
_exhaust pipe_. The cycle of operations is as follows:—
[Illustration: FIG. 6.—OTTO CYCLE. THE EXHAUST STROKE.]
(1) =On the first downstroke= made by the piston a suction effect or
partial vacuum is produced in the cylinder; the air and petrol vapour
in the induction pipe being at atmospheric pressure, which is in excess
of that now existing in the cylinder, flow into the cylinder as soon
as the inlet valve I is opened by the engine mechanism. At the end of
this, _the suction stroke_, the inlet valve closes and traps the charge
of explosive mixture in the engine cylinder. This is shown in Fig. 3.
(2) =On the first upstroke= made by the piston the charge of explosive
mixture is compressed ready for firing. Both valves are shut. This is
shown in Fig. 4.
(3) =On the second downstroke= made by the piston the sparking plug S
passes a spark which explodes the charge at the very commencement of
the downward movement of the piston. The force of the explosion drives
the piston downwards, doing useful work. Both valves are shut. This
is the _power stroke_, and sufficient power must be developed on this
stroke not only to do the work required from the engine but also to
tide it over the other three _idle_ strokes. On this stroke the piston
drives the crank by means of the connecting rod, but on the other three
strokes of the cycle the crank has to drive the piston by means of the
power or energy stored in the engine flywheel on the power stroke.
Towards the end of the power stroke (or explosion stroke) the engine
mechanism opens the exhaust valve E and allows part of the burnt gases
to escape to the silencer along the exhaust pipe. This is shown in Fig.
5.
(4) =On the second upstroke= of the cycle the piston pushes the
remaining burnt gases out of the cylinder through the exhaust valve.
When the piston reaches the top of its stroke the exhaust valve closes.
This is shown in Fig. 6. The cycle of operations then begins again,
giving one power stroke and three idle strokes each time as already
described.
CHAPTER II
DESCRIPTION OF A TYPICAL PETROL ENGINE
For the purpose of explaining the cycle of operations we have
considered only a diagrammatic sketch of an imaginary motor-car
engine, but in Fig. 7 we illustrate an up-to-date motor-car engine.
In the first place we note the position and arrangement of the four
water-cooled cylinders, A_{1}, A_{2}, A_{3}, A_{4}, containing their
pistons and mushroom type valves. These are conveniently placed in a
vertical position and mounted on top of the crankchamber C, to the
bottom of which is attached the oil-base B. At the front of the engine
are shown the timing wheels in their casing E, and at the rear end the
flywheel F. The starting-handle connexion is at S, the fan pulley being
shown at M. The high tension magneto which supplies the current to the
sparking plugs is shown at H, and I is the induction pipe connected to
the carburettor K. The water circulating pump is on the off side of the
engine and does not appear in the illustration, but L_{1} is the inlet
water pipe leading from the radiator (not shown) to the water pump, and
L_{2} is the delivery pipe from the pump to the respective cylinder
jackets, L_{3} being the outlet water pipe. The exhaust pipe is shown
at D, and the oil pump at P. The valve springs, valve tappets and
guides can also be clearly seen. In examining the several parts of the
engine in detail we must not lose sight of their respective positions
in the general arrangement view of Fig. 7.
[Illustration: FIG. 7.—GENERAL ARRANGEMENT OF A MODERN PETROL ENGINE.]
=The Cylinder.=—Probably one of the most important parts of an engine
is the cylinder. As we have already seen, it is inside the cylinder
that the charge of petrol vapour and air is exploded and completely
burnt. The heat energy of the petrol mixture which is liberated by
the explosion is immediately transformed into mechanical work and
propels the piston forward like a projectile from a gun. But we must
also notice that our present-day arrangements (clever as they are) are
by no means perfect, and we cannot, even under the most favourable
circumstances, convert more than about _one-third_ of the heat energy
of the petrol mixture into the mechanical energy of the moving piston.
Of the remaining two-thirds of the heat, part is used up in heating the
cylinder walls, the piston and the valves, and the remainder goes out
with the exhaust gases to the silencer, finally escaping to the outside
air. Thus two important facts are brought to our notice:—
(1) The reason why we use petrol to drive our motor-cars is because
petrol (and certain other liquid fuels such as benzol, etc.) contains
within itself a store of energy which can be liberated as heat when the
fuel is burnt or exploded in the presence of air in the engine cylinder.
(2) At the present day, even with our most up-to-date contrivances,
we cannot make use of two-thirds of the available heat in our petrol.
Instead of being able to turn this heat into useful mechanical work,
we are compelled to throw it away—to waste it. Further than that, we
have to make special provision to ensure that it shall be wasted as
quickly as possible and as easily as possible. We take out the greatest
amount that we can possibly turn into work and then hasten to dissipate
the remaining two-thirds. We cast hollow chambers on the outside of
our cylinders through which we circulate cold water to keep down the
heat in the cylinder walls; if our cylinder walls and piston get too
hot our engine may seize up, therefore we must cool them to ensure
satisfactory running. Again we make large exhaust valves and provide a
free escape through the silencer for the exhaust gases, so that when we
have snatched our useful one-third of the heat supply we may throw the
remainder away into the atmosphere as rapidly as possible.—this part is
of no use to us, we cannot turn it into work, then why let it stay here
and heat our cylinder walls and piston still further?
It is a good plan to extend this hollow chamber, containing the water
in circulation, at least round the whole of the combustion chamber
and all round the inlet and exhaust valve passages and down the
barrel of the cylinder as far as the walls are likely to come into
contact with the hot gases from the explosions. We refer to this
hollow chamber, with its circulating water, as the _water-jacket_ of
the cylinder. It is not absolutely _essential_ to have our cylinder
water-jacketed, especially with small engines for motor-cycles and
engines for aeroplanes which have revolving cylinders, but it is
practically essential in nearly all other cases. Even in the special
cases mentioned it is found necessary to form special heat radiating
fins on the outside of the heated walls to assist in dissipating or
getting rid of the surplus heat and preventing seizure of the piston
within the cylinder. These fins are clearly seen on the cylinder of the
motor-cycle engine shown in Fig. 13.
Thus we may say that motor-car engine cylinders are bound to be
water-jacketed, i.e., to have a hollow space round them containing
water in circulation. The cylinders themselves are nearly always made
in the form of iron castings and the jacket spaces form part of the
_cylinder casting_ as a general rule, but occasionally the water-jacket
space is formed by attaching plates or tubes to the cylinder casting by
means of bolts or screws—not an easy thing to arrange successfully, as
it requires water-tight joints.
The procedure for manufacturing a motor-car cylinder is first of all
to design and calculate the proportions of the various parts and get
out a set of working drawings. From these drawings we get _patterns_
and _core-boxes_ made in wood. The patterns are the exact shape of the
finished cylinder on the _outside_, and the core-boxes are the exact
shape of the _inside_ of the finished cylinder (except in so far as
allowance has to be made for parts which must afterwards be machined).
The patterns are pushed down into the moulding sand in the foundry,
and when withdrawn leave their impression, thus forming _moulds_. The
core-boxes are filled with sand, which when withdrawn furnishes us with
masses of sand that are the counterpart of the interior of the cylinder
in shape. These _cores_ are supported centrally in the mould (which is
usually in halves, or _more_ than two parts), while the molten iron
is poured into the intervening space to form the iron _casting_. When
the casting has cooled down the sand can be cleaned off quite easily.
One set of patterns and core-boxes will thus produce quite a number of
cylinder castings, each being similar in every respect to the other,
the process being a quick and fairly cheap method of reproduction.
Later on the cylinder barrel has to be machined and bored out true to
very fine limits by the use of boring tools and some kind of boring
machine or lathe. The flanges or flat faces have to be planed true in
a planing machine and the valve stem guides and valve seatings must be
carefully and truly machined to correct size and shape.
[Illustration: FIG. 8.—SECTIONAL DRAWING OF A T-HEADED WATER-JACKETED
CYLINDER. VALVES ON OPPOSITE SIDES OF CYLINDER.]
[Illustration: FIG. 9.—OUTSIDE VIEW OF A WATER-JACKETED CYLINDER.]
Figs. 8 and 9 show two views of a single motor-car engine cylinder,
the water-jacket forming part of the cylinder casting. In the figures
C is the cylinder barrel or bore; J the water-jacket; I the inlet for
the jacket water; O the outlet for the jacket water; D is for the
compression tap; S for the sparking plug; V_{1}, V_{2} are the valve
seats; G_{1}, G_{2} are the valve stem guides; H_{1}, H_{2} are caps
which may be removed when the valves are being put in or taken out;
f_{1}, f_{2}, f_{3}, f_{4}, f_{5} are called flanges. The flange f_{1}
is used for attaching the cylinder to the crankchamber; while it is
quite true that the force of the explosion within the cylinder drives
the piston downwards, it is equally true that it also tends to force
the cylinder head off or to blow the cylinder casting upwards off the
crankchamber, unless it is securely fastened to it by means of screws
or bolts passing through the flange f_{1}. The flanges, f_{2}, f_{3}
are for the inlet and outlet water pipe attachments, and f_{4}, f_{5}
are for the induction pipe and exhaust pipe connexions. Generally the
pipes will have flanges and be held tight against the flanges on the
cylinder casting by means of screws or studs. Figs. 10, 11, and 12 show
how two metal flanges are held in contact by means of screws or studs
or bolts, and they also show the _packing materials_ between the metal
surfaces which keep the joint _tight_ and prevent water or gas leaking
across the flanges and escaping to the outside air, or air leaking in
if the internal pressure is below that of the atmosphere.
[Illustration: STUD FIG. 10.]
[Illustration: BOLT FIG. 11.]
[Illustration: SETSCREW FIG. 12.]
[Illustration: FIG. 13.—MOTOR-CYCLE ENGINE WITH AN L-HEADED AIR-COOLED
CYLINDER. VALVES BOTH ON SAME SIDE OF CYLINDER.]
[Illustration: FIG. 14.—SECTIONAL DRAWING OF A BUILT-UP CYLINDER
SUITABLE FOR AN AEROPLANE ENGINE.]
In Figs. 8 and 9 the valves are placed one on each side of the
cylinder, this form of cylinder being known as a =T=-headed cylinder,
but it is rather more usual here in England to place both valves on
the same side of the cylinder and next to each other as indicated in
Fig. 13, this form of cylinder being known as an =L=-headed cylinder.
The chief object is of course to avoid the use of two valve shafts and
also to produce a neater looking engine, but the _T_-headed design is
better cleaned or scavenged by the passage of the inlet and exhaust
gases. When a motor-car engine has two cylinders we frequently find
them both in a single casting, having a common water-jacket, and then
we say they are _cast in pairs_. A four-cylinder engine may thus have:
(1) Cylinders _cast separately_; (2) Cylinders _cast in pairs_; (3)
Cylinders _cast en bloc_; or all four in a single large casting. The
third method is cheapest in first cost, but in the event of breakage
will become the most expensive. The second method is a sound compromise.
An example of a _built-up_ cylinder and water-jacket is shown in Fig.
14, the cylinder barrel being of steel tube with steel flanges, and
the water-jacket being formed by copper tube slipped over the outside
of the steel cylinder. Its great advantage lies in the reduction of
weight, and it is thus largely used for aeroplane work. The valves
would then be fitted in the top cover of the cylinder and driven by
overhead gearing.
CHAPTER III
ENGINE DETAILS
=The Piston= is perhaps the most important detail to consider, for
it is on the piston that the force of the explosion acts when the
heat energy is converted into mechanical energy. It must be made
sufficiently strong to withstand the bursting effect of successive
explosions, and yet if we make the metal too thick it will retain too
much of the _waste heat_ and the piston may _seize_ in the cylinder
due to expansion. To understand why the piston is likely to seize in
the cylinder we have only to remember that when a metal body is heated
it gets larger in every direction, but if cooled it returns to its
original size. Now if we make the metal of the piston too thick so
that the waste heat cannot pass _quickly_ through it and dissipate
itself at cooler parts of the engine, then the bulk of this heat will
be concentrated in the piston head, causing it to expand and become a
tight fit in the cylinder, as the cylinder walls are fairly thin and in
contact with the jacket water which keeps them fairly cool and prevents
them expanding much above their normal size. The actual amount of
expansion is very small of course, but there is very little _clearance_
between the piston and the cylinder walls, even when the engine is all
cold—perhaps five-thousandths of an inch. The piston therefore must be
strong, yet as light as we can make it, having regard to the necessity
for its being amply stiff and rigid and able to _stand up to_ its work.
[Illustration: FIG. 15.—TWO VIEWS OF A CAST IRON PISTON WITH GUDGEON
PIN AND PACKING RINGS.]
Generally it will be an iron casting in the form of a small cylinder
(see Fig. 15), having provision in it for the packing rings P, and the
gudgeon pin G, with its fastening screws S_{1}, S_{2}. The piston
itself, as we have observed, must be a nice sliding fit in the bore of
the cylinder without any shake or side play when there are no packing
rings in the grooves. The packing rings are turned to size so as to fit
the cylinder exactly and prevent any gas leaking past the piston into
the crankchamber. These rings are very light, are made from cast iron,
and arranged to break joint, as indicated, by cutting the middle ring
in the opposite direction to the two outer ones. Bosses are cast on
the inside of the piston and afterwards bored out to receive the steel
_gudgeon pin_ or wrist pin G. This pin is best made of plain parallel
cylindrical form _ground_ true, and the bosses in the piston should
be reamered out to the exact size of the pin. When the pin has been
inserted the tapered screws are screwed hard up by means of a special
spanner and bear against the pin, preventing it from coming loose or
from shaking or knocking. There are many other methods of fixing the
gudgeon pin which are not shown here; each has some special point in
its favour, but the one illustrated is undoubtedly the best and affords
a positive adjustment for wear.
[Illustration: FIG. 16.—METHOD OF FIXING THE GUDGEON PIN WHICH ALLOWS
FOR ADJUSTMENT AFTER WEAR.]
[Illustration: FIG. 17.—THREE FORMS OF PISTON HEAD.]
An enlarged view of one of the bosses, showing the taper pin in detail
and how the split pin Q prevents it from slacking back by contact with
the wall of the piston, is shown in Fig. 16. Sometimes the lower part
of the piston is made lighter by drilling holes through the walls.
It is very important to reduce the weight of the piston as much as
possible, otherwise the engine cannot attain a high speed, so that it
becomes essential to bear this in mind when constructing engines for
racing purposes. Frequently we find _steel_ pistons used, as they may
be made lighter for the same strength, and then steel piston rings may
be used; they are not much in favour for ordinary motor-car engines
because the steel pistons expand at a greater rate than the cast iron
of the cylinder, so that there is more liability to seizure. The
crown of the piston is sometimes curved upwards and at other times
curved downwards, but more often it is flat as shown in Fig. 17. The
gudgeon pin is sometimes made of mild steel, and the surface is then
case-hardened in the centre where the connecting rod end bears. At the
present time it is quite as common to find gudgeon pins made of special
nickel steel or other steel alloys that do not require case-hardening.
On the whole these special steels make the best gudgeon pins and stand
the hardest wear.
[Illustration: FIG. 18.—CONNECTING ROD IN THE FORM OF A PHOSPHOR BRONZE
STAMPING.]
=The Connecting Rod= is another very important detail of the engine
mechanism, its function being to transmit the force of the explosion
from the piston to the crankshaft.
[Illustration: FIG. 19.—STEEL CONNECTING ROD TURNED OUT OF THE SOLID
BAR.]
One end of the connecting rod moves up and down with the piston and
oscillates (or swings to and fro) on the gudgeon pin, while the other
end of the connecting rod travels in a circle, being pivoted at the
crankpin and rotating in a circle which has for its centre the centre
line of the engine crankshaft. This is clearly indicated in Fig. 18.
On the _suction stroke_ of the engine the piston has to be _pulled_
down, as we have already seen; on the _explosion stroke_ the greatest
pressure acts on the piston and _pushes_ the connecting rod down. Thus
sometimes the connecting rod is being pulled and at other times it is
being pushed; in each case it has to overcome the resistance of the
engine and drive the car. It is evident, therefore, that the character
of the load carried by a connecting rod is just about as complex and
dangerous as it is possible for a system of loading to be, and great
care has to be taken in the design of such rods to ensure adequate
strength without undue weight, as this would tend to keep down the
maximum speed of the engine. Another important consideration is the
_cost of production_, and for this reason one often finds it in the
form of a phosphor bronze _stamping_ of I section, although the ideal
form is a round section of steel with a straight taper from gudgeon
pin to crankpin end, and having a hole bored right up the centre to
reduce the weight without sacrificing much strength. When the rod is
made in the form of a stamping between _dies_ it is possible to turn
out great quantities at very low cost and at a very rapid rate, whereas
the round steel rods would require to be machined from the solid bar
to compete in price with the others. When phosphor bronze is used it
is only necessary to bore out carefully and face the bearings at the
two ends for the gudgeon pin and crankpin; the bearing at the crankpin
end is always formed with a removable cap to facilitate fitting it
nicely to the crankpin, journal and also to allow for adjustment as the
bearing wears. With steel rods it is necessary to cast a white-metal
lining in the crankpin end and then bore it out to form the bearing,
but the crosshead bearing is usually formed by a phosphor bronze bush.
It is evident, therefore, that the steel rods are more expensive, but
they make a splendid mechanical job. A steel connecting rod is shown
complete in Fig. 19. Stamped steel rods of I section are also commonly
used and are much better and stronger than those made entirely of
phosphor bronze.
[Illustration: FIG. 20.—CRANKPIN AND CRANKWEBS.]
[Illustration: FIG. 21.—FOUR-THROW CRANKSHAFT]
=The Crankshaft=, as its name implies, is a shaft with one or more
_cranks_ or right-angled bends in it. Its function is to convert the
sliding motion of the piston into the rotary motion of the flywheel and
revolving shaft. A crankshaft with a single throw (or single crank)
is shown in Fig. 20; a four-throw crankshaft is shown in Fig. 21; and
Fig. 22 shows how an equivalent motion can be obtained by a single pin
fixed into the face of a flywheel. This device (Fig. 22) is frequently
used for motor-cycle engines. Crankshafts are always made of steel;
sometimes _mild_ steel is used, but more usually special alloys of
steel containing chrome, nickel, vanadium, etc., are used. The general
practice at the present time is to machine the crankshaft direct out of
a solid bar of steel; this requires special _jigs_ for holding the work
and special tools for cutting the metal, but is the quickest, cheapest,
and most satisfactory method to adopt. A few firms specialize in this
class of work with high-grade steel and can supply crankshafts from
stock.
[Illustration: FIG. 22.—MOTOR-CYCLE CRANKPIN FIXED INTO THE FLYWHEEL.]
It is easily seen by examining Fig. 18 that the crankshaft is being
twisted in overcoming the engine resistance, while Fig. 20 shows that
the crankshaft is being _bent_ under the push from the connecting rod,
so that we say the material of a crankshaft is subjected to _combined
bending and twisting_, and as such a combination is not easy to resist
we see now why special steel alloys are required for safety, combined
with economy in material and reduction of weight. In Fig. 20 the
_crankpin_ is shown at A, the crank cheeks or _webs_ at B_{1}, B_{2}
and the crankshaft proper at C. The portions of the crankshaft C which
work in the bearings D_{1}, D_{2} are termed _journals_. A crankshaft
must be very stiff and not bend or twist sensibly, otherwise the shaft
will vibrate when the engine runs up to speed—which would be very
undesirable. It must be perfectly true with all the bearings absolutely
in line and the journals well bedded down in their respective brasses
(or bearings), otherwise mechanical troubles will arise. Each crank
with its crankpin and webs forms a lop-sided or unbalanced mass, so
that either (1) each crank must have its own _balance weight_ as in
Fig. 23, or (2) special balancing masses must be fitted at each end of
the crankshaft. A convenient method of balancing the crankshaft is to
have a fan pulley at one end and the flywheel at the opposite end, so
that by drilling holes in the faces of these discs an amount of metal
may be removed from them sufficient to balance the excess weight of the
respective crankpins and webs. In Fig. 24 the shaded area indicates
that portion of the crank which constitutes an unbalanced mass.
Crankshafts for high-speed engines have always to be very carefully
balanced, otherwise the engines will never run satisfactorily, the want
of balance being greatly aggravated as the speed of rotation increases.
Fig. 25 shows how the crankshaft of a two-cylinder engine may be
balanced by drilling holes in the flywheel and fan pulley respectively,
but the same effect may be produced by attaching balancing masses—this
latter method would, however, be more inconvenient and expensive. The
crankpins and journals are _ground_ truly circular after being turned
in the lathe as true as possible.
[Illustration: FIG. 23.—SKETCH OF A BALANCED CRANK.]
[Illustration: FIG. 24.—SKETCH SHOWING THE UNBALANCED PORTION OF THE
CRANK BY SHADED LINES.]
[Illustration: FIG. 25.—TWO-THROW CRANKSHAFT BALANCED BY DRILLING HOLES
IN FLYWHEEL AND FAN PULLEY.]
[Illustration: FIG. 26.—SKETCH TO ILLUSTRATE THE FORCES ACTING ON A
FLYWHEEL RIM.]
=The Flywheel.=—We have already described how the force driving the
piston of a motor-car engine varies during the four strokes of the
cycle, but we must note that it also varies considerably during each
individual stroke. Thus, on what is known as the _explosion stroke_ (or
power stroke) of the cycle, the pressure at the commencement of this
stroke may be exceedingly great and yet towards the end of the stroke
the gases have expanded and the exhaust valve has been opened, so that
the pressure acting on the piston is then very small. Again, on the
compression, suction, and exhaust strokes, the piston has to be pushed
or pulled by some means, as _no power is being generated_. Therefore,
if the engine is to be self-acting and run continuously, some means
must be provided for storing up the great force of the explosions and
giving it out again on the idle strokes. The function of the flywheel
is to _store_ any energy given to it over and above that required to
drive the car and to give it out again when required for performing the
functions of compressing, exhausting, and sucking in gas, as well as
to keep the car running steadily. It is simply a heavy wheel mounted
on the end of the crankshaft which, when once started revolving at a
high speed, is not easily stopped, and which will give up part of
its energy each time its speed is reduced owing to the demands of the
engine; but when the engine is generating power the wheel will speed
up and store the excess—the mere fact that the wheel is heavy causes
these changes in speed to occur slowly, and therefore on the whole the
_fluctuation_ of speed is not great when a suitable flywheel is fitted.
The flywheel does _not_ limit the _maximum_ speed of the engine, as it
could go on slowly increasing in speed if no resistance was encountered
until the wheel finally _burst_ or flew to pieces. Thus the flywheel
does not regulate the _speed_ of the engine; it merely smooths out the
inequalities in the several _strokes_ of the “cycle.” Flywheels of
motor-car engines are now always made of steel, so that they can be
safely run at speeds up to 3,000 revolutions per minute without fear
of the rim bursting. All parts of the rim tend to fly off radially in
the direction of the arrows as shown in Fig. 26 under the action of
_centrifugal force_. A built-up flywheel is shown in Fig. 27, and one
made from a single stamping of steel is shown in Fig. 28. Generally
speaking, when a coned clutch is fitted one portion of it is formed
on the inside of the flywheel rim as indicated in these two figures.
When the construction shown in Fig. 28 is adopted the lining would be
inserted _after_ the clutch cone had been put into place; very often
the lining is made up of sections which can be readily inserted or
withdrawn after the cone is in position.
[Illustration: FIG. 27.—A FLYWHEEL BUILT UP FROM STEEL FORGINGS.]
[Illustration: FIG. 28.—A FLYWHEEL TURNED FROM A STEEL STAMPING.]
CHAPTER IV
THE VALVES
=Poppet Valves.=—Valves are provided for the purpose of controlling
the admission of the mixture to the cylinder and also for controlling
the exhaust or ejection of the burnt gases at the end of the firing
stroke. The most common form of valve is the mushroom or _poppet_ type
of valve shown in Fig. 29, in which A is the valve head, B is the
valve stem, C is the valve seating, and D is the cotter hole for the
cotter E. It will be seen that the general appearance of the valve is a
disc of steel with a fine stem to it similar to a mushroom in general
outline—hence its name. The valve has a coned face which is kept
pressed down on a coned seating by means of the pressure of a powerful
spring F acting on the washer G, which bears against the cotter E and
thus presses down the valve stem. To ensure that the valve shall always
come down correctly on its seating and make a gas-tight joint, the
valve stem guide M is provided.
[Illustration: FIG. 29.—GENERAL ARRANGEMENT OF A POPPET VALVE (A) WITH
TAPPET (K) AND CAM (H).]
The cam H raises the valve off its seat at the required instant when
the motion of the camshaft brings the cam under the roller R. The cam
lifts the roller vertically and with it the tappet or push rod K, which
slides vertically upwards in the guide P and lifts the valve. The
tappet is provided with an adjustable head S kept in position by the
locknut T. To adjust the _clearance_ between the head of the tappet and
the underside of the valve stem the locknut T must first be slackened
back and then the head S can be screwed up or down as desired, the best
clearance being about 1/64 of an inch; the locknut is then tightened
down again. During this operation the valve must be down on its seat.
Sometimes to reduce the noise arising from the tappet striking the
valve stem, the head of the tappet is padded with some material such as
hard vulcanite fibre, but this wears down more quickly than steel and
requires frequent adjustment. The latest device for reducing the noise
arising from the valve mechanism consists in totally enclosing the
valve gear and springs either by metal plates bolted to the cylinder
casting or by extending the crankchamber to cover it all in, and then
it is certain to be well lubricated. The exhaust valve is always liable
to give trouble either from leakage or seizure or other causes due to
the great heat of the exhaust gases, so that the valves are often made
now of tungsten steel alloy which is not much affected by heat. If a
mushroom type valve leaks it can be _ground in_ and made a tight fit
on its seating, provision usually being made for this in the form of
a slot cut in the valve head, as shown in Fig. 32, for the insertion
of a screwdriver or special tool. To grind in a valve, remove the cap
Q by unscrewing it, raise the spring F by pushing up the washer G and
then withdraw the cotter E. Lift out the valve and smear the coned face
with fine emery powder and oil (or water). Put the valve back and turn
it to and fro on its seating by means of the screwdriver, keeping a
firm pressure down on it; continue the operation until by examining the
valve you ascertain that it touches on the seating all the way round,
then couple up the spring again, after carefully removing all traces of
the emery powder.
[Illustration: FIG. 30.—SECTIONAL DRAWING OF THE CYLINDER OF A SLEEVE
VALVE ENGINE, WITH INLET FORTS UNCOVERED.]
[Illustration: FIG. 31.—SECTIONAL DRAWING OF THE CYLINDER OF A SLEEVE
VALVE ENGINE, WITH _EXHAUST_ PORTS UNCOVERED.]
=Sleeve Valves.=—Another form of valve which has come very much into
favour is the sleeve valve, two views of which are shown in Figs. 30
and 31. In this case the gases enter the cylinder through ports or
slots P cut in the cylindrical cast iron sleeves S_{1}, S_{2}, which
are placed between the piston K and the walls of the water-jacketed
cylinder C. These sleeves are moved up and down inside the cylinder,
while the piston travels up and down inside the inner sleeve S_{2} just
as though it constituted the cylinder C. Some engines have two sleeves,
as shown in the figure, but others have only one sleeve, and there is
very little to choose between the two types on the score of efficiency.
The great claim made for the sleeve valve is that it is almost
noiseless in action and gives very much fuller openings for inlet and
outlet of the gases. The piston has the usual number of packing rings
to keep it gas-tight, and there is also a deep packing ring provided
in the head of the cylinder H to keep the sleeve S_{2} gas-tight and
prevent loss of compression pressure. The head of the cylinder is
usually detachable, and has often separate water connexions in the form
of pipes leading from the cylinder jackets. The sleeves receive their
reciprocating motion from eccentrics and rods attached to pins shown
at the bottom right-hand corner of each sleeve. It might be expected
that the sleeves would get very hot or very dry and seize up, or the
piston might seize, but in actual practice this has not occurred to any
great extent, and on the whole they have been very successful. It is,
however, necessary to keep the engine well lubricated, especially when
the sleeves get worn, as the oil prevents loss of gas by leakage past
the sleeves and piston. In Fig. 31 the two sleeves have come together
in such a position that the ports coincide with the exhaust ports cut
in the cylinder walls and therefore the exhaust is _full open_, and
as the sleeves travel at times in opposite directions quick opening
and closing of the ports is secured. The cylinder head is held down to
the cylinder casting by screws or bolts and can be readily detached
for cleaning or inspecting the interior of the cylinder. The great
objection raised against the sleeve valves is their excessive weight
and the unmechanical manner in which they are operated.
[Illustration: FIG. 32.—SKETCH SHOWING SLOT IN A POPPET VALVE HEAD FOR
GRINDING-IN PURPOSES.]
[Illustration: FIG. 33.—INLET (A) AND EXHAUST (B) VALVE CAMS FOR A
SLOW-RUNNING PETROL ENGINE.]
=The Camshafts and Eccentric Shafts.=—These are usually made from the
same material as the crankshaft and machined from the solid bar, the
projecting cams or eccentrics being afterwards cut to the correct
shape. In the case of a camshaft it is very important that the shape
of the cams should be such that they lift the valves quickly off their
seats to the full extent of their opening (or _lift_), keep them open
for as long a period as desirable, and then allow them to close quickly
but without shock. Cams which have straight sides are more in favour
than those with curved sides, but if the action of the cams is to be
theoretically correct the side of the cam should be curved in such a
manner that the valve is lifted at first with a uniformly increasing
speed and afterwards with a uniformly decreasing speed, so that it will
be at rest in its top position. If this is not done the valve tappet
may jump a little above the cam each time the valve is lifted. In
Fig. 33 the cam A is intended for the _inlet_ valve and the cam B for
the _exhaust_ valve, the essential difference being that the exhaust
valve must be kept open longer than the inlet valve, and therefore
the exhaust valve cam is the wider of the two. The _timing_ of the
inlet and exhaust valves of an up-to-date engine may be explained by
considering the crankpin circle as divided into 360 parts or _degrees_.
If there were no _lag_ or _lead_ in the opening of the valves, then
they would open when the crank was on its dead-centre and close when
the crank was on its dead-centre. The inlet valve would open when
the crank was on its top dead-centre and close when it had reached
its bottom dead-centre, this representing the _suction_ stroke of
the engine. Then would follow compression and explosion, giving two
strokes or one revolution before the exhaust valve commenced to open.
The exhaust valve would then open when the crank was on its bottom
dead-centre and close when the crank reached its top dead-centre
corresponding to the completion of the exhaust stroke. It is very
important that the pressure of the gases above the piston when it
commences to move upwards on the exhaust stroke should be as low as
possible, and this can only be secured by opening the exhaust valve
towards the end of the _explosion_ or power stroke, thus allowing
the bulk of the gases to escape and leaving the piston with little
resistance to encounter on its upward exhaust stroke. Therefore we give
the exhaust valve a _lead_ of about 30 _degrees_, which means that it
begins to open when the engine crank is 30 degrees from the bottom
dead-centre on the downward explosion stroke, and we give it a _lag_
of about 5 degrees in closing. This means that we keep the exhaust
valve open until the crank has moved 5 degrees over the top centre, so
that we may fully utilize the _momentum_ of the gases to clear out the
cylinder or _scavenge_ it. As the piston moves rapidly up the cylinder
on the exhaust stroke it pushes the gases in front of it out through
the exhaust opening, but when it gets to the top of its stroke the
piston stops and then comes down again for the suction stroke, whereas
the gases will tend to keep on moving if they are not unduly restricted
in their passage through the exhaust system, so that we can generally
obtain some slight advantage by giving the exhaust valve a small amount
of _lag_ in closing.
The pressure of the gases in the cylinder after the exhaust valve
closes will nearly always be a little above atmospheric pressure, and
therefore nothing is gained by opening the inlet valve immediately the
exhaust closes—we generally allow an interval of 5 degrees, which means
that the total _lag of the inlet_ valve is 10 degrees in opening, or
the inlet valve does not begin to open until the crank has moved 10
degrees off its top dead-centre on the downward suction stroke. At the
end of the suction stroke the piston will again come to rest before
moving up on the compression stroke, but the gases will continue to
rush into the cylinder from the carburettor owing to their _momentum_
if we leave the inlet valve open a little longer, hence we generally
give it a _lag_ of 20 degrees in closing, which means that the inlet
valve does not close until the crank has moved 20 degrees up from the
bottom dead-centre on the compression stroke.
[Illustration: FIG. 34.—ECCENTRIC SHEAVE (A) AND ECCENTRIC ROD (B) FOR
OPERATING A SLEEVE VALVE.]
The camshaft requires to be well supported in bearings to prevent
it from sagging or bending under its load. If the shaft and the
cams are not made from nickel steel or high-grade steel alloy, they
require to be case-hardened (hardened on the surface) to prevent wear
on the surfaces due to the pressure of the valve springs, which is
considerable and may reach 100 lb. per valve easily; the same applies
to the rollers of the tappets. When sleeve valves are fitted to the
engine, _eccentric sheaves_ must be used instead of cams, as no springs
are employed. An eccentric sheave with its _strap_ and rod are shown
in Fig. 34. The valve shaft or lay shaft is shown at C, and the sheave
with the hole bored eccentrically is shown at A, and B is the combined
eccentric strap and rod. The pin D operates the sleeve valve, giving it
a reciprocating motion in a vertical direction, the angular movement
being taken up by the oscillation of the rod about the pin D, which
would be fixed into the sleeve. Sometimes a groove is formed round
the periphery of the eccentric disc or sheave to keep the strap in
position and prevent end movement. As the weight of the sleeves is very
considerable, the pin D and the eccentric rod must be well proportioned
to prevent breakage or undue wear.
[Illustration: FIG. 35.—A PAIR OF TIMING WHEELS.]
=The Timing Wheels=.—As there is only one suction stroke and one
exhaust stroke in every two revolutions of the engine crankshaft, it
will be clear that the camshaft or eccentric shaft must be driven at
_half the speed_ of the engine crankshaft. This may be done by the use
of two _gear wheels_ or wheels having teeth cut on their periphery,
such wheels when used for this purpose being called _timing wheels_,
because the positions of the cams on the camshaft (or the eccentrics on
the eccentric shaft) _relative_ to the engine crankshaft when the teeth
of the timing wheels are put into mesh determines the _timing_ of the
inlet and exhaust valves, i.e., the instant at which they will open or
close. A pair of timing wheels is shown in Fig. 35. The pinion A has
twelve teeth and is keyed to the engine crankshaft, but the wheel B,
which is keyed to the valve shaft, has twenty-four teeth, and hence
the valve shaft runs at half the speed of the crankshaft. The wheels
shown are _spur_ gears, and the teeth run straight across the rim of
the wheel; it is, however, quite common to find wheels with curved or
_helical_ teeth, as these run quieter. Sometimes when spur gearing
is used, one of the wheels is made of fibre and the other of steel,
but when helical gears are used the wheels are generally made from
nickel steel of high tensile strength. The finer the _pitch_ of the
teeth (i.e. the distance between the centres of consecutive teeth) the
quieter the gears will run, but the question of strength and the cost
of production must also be considered. The latest practice is to use
a silent _chain_ drive; this originated with the introduction of the
sleeve valve and eccentric shaft. When chains are used for the timing
wheels provision must be made for taking up slack in the chain owing
to stretching of the links, and as this cannot be done in the usual
manner (by sliding the _sprocket wheels_ further apart) owing to the
centres of the crankshaft and the valve shaft being rigidly fixed by
the bearings, a small jockey pulley (with teeth on it similar to those
on the chain sprocket wheels) is provided attached to a short shaft
or spindle, which can be raised or lowered at will, and thus keep the
correct tension on the chain. The chain drive must be more expensive
and require more attention; moreover, it cannot be so very much quieter
in action than good well-cut helical gearing.
[Illustration: FIG. 36.]
[Illustration: FIG. 37. TWO VIEWS OF A CRANKCHAMBER.]
=The Crankchamber.=—The crankchamber, as its name implies, is the
receptacle which contains and supports the crankshaft and also the
camshaft. It is generally an aluminium casting, but frequently for
commercial vehicle engines the top portion is made of cast iron and
the bottom portion of sheet steel. In either case brass or gunmetal
bearings, often lined with white metal, are fitted for the shafts
to revolve in, and the engine cylinders are mounted on the top of
the chamber. Provision should be made on the sides and ends of the
crankchamber for fitting the _magneto_ and _oil pump_ and also the
_water pump_, if required. There must also be some form of housing or
extension of the chamber to enclose the timing wheels, and sometimes
the whole of the valve gear is contained within the crankchamber to
ensure proper lubrication for it and stop any noise from it reaching
the outside world. It is also important that there should be large
inspection openings fitted with proper oil-tight covers and some
provision for easily pouring large quantities of oil down into the
lower portion of the chamber. The design of a crankchamber necessitates
careful forethought to ensure ample provision for all the necessary
attachments and fittings and to secure the maximum accessibility of all
parts. One or two _vent_ pipes, consisting of upwardly projecting pipes
having their outer end covered with wire gauze and screened from dust
should be provided to allow hot air and gas to escape from the chamber.
Two views of a crankchamber of modern design are shown in Figs. 36
and 37. In these figures A is the top half of the crankchamber which
rests upon the _chassis_ or framework of the car, being bolted to an
_underframe_ at B and C. The cylinders are attached to the chamber at
the flange H by means of studs and nuts. This portion, the top half of
the crankchamber, requires to be very strong and stiff, because the
upward pressure of the explosions acts on the crown of the cylinder and
tends to tear the cylinder off the flange H, while at the same time it
exerts a great force on the piston, pushing it downwards and tending to
force the crankshaft down out of its bearings. In the best practice the
whole weight of the crankshaft is supported from the top half of the
crankchamber and is carried on the bearing bolts as shown at S, so that
they also receive the downward thrust of the piston and in their turn
transmit it to the main casting.
The bottom half of the crankchamber then becomes merely an oil
container, or reservoir, and dust cover; it should be so arranged and
situated that it may be readily removed for inspection of shaft and
bearings from underneath. Sometimes the crankchamber has long arms,
which can be attached directly to the side members of the chassis, or
it may be supported in the chassis by a tubular cross member.
In Fig. 37 the camshaft is shown at T; the magneto would be carried on
the bracket E and driven by gearing from the crankshaft. The facing at
G is for the water pump, which, in this case, is intended to be mounted
on an extension of the camshaft T. The oil pump would be fixed at F,
preferably towards the rear of the engine, so as to secure an adequate
supply of oil for the pump when the car is climbing a steep hill.
The oil could be drawn off and the reservoir emptied by unscrewing
the large plug shown in the centre of D in Fig. 37. The timing wheel
housing or casing is shown at Q; the oil ducts and connexions for
supplying the main bearings with oil are not shown in these drawings,
nor are the inspection openings and covers. The upper half of the
crankchamber frequently becomes very hot, due to conduction of heat
from the metal of the cylinders, and for this reason it has from time
to time been proposed to draw the air supply of the carburettor through
the crankchamber to serve the dual purpose of cooling the bearings and
heating the air supply to the carburettor; but the idea has not found
favour, as there is considerable risk of dust and grit finding its way
into the bearings and causing trouble due to abrasion.
CHAPTER V
THE CARBURETTOR AND CARBURATION
A carburettor is a contrivance for supplying an explosive mixture of
air and petrol vapour to a petrol engine. Petrol, although a liquid
fuel, is a combination of carbon and hydrogen which, when supplied with
the necessary air, can be burnt and thus evolve heat, which heat is
turned into work inside the engine cylinder. What we have to supply to
the engine is really a mixture of _air_ and _petrol vapour_ in certain
proportions, such a mixture being often spoken of as _carburetted
air_ on account of the carbon contained in it. About two parts of
petrol vapour (by volume) are required to every one hundred parts of
mixture, or fifteen _pounds_ of air to every pound of petrol vapour
(by weight). This carburetted air must be of the required strength and
form a homogeneous mixture in the form of a vapour. The problem of
_carburation_ consists in forming a mixture of the correct strength
and character. Air may be carburetted by passing it over the surface
of liquid petrol in a _surface carburettor_, or by drawing it over
or among wicks saturated with liquid petrol as in the _wick type of
carburettor_, but both these methods have been largely superseded by
the use of what is now known as a _jet_ or _spray type_ of carburettor,
in which the petrol is sprayed from a fine jet and mixes with air which
is passing up rapidly round the outside of the jet. In all cases,
however, the liquid petrol must be _vaporized_ before entering the
engine, and to do this _heat_ must be supplied to the mixture, just
as water has to be heated before it can be vaporized and turned into
steam. Under ordinary circumstances sufficient heat can be obtained
from the incoming air to effect vaporization of the liquid petrol if
it issues in the form of a very finely divided spray, but when the
demand for mixture, from the engine, is great the air cannot supply the
requisite heat without its temperature falling below the vaporization
point; hence most carburettors of up-to-date pattern are fitted with
a _mixing chamber_ surrounded by a hot-water jacket. The essential
features of the carburetting plant are shown diagrammatically in Fig.
38, in which A is the _petrol tank_ fitted with the _petrol tap_ G, to
which is coupled the _petrol pipe_ F. Some form of _petrol filter_ as
indicated at B should be placed between the tank and the carburettor
C. The throttle valve of the carburettor is shown at H, the _extra-air
valve_ at E, and the engine induction pipe at D.
[Illustration: FIG. 38.—GENERAL ARRANGEMENT OF THE CARBURETTING PLANT,
SHOWING PETROL TANK (A), PETROL FILTER (B), CARBURETTOR (C), AND
EXTRA-AIR VALVE (E).]
The carburettor proper may be constructed in a variety of forms, but
the elements of which it is composed are: (1) the float chamber A, (2)
the petrol jet B, (3) the choke tube C, (4) the mixing chamber D, and
(5) the throttle valve E, as shown in Fig. 39.
[Illustration: FIG. 39.—SECTIONAL DRAWING OF A CARBURETTOR OF THE _JET_
TYPE.]
=The Float Chamber= is generally cylindrical in form and the liquid
enters at the bottom, the flow being regulated by a pointed rod called
a _needle valve_. A hollow metal float which can slide freely up and
down the needle valve stem operates two levers which are pivoted on
the float chamber cover. It is well known that when a body is immersed
in a liquid the liquid exerts an upward pressure on the body equal to
the weight of liquid displaced by the body. The float being hollow
and made of very thin sheet metal, displaces a very large quantity
of liquid in proportion to its own weight, and is therefore very
_buoyant_. The buoyancy of the float will, of course, depend on the
_density_ of the liquid in the float chamber, and it will naturally
sink deeper down into petrol than it would into a heavier spirit such
as paraffin or benzol. The action of the float is as follows:—Supposing
the petrol to be turned off and the needle valve lifted up off its
seating, then on turning on the petrol supply the petrol will run into
the float chamber, and as the level of the liquid rises the float will
rise too, lifting up the outer ends of the levers and depressing the
needle valve down on to its seating by means of the collar which is
rigidly attached to the spindle of the needle valve. If at any time the
level of the liquid in the chamber falls, the float will fall also,
thus allowing the outer ends of the levers to drop and raise up the
needle valve from its seating; this allows more petrol to enter the
chamber and raises the float again, thus keeping a constant level in
the chamber.
The height of the orifice in the top of the petrol jet above the bottom
of the float chamber determines the height at which we require the
liquid to stand in the chamber. As a general rule the level of the
liquid in the float chamber should be slightly below the top of the jet
orifice to prevent the liquid oozing over and causing _flooding_ or
continuous dripping of petrol from the jet, even when the engine is not
running. The height of the collar on the needle valve spindle must be
adjusted until the float closes the valve down on its seating when the
liquid has risen to the desired height in the float chamber. Hence, if
a carburettor has been adjusted to work with petrol, it will require
to have some slight extra weight added to the float when working with
heavier spirits to cause it to sink to the required depth in these
denser spirits.
=The Petrol Jet and Choke Tube.=—The _petrol jet_ generally consists
of a short tube of fine bore, one end of which contains a very small
orifice for the purpose of spraying the petrol into the choke tube.
When the engine is at rest it is easily seen that the pressure of the
air in the choke tube is atmospheric, and that the pressure above the
liquid in the float chamber is also atmospheric, but when the engine is
running it draws air up the choke tube at a very high speed and thus
causes a partial vacuum round the petrol jet, and therefore the petrol
spurts out of the jet under the pressure difference which then exists
and issues in the form of a fine spray which is readily vaporized. The
choke tube is purposely made of rather small diameter, in order to
get a high air speed, which results in a low pressure round the jet
and ensures a good driving force to spray the petrol out of the jet.
The speed of the engine is controlled by the position of the throttle
valve or disc E, which regulates the amount of air flowing up the choke
tube, and therefore incidentally checks the quantity of petrol issuing
from the jet by regulating the vacuum in the neighbourhood of the jet
orifice. At low engine speeds there is very little suction or vacuum
effect on the jet, but at high engine speeds with full throttle opening
the maximum suction of the engine is exerted upon the jet. Thus at
low speeds with this type of carburettor we do not get enough petrol
out of the jet, and at high speeds we get too much, which results in
_too weak_ a mixture at low speeds and _too rich_ a mixture at high
speeds. One reason for this is that the air flows out of the choke
tube _faster_ than it flows into it, owing to the fact that its volume
increases as the pressure decreases, and hence the pressure round the
jet falls very rapidly indeed as the air velocity increases and causes
too much petrol to issue from the jet in proportion to the quantity of
air flowing through the tube. The _choke tube_ is often a plain piece
of pipe, as shown in Fig. 40, instead of being tapered as in Fig. 39.
=The Mixing Chamber and Throttle Valve.=—The throttle valve is usually
a plain flat disc of metal mounted on a spindle which can be rotated
and thus regulate the size of the air passage to the engine. It is
placed above the petrol jet and situated in the mixing chamber, which
is simply a short length of pipe (of the same bore as the engine
induction pipe) surrounded by a hot-water jacket, the supply of hot
water being drawn from the engine cooling system. The heat from this
jacket should be sufficient to make up for the fall in temperature
that would otherwise result due to the vaporization of the petrol as
explained above.
[Illustration: FIG. 40.—PLAIN FORM OF CHOKE TUBE.]
=Recent Improvements in Carburettors.=—Another defect of this simple
type of carburettor becomes apparent in the larger sizes required for
multi-cylinder engines. To pass the requisite quantity of petrol to
keep the engine running at high speeds without creating too great a
suction effort and thereby hampering the engine, necessitates the use
of a jet of larger calibre, so that the liquid is no longer _sprayed_
but issues in the form of a fine stream which is not readily vaporized.
This has been overcome by the use of _multiple-jet carburettors_
which have several jets each surrounded by its own choke tube, but
all controlled by one throttle valve and supplied from one common
float chamber. In this case the total cross-sectional area of all the
jet orifices together could be made sufficient to pass the necessary
quantity of fuel, but the bore of each individual jet orifice would
be comparatively small and spraying would result as before. Another
very successful device is shown in Fig. 41, in which A is the petrol
jet which, in this case, has no special orifice and is surrounded by a
larger tube B containing small holes for the inlet of air and outflow
of petrol. As the petrol issues from the jet it strikes against the
pointed cone on the end of the screw C, and is thus very successfully
_atomized_ and broken into small particles which can be readily
vaporized.
[Illustration: FIG. 41.—PETROL JET, SPECIALLY ARRANGED FOR _ATOMISING_
THE PETROL.]
[Illustration: FIG. 42.—COMPENSATED PETROL JET. A IS THE MAIN JET AND B
THE COMPENSATING JET SUPPLIED THROUGH THE ORIFICE C.]
There are several devices for keeping the strength of the mixture
constant at all engine speeds irrespective of the amount of vacuum in
the choke tube. One of the best of these is illustrated in Fig. 42,
and consists in the use of a _compensating jet_. The main petrol jet A
is of sufficient size to supply the requirements of the engine under
full speed and with the resulting high vacuum; it is fed directly
from the float chamber in the usual manner. The compensating jet B
surrounds the main jet and is supplied with petrol through an orifice
C, so arranged that it offers a greater resistance to flow than the
passage up the centre of the main jet. At all engine speeds up to a
certain predetermined maximum the compensating jet will supply most of
the petrol, but as the demand increases the main jet will also begin
to supply, and simultaneously the compensating jet will commence to go
out of action owing to its supply of petrol becoming partly or wholly
exhausted due to the restriction of the orifice C.
The simple jet-in-tube carburettor has been greatly improved by the
addition of an _automatic extra-air valve_, of which a simple form
is shown in Figs. 43 and 44. It consists of a small mushroom type
valve A, with its seating B so arranged that it can be screwed into
the induction pipe of the engine. The valve is held up against its
seating by a light spring C, so that at high engine speeds when there
is a good vacuum in the induction pipe the pressure of the atmosphere
will open the valve against the tension of the spring and allow air to
pass into the induction pipe, thus reducing the amount of vacuum and
simultaneously weakening the mixture.
[Illustration: FIG. 43.—AUTOMATIC SPRING CONTROLLED EXTRA-AIR VALVE.]
[Illustration: FIG. 44.—PLAN VIEW OF AUTOMATIC EXTRA-AIR VALVE.]
The points of a =good carburettor=:—
These may be set out in the following order—
(1) Complete _atomization_ and _vaporization_ of the liquid fuel at
all engine speeds.
(2) The supply of an _adequate quantity_ of gas of the _correct
proportions_ with all throttle openings and at all temperatures.
(3) Sufficient _mechanical strength_ and _durability_ to withstand
road shocks and to ensure freedom from breakdowns without undue
_weight_ or _complications_.
(4) Ability to continue working correctly when the car is on an
incline or affected by the camber of the highway.
(5) Moderate first cost.
=Pressure Feed and Gravity Feed.=—In Fig. 38 we showed a gravity-fed
system or one in which the petrol is fed from the tank to the float
chamber of the carburettor by the action of gravity only. For this
system to be successful at all times the carburettor must be placed low
down to obtain a good _head_ for the flow of petrol in the connecting
pipes, as there is a practical limit to the height at which the petrol
tank can be fixed. Also the pipes must have a continuous run down
towards the float chamber to prevent air-locks in them, and they must
be kept away from the hot exhaust system. When all these points can be
secured this system is perfect. An alternative system is to force the
petrol into the float chamber by maintaining an air pressure (of 2 or 3
lb. per square inch) on the surface of the liquid in the petrol tank.
With this arrangement the carburettor may, if desired, be situated
_above_ the level of the petrol tank in a more _accessible_ position,
but it necessitates the fitting of a small _air pump_ on the engine and
the use of a _hand air pump_ for starting.
CHAPTER VI
IGNITION AND IGNITION DEVICES
We have already stated that the charge of explosive mixture is ignited
in the cylinder at the end of the compression stroke by means of an
electric spark. The electric spark takes place as the result of an
electric discharge across the _gap_ between the _electrodes_ of the
_sparking plug_.
[Illustration: FIG. 45.—SECTIONAL DRAWING OF A SPARKING PLUG.]
=The Sparking Plug.=—Two views of a typical sparking plug are shown in
Figs. 45 and 46, in which A is the _high tension_ electrode which is
periodically charged with electricity at high voltage (or electrical
pressure) from a high tension magneto or a high tension coil, and
B_{1}, B_{2} are electrodes which, being in metallic contact with the
cylinders and framework of the engine, are thus at zero potential. The
electric discharge occurs across the gap C_{1}, C_{2} in the form of a
spark or flash. The electrode A is heavily _insulated_ from the metal
casing D of the sparking plug by porcelain insulators E and F. The
locknuts G and H serve to keep the plug gas-tight and hold the several
portions together mechanically. The terminal K is used for clamping the
wire (or lead) which brings the supply of high tension electricity.
The high tension electric current may be supplied either by (1) a
magneto machine or (2), a coil and accumulator ignition system.
[Illustration: FIG. 46.—A SPARKING PLUG.]
[Illustration: FIG. 47.—OUTSIDE VIEW OF A HIGH TENSION MAGNETO.]
[Illustration: FIG. 48.—END VIEW OF A HIGH TENSION MAGNETO, SHOWING
HIGH TENSION DISTRIBUTOR AND LOW TENSION CONTACT BREAKER.]
=The High Tension Magneto.=—In Figs. 47, 48 and 49 we show a modern
high tension magneto suitable for a four-cylinder engine. It consists
of the stationary magnets A, the driving spindle B, the high tension
electrode D, the high tension distributor C, and the low tension
contact breaker E. The armature, condenser, and distributor gear wheels
are not shown in the drawings, but are situated _inside_ the machine
in the space between the high tension electrode D and the low tension
contact breaker E. As the spindle B is rotated by gearing driven from
the engine crankshaft the armature attached to it generates a high
tension current and a low tension current. The high tension current
passes to the high tension electrode D and thence across the machine to
the carbon brush H of the high tension distributor C. The low tension
current passes through the platinum-tipped contact screws F_{1}, F_{2}
of the low tension contact breaker. Twice during each revolution of the
armature these contacts are separated owing to the fibre block attached
to the bell crank lever G passing over the stationary cams T_{1},
T_{2}; this constitutes the _make-and-break_ device for interrupting
the primary current. The momentary interruption of the primary current
in this way causes a very great increase in the electrical pressure (or
voltage) of the secondary or high tension current which is sufficient
to bring about the spark discharge across the gap between the
electrodes of the sparking plug. Since there are two of these cams on
the low tension contact breaker it will be understood that the armature
can supply current for two sparks in every revolution it makes. If
we bear this fact in mind we will have no difficulty in determining
the relative speeds of the magneto armature and the engine crankshaft
for any type of engine. A four-stroke engine requires one spark in
every two revolutions made by the crankshaft, so that a four-cylinder
engine of this type requires two sparks per revolution, and the magneto
armature must run at crankshaft speed. A six-cylinder engine working on
the four-stroke cycle would require three sparks per revolution, but
the armature of the magneto only supplies two, therefore it must be
driven at one-and-a-half times the crankshaft speed.
[Illustration: FIG. 49.—END VIEW OF A HIGH TENSION MAGNETO, SHOWING THE
EARTHING TERMINAL (P).]
The high tension distributor consists of the carbon brush H driven by
gearing from the magneto armature and the metal segments M_{1}, M_{2},
M_{3}, M_{4}, which are mounted in a block of insulating fibre. There
must be as many segments on the distributor as there are cylinders on
the engine, one segment for each sparking plug; but the armature cannot
supply more than _two_ sparks per revolution, and therefore if the
distributor has four segments it must be driven at half the armature
speed, and if it has six segments it must be driven at one-third of
the armature speed. Each metal segment is electrically connected to
a sparking plug lead such as L_{1}, L_{2}, L_{3}, L_{4}. The high
tension electrode D is attached to a light carbon brush which presses
on a gunmetal collector ring at the high tension end of the armature
winding. A special terminal is provided at P, so that when a wire
is attached to it and connected to the frame of the engine (usually
through a switch) the low tension windings are _short-circuited_ or
closed on themselves, and the make-and-break has no effect, because
there is always the path through the switch until it is opened again.
Under these circumstances the voltage of the high tension circuit is
not sufficient to cause the spark discharge, and the ignition is then
said to be _switched off_. The instant at which the spark occurs may be
_advanced_ or made earlier by moving the rocker arm K, which carries
the stationary cams T_{1}, T_{2} _backwards_, whereas if it is moved
_forward_ the ignition is _retarded_ and occurs later in the stroke.
_Normal_ ignition occurs when the lever is midway in its range of
movement and corresponds to the position of the piston when the crank
is on the top dead-centre, whereas advanced ignition occurs just before
the piston has completed the compression stroke, and retarded ignition
will take place after the crank has passed the dead-centre and when
the piston has moved down a little on the power (or explosion) stroke.
Advancing the ignition increases the speed, and retarding the ignition
reduces the speed, except when the engine is overloaded, and then it
may pick up speed a little or run better if the ignition is slightly
retarded—but the exact behaviour will depend on the temperature of the
metal walls and piston within the cylinder.
[Illustration: FIG. 50.—AN IGNITION COIL, SHOWING THE TREMBLER
MECHANISM.]
We have mentioned that normal ignition occurs when the crank is exactly
on the dead-centre and the piston at the top of its stroke. If we set
the magneto when the engine is at rest so that ignition _ought_ to
occur on dead-centre when the arm K is in its mid position the actual
sparking will be _late_ on account of the _time lag_ of the electric
current. The current takes time to flow and in that brief element of
time the crank has moved a few degrees off the dead-centre, at high
speeds. Hence the ignition _must be advanced_ if the charge is to be
correctly fired when the engine is running fast. If the ignition is
too far advanced it will cause the engine to “_knock_,” especially
under heavy loads. If the ignition is _retarded_ the charge is not
fired at the commencement of the stroke so that a portion of the power
theoretically available in the fuel is lost to exhaust at the end of
the stroke. Retarded ignition always causes overheating of the exhaust
system.
If the arm K is fixed mechanically in its mid position so that the
ignition can neither be advanced nor retarded, we have what is known as
_fixed_ ignition.
[Illustration: FIG. 51.—IGNITION COIL CASE.]
[Illustration: FIG. 52.—LOW TENSION CONTACT BREAKER FOR SINGLE CYLINDER
COIL IGNITION SYSTEM (WIPE CONTACT).]
=An Ignition Coil= suitable for a single cylinder engine is shown in
Figs. 50 and 51, in which A and B are the low tension terminals and
C is the high tension terminal. The trembler blade is shown at D,
with the adjusting screw F and the platinum-tipped contacts G_{1},
G_{2}. The iron core of the coil projects a little above the case,
as shown at E in Fig. 50. The strength and character of the spark
may be varied considerably by slightly screwing F up or down. When
current is supplied to the low tension terminals of the coil it flows
through the primary winding and magnetizes the iron core, completing
its circuit by passing across the platinum contacts. When the trembler
blade is attracted to the iron core the primary circuit is broken
by the temporary separation of the platinum contacts, and therefore
the magnetism ceases, the trembler is released, and the circuit is
completed again. Thus the trembler blade is set rapidly vibrating and
making and breaking the primary circuit as long as the roller attached
to the rotating arm H of the low tension contact breaker shown in
Fig. 52 is in contact with the metal segment K, and this results in
the production of a _succession_ of sparks at the sparking plug which
is connected to the terminal C of the high tension winding. This
is very useful especially when starting an engine, but with modern
high-speed engines the trembler has only time to give one spark at
high engine speeds, and therefore the magneto has the advantage except
for easy starting. This has led to the introduction of _dual_ ignition
systems, and in particular to that system in which the main ignition
is by magneto, but there is a supplementary coil fitted to supply
high tension current to the ordinary high tension magneto distributor
when the engine is at rest, the coil being cut out after the engine
has got up speed. But this has been largely superseded by the use of
electric motors for starting the engine, although the _magneto_ is
still relied upon for the ignition of the charge in the cylinders. The
contact breaker and coil just described would be very suitable for a
single cylinder petrol engine, or a non-trembler coil might be used
in conjunction with a contact breaker of the quick break type used on
magnetos and illustrated in Fig. 48. In the case of a multi-cylinder
engine having coil ignition we may use separate coils without a high
tension distributor, or a single coil _and_ a high tension distributor
having as many segments as there are engine cylinders and arranged
similarly to the magneto distributor of Fig. 48. When no high tension
distributor is fitted there must be a separate coil for each cylinder,
and the high tension wire runs direct from the coil to the sparking
plug, so that the character of the spark as well as the exact instant
at which it occurs may not be the same in each of the cylinders. If
there is a high tension distributor it should be mounted on the same
driving spindle as the low tension contact breaker, in order that the
ignition may be _synchronized_, i.e., the spark will occur at the
_same_ point in the piston’s stroke for all the cylinders. The ignition
may be advanced or retarded by moving the casing of the low tension
contact breaker relative to the roller arm, thus causing it to make
contact either earlier or later in the revolution.
At one time it was thought that _two-point ignition_ gave increased
power and efficiency. Two-point ignition means _simultaneous_ firing of
the charge from more than one plug. Sometimes two high tension leads
were led from each distributor segment and connected to the two plugs
in the corresponding cylinder—this constituted the _parallel_ system.
Another system employed a special plug with _both_ electrodes insulated
from the engine frame; this was coupled in _series_ with an ordinary
plug so that the spark jumped the gaps in succession. It is quite
evident, however, that if the gas is thoroughly mixed up and in a
state of violent agitation as the result of rapid compression, a single
well-placed spark will fire it successfully and so no gain results from
simultaneous ignition at another and less favoured point.
[Illustration: FIG. 53.—WIRING DIAGRAM FOR FOUR CYLINDER ENGINE WITH
HIGH TENSION MAGNETO IGNITION.]
=Wiring Diagram for Magneto Ignition System.=—The electrical connexions
are extremely simple in the case of a high tension magneto ignition
system. In Fig. 53 we show a four-cylinder engine fitted with high
tension magneto. The only wires required are the four high tension
cables from the high tension distributor to the sparking plugs and
the _earthing_ wire leading from the short circuiting terminal to the
frame of the engine through a switch as indicated. The firing order
of the cylinders may be _either_ 1, 3, 4, 2 or 1, 2, 4, 3, as desired
(provided the cranks are arranged in the usual manner, that is, in the
order shown in Fig. 21). In determining the order of firing of the
respective cylinders the engine should be turned round very slowly by
hand and careful note made of the order in which the firing strokes
occur. To determine the _firing_ stroke the piston should be moving
downwards and the position of the valves noted; if _both_ valves are
_shut_ then this is the firing stroke, but if the inlet valve is
opening it is the suction stroke.
=Wiring Diagram for a Coil Ignition System.=—The electrical connexions
for a coil ignition system are slightly more difficult to follow out;
they are shown in Fig. 54 for the same engine illustrated in Fig. 53.
In the diagram we show four separate trembler pattern coils, each of
which can give a succession of sparks as long as contact is being made
on any one segment of the low tension contact breaker connected to it.
All the low tension terminals of the coils are connected together to
a common busbar, which is supplied with current from the accumulator
direct. The current flows from the busbar through the low tension
windings of each of the coils in turn, as it comes into operation
through the engine-driven contact breaker, and returns to the battery
through the frame of the engine. High tension cables lead from the
high tension terminal of each coil direct to the sparking plugs, and
therefore the ignition is not necessarily synchronized.
[Illustration: FIG. 54.—WIRING DIAGRAM FOR FOUR CYLINDER ENGINE WITH
TREMBLER COIL IGNITION.]
When the switch in the low tension circuit is opened the ignition is
_off_, because the current is then permanently interrupted; when the
switch is closed the ignition is _on_. To economize current a quick
make-and-break device should be used instead of the _wipe_ form of
contact breaker illustrated, and a non-trembler coil used. It is very
important to fully _retard_ the ignition lever when starting an engine
having coil ignition, because it is very liable to backfire and injure
the operator’s wrist; with magneto ignition this is less liable to
happen.
=Timing the Ignition.=—Various instructions are given from time to time
for correctly timing magneto ignition, but the following will be found
to give satisfactory results. First ascertain the firing order of the
cylinders as explained above, and then bring No. 1 piston on to the top
dead-centre. Rotate the driving spindle of the magneto until the carbon
brush H of the high tension distributor makes contact on the segment
connected to the lead marked (1). If the leads are not marked it will
be necessary to determine which is No. 1 by observing the direction of
rotation of the brush. Next adjust the position of the driving spindle
very carefully by turning it to and fro, so that when the ignition
lever K (see Fig. 48) is in its mid position the platinum contacts
F_{l}, F_{2} are fully separated, the brush H still being on segment
No. 1. Then push the magneto gear wheel into mesh with the engine gear
wheel which is to drive it, and firmly bolt down the magneto to its
bracket. Similar instructions may be followed out for the coil ignition
system.
CHAPTER VII
LUBRICATION
=Properties of Oils.=—Owing to the very high speed at which the modern
petrol engine runs great attention must be paid to lubricating the
moving parts, otherwise undue wear or even _seizure_ will result. We
must be very careful to choose a suitable oil, one which is chemically
pure and retains its lubricating properties at high temperatures. A
considerable amount of oil finds its way into the cylinder, where it
comes into direct contact with the hot gases. If an oil is heated
a temperature will sooner or later be attained, when the oil will
give off an _inflammable_ vapour, i.e., one which will burn. This
temperature is called the _flash point_ of the oil. If the oil is
likely to get into the cylinder of a petrol engine it should have a
very high flash point; in fact, most of these oils do not flash until
well over 400° Fahrenheit. Also when the oil is burnt it must not leave
any appreciable residue. Some oils are very defective in this respect,
and leave large quantities of _carbon deposit_ on the metal walls of
the cylinder and the valves; others again are gummy or too _viscous_
even at high temperatures. Such oils must be avoided equally with those
which lose their _viscosity_ too much under heat.
=Splash System of Lubrication=.—One method of lubricating the working
parts is known as the splash system. In this system oil is poured
into the crankchamber and the moving parts dip into it, splashing it
all over the interior of the crankcase and the lower portions of the
cylinder walls. Oil holes are drilled in such positions that as the
oil drops down again after being splashed upwards some of it will
fall into these holes and lubricate the bearings. This is a very
cheap method of lubrication in _first cost_, but very wasteful and
unsatisfactory in regular use, hence it has practically died out. As
the oil is used up a fresh supply must be admitted by some form of
continuous drip-feed arrangement, the oil being forced over very often
from a small tank on the footboard by means of air pressure or the
pressure of the exhaust gases from the engine. It is very difficult
under these circumstances to estimate how much oil is present in
the crankchamber at any given instant, so that there was usually
alternately too much or too little. Too little oil meant undue wear
on bearings (perhaps seizure), and too much oil meant a smoky exhaust
which became very obnoxious when the engine was suddenly accelerated.
[Illustration: FIG. 55.—IMPROVED SYSTEM OF SPLASH LUBRICATION.]
=Improved System of Splash Lubrication.=—This is a combination of the
splash system and the _forced_ system, and is shown in Figs. 55 and
56. In these figures A_{2} and A_{3} represent two of the main engine
bearings which support the crankshaft; C_{1}, C_{2}, C_{3} are three
of the crankpins; F_{1}, F_{2}, F_{3} are oil troughs placed under
the crankpins; D_{2}, D_{3} are oil feed pipes to the main bearings.
Generally speaking, the oil is drawn from the bottom of the crankcase
by means of a pump, and this pump delivers the oil to some form of
indicator mounted on the dashboard of the car. After passing through
the indicator the oil flows by two main pipes, one of which feeds the
main bearings by means of branches D_{2}, D_{3}, etc., and the other
feeds oil troughs by means of branches such as G_{2}. When the troughs
are _full_ the oil overflows into the bottom of the crankchamber, and
so there is always a constant depth of oil for the scoops attached to
the connecting rod ends to dip into, and one great drawback to the
splash system is overcome; also the main bearings are always sure of
being amply supplied. The oil pump may be an ordinary plunger type pump
or a rotary pump.
[Illustration: FIG. 56.—SECTIONAL VIEW OF END OF CONNECTING ROD,
SHOWING ARRANGEMENT OF SCOOP AND OIL TROUGH.]
=Forced Lubrication.=—One system of forced lubrication is shown in
Fig. 57. The general arrangement of the system is very similar to the
preceding one, except that there are no troughs in the crankchamber and
_all the bearings_ receive an ample supply of oil under _pressure_ so
that the journals are supported in their bearings on a _film of oil_
and the metals never come in direct contact with each other. After
entering the main bearings the oil passes through holes drilled in
the crankshaft and thus positively lubricates the crankpin bearing,
passing up the connecting rod either internally as shown or by an
external pipe it lubricates the gudgeon pin and then falls down
into the crankchamber. On its way down it gets splashed about and
thus lubricates the cylinder walls and piston; sometimes these are
positively lubricated by leading the oil through the centre of the
gudgeon pin direct to the surface of the cylinder walls—but this often
gives an excess of oil and causes a smoky exhaust. In Figs. 58 and
59 we show two views of a very popular form of oil pump for forced
lubrication systems. It consists of two gear wheels, one of which is
driven by a spindle from the engine crankshaft, and it drives the
second wheel by means of the projecting teeth. The oil is picked up by
the teeth and passed round from the suction to the delivery side of
the pump on the _outer_ edge of the wheels; no liquid can pass direct
across between the teeth which are in mesh, and hence the direction of
rotation is as shown by the arrows.
[Illustration: FIG. 57.—FORCED LUBRICATION SYSTEM.]
[Illustration: FIG. 58.]
[Illustration: FIG. 59. TWO VIEWS OF A ROTARY OIL PUMP FOR FORCED
LUBRICATION.]
The difficulty of securing a really good lubricant for petrol engines
must be apparent from a study of the prices of the various oils. It
will be observed that they are all considerably more expensive than
petrol, and therefore we must economize in their use. The old splash
system was very wasteful and consumed oil at the rate of one gallon
every hundred miles at least, but a modern system of forced lubrication
will not require more than one gallon of oil every thousand miles.
Perhaps an average everyday figure for ordinary motor-car engines would
be one gallon every 250 miles. The pressure of the oil in a forced feed
system varies in different makes of engines from 5 up to 40 pounds per
square inch—a very common figure, however, is 10 pounds per square
inch. The speed of the oil pump also varies considerably, and ranges
from 500 up to 2,000 revolutions per minute at normal engine speed.
Generally a small relief valve is fitted in the pump casing, which
returns oil to the crankchamber if the pressure tends to rise above the
desired limit due to the engine speed increasing. We have mentioned
already that the flash point is generally over 400° Fahrenheit when the
oil is new, but after it has been in the crankcase some time and got
used over and over again it is found that the petrol vapour leaking
past the piston rings of the engine condenses when the engine cools
down after a run and drops into the oil in the sump, thus lowering its
viscosity and its flash point. According to Mr. Morcom it may come
down as low as 200° Fahrenheit (about), but if the oil is heated and
the petrol driven off the flash point goes up again. Therefore it is
a good plan with forced lubrication systems to empty the old oil out
periodically and fill up entirely with fresh oil.
CHAPTER VIII
COOLING
We have already explained the necessity for cooling the cylinders of a
petrol engine by means of a water-jacket, and we now proceed to show
how the circulation system may be arranged. There are two forms of
circulation in use: (1) Natural; (2) Forced.
[Illustration: FIG. 60.—THERMO-SYPHON WATER COOLING SYSTEM.]
=Natural or Thermo-Syphon Circulation.=—This system is shown in
Fig. 60, and may be explained as follows:—The heat generated by the
successive explosions within the cylinder causes the water at the
top of the cylinder jacket A to get hot. As a column of hot water is
lighter than one of cold water of equal height, the heated water rises
up the pipe B and flows into the top of the radiator D, while colder
water from the bottom of the radiator flows up the pipe C and into the
cylinder jacket A. It is important that the height of the water in the
radiator D should be at such a level that the outlet from the pipe B is
submerged.
[Illustration: FIG. 61.—FORCED WATER CIRCULATION BY MEANS OF A PUMP
(P).]
In the radiator the water falls through a series of tubes E, having
_gills_ or fins on the outside for the purpose of dissipating the heat.
The cooling of the water is also assisted by the fan F, which is driven
from the fan pulley G and draws air past the radiator tubes at high
speed. Sometimes the water in the radiator is made to fall through a
series of cells which are formed of cast aluminium; such a radiator is
called a _honeycomb_ radiator. It is important that the pipe C should
not have any sharp bends and it should not rise very much in height,
but the outlet pipe B may have a considerable rise with advantage. Both
the inlet and outlet pipes should be of large diameter with this system
of circulation, and the radiator should be so arranged that there is
a good _head_ of water above the cylinders. In the drawings H is the
front cross-member of the chassis, K is the starting-handle clutch, and
L is the starting handle.
=Forced or Pump Circulation.=—With this system the water is positively
circulated through the jackets; it is drawn from the bottom of the
radiator by the pump P (Fig. 61), which is mechanically driven from
the valve shaft of the engine, and delivered under pressure to the
jacket A. The outlet of the pipe B need not be drowned, and the pipe C
may be arranged in any way most convenient to the chassis. Sometimes
when a pump is fitted the pipes are arranged so that the system may be
operated as a thermo-syphon in the event of a breakdown of the pump. It
is not uncommon to experience trouble due to leakage at the pump gland,
which results in gradual loss of water from the system, and therefore
the thermo-syphon or natural circulation has much to recommend it. Also
it may be said that the pump represents an additional _complication_
to the engine and means increased first cost. Every moving part we add
to the engine is of course an additional potential source of trouble,
but the addition of a really first-class water circulating pump of the
type shown in Fig. 58 cannot be said to be anything but a reasonable
precaution. The weight and size of every part of a motor-car engine and
chassis have been so much reduced recently, owing to competition with
American firms, that many manufacturers who adopted the thermo-syphon
principle experienced great trouble with it owing to the small size
of radiator fitted, as well as faulty arrangement of the connexions.
Considering any one engine, it follows that if a certain size of
radiator and a given quantity of water in the circulating system
will keep the engine cool when a pump is used to give a positive
circulation, then a larger radiator and greater quantity of water will
be required for natural circulation. Thermo-syphon circulation also
means a _high_ radiator and bonnet, which many people object to on
the score of appearance, without considering its utility. With natural
circulation greater care must be exercised to keep the radiator well
filled, but this often leads to other difficulties on bad roads owing
to the water splashing from the overflow pipe and finding its way on to
ignition appliances. Before starting an engine it is always advisable
to remove the radiator filling cap and examine the water level; if
it should happen that at any time while the engine is running the
circulating system runs quite dry, owing to a breakdown or leakage,
do not attempt to pour water into the radiator, but simply raise both
sides of the bonnet and leave the engine to cool down first. Again,
when filling the radiator for a forced circulating system, it is
desirable to give the engine a turn or two with the starting handle
occasionally to operate the pump and prevent air locks; very often the
radiator appears to be full, but as soon as the engine commences to run
the water disappears owing to the system not being full, due to the
above-mentioned cause. In cold or frosty weather all the water should
be drained off from the circulating system when the car is in the
garage, unless the garage is heated or some anti-freezing solution is
used. Glycerine or alcohol added to the water will prevent it freezing,
but as an additional precaution in cold countries one often sees
travelling rugs strapped over the radiator and bonnet.
Occasionally one gets trouble due to the water boiling in the jackets,
and on this account reasonable care should always be exercised in
unscrewing the radiator filling cap if the presence of steam is
suspected. An engine may have been running well for a long time without
trouble and then develop symptoms of overheating in the circulation
system. This overheating may be either _local_ or _general_. Local
overheating may result from some partial seizure of the piston in the
cylinder due to dirt on the walls, or from the presence of grease on
the _outside_ of the cylinder walls, in the jacket space. If grease is
suspected or there is _furring_ up in the passages of the jacket due
to bad water supply, the trouble may be cured by adding some common
washing soda to the water in the radiator and running the engine with
the car at standstill for half-an-hour or so. After this drain off all
the water and sludge, allow the engine to cool down, and then fill up
again with clean water.
General overheating may result from leaky pistons and pistons rings,
or from the use of _too weak_ a mixture in the carburettor, or from
overloading the engine. If the mixture supplied to the engine is very
weak, the overheating will be very marked on the _exhaust side_ of the
engine. Local overheating causes the engine to “_knock_” badly.
In arranging the jackets and the pipes care must be taken to arrange
that a cock is placed at the _lowest_ point in the system, so that
the whole may be completely emptied, and the inlet pipe to the jacket
should enter at the very bottom of the jacket chamber for the same
reason. It may be thought that all that is necessary is to provide
plenty of space in the jackets round the cylinders and plenty of water
in the whole system, but experience shows that it is very important
not to make the jacket space _too large_, so as to ensure positive
circulation and avoid _local circulation_ in any one portion of the
jacket. When cylinders are cast in pairs the back pair have a tendency
to discharge their hot water into the front pair and so back to the
inlet pipe again, hence this should be guarded against in arranging the
outlet pipes.
Pipes suitable for use with multi-cylinder engines are shown in Fig.
62, in which (_a_) is an outlet pipe for a monobloc casting, and
(_b_) and (_c_) are inlet and outlet pipes respectively for engines
having separate cylinders. It is advisable to modify the diameter of
the branches by the insertion of metal orifice plates at the flanges
to ensure an equitable distribution of the water among the several
cylinders.
[Illustration: FIG. 62.—FORMS OF WATER PIPING.]
The weight of water carried in the circulation system for a fifteen
horse-power engine would be about 30 lb. with pump circulation, whereas
60 lb. would be required for thermo-syphon cooling. It is not desirable
to cool the engine too much. The jacket water temperature may be
allowed to reach 180° Fahrenheit at full load, but if this is exceeded
there is liability to boiling. Given two similar engines of equal power
and equally loaded, one of which was operated with a jacket temperature
of 100° Fahrenheit and the other at 180° Fahrenheit, the hotter engine
would show a gain in economy of from five to ten per cent. in fuel
consumption. In considering the type of radiator to adopt, one would
not recommend the honeycomb variety (except for appearance) owing to
the difficulty of cleaning the passages after it has been in use some
time; and the gilled tube would be more efficient than the plain tube.
The amount of tube required depends of course on its diameter, but a
rough approximation would be twelve feet of gilled tube or eighteen
feet of plain tube (of half-inch diameter) per brake horse-power.
CHAPTER IX
THE POINTS OF A GOOD ENGINE
=Choosing the Number of Cylinders.=—It is a very difficult problem to
select the _best_ engine for a particular purpose, as there are so
many factors which influence one’s choice. A single cylinder engine
would only be used for a motor-cycle or a small car of low power; the
vibration and noise resulting from the use of a single cylinder petrol
engine of even six horse-power are most objectionable, and difficulties
of starting and risk of engine unexpectedly pulling up and stopping
are greatly enhanced. The two-cylinder engine offers better prospects,
and was for some time considered quite good enough for most purposes,
but owing to its _comparatively_ bad balance and its low torque it
has fallen into disfavour. We have seen how the rotating parts of the
engine can be balanced, but we have not considered the reciprocating
parts. To understand this question of balancing we must talk about
“_inertia forces_.” All bodies possess inertia, that is, they resent
changes in their state of rest or motion. If a body is moving uniformly
it tends to keep on doing so, whereas if it is at rest it tends to
remain so. To start the body off from rest, or to stop the body and
bring it to rest, requires a _force_ to be exerted, and this force
may be called the _inertia force_. When a petrol engine is running at
high speed the piston has to be started and stopped at the top and
bottom of its stroke every time the crankshaft revolves once, and to
do this very large forces are needed, because it has to be done so
quickly. These inertia forces take the form of pushes or pulls on the
shaft and framework of the engine, and thus cause _vibrations_ to be
set up. If the periodicity or _frequency_ of these forced vibrations
happens to coincide with the natural period of vibration of the shaft
material the shaft will commence to _whip_, and may possibly break
under the excessive strain. In a two-cylinder engine with cranks 180
degrees apart (or half a revolution) one piston is moving upwards and
the other piston is moving downwards, both at very high speed; and both
have to be brought to rest when the cranks come on their respective
dead-centres. The piston which is moving up tends to lift the shaft up
with it, and the one which is moving down tends to pull the shaft down
with it, because the connecting rods check the progress of the pistons
and bring them to rest at the top and bottom of their strokes. If these
two pulls acted in line with each other they would _balance_, but the
cylinders are usually mounted side by side, and then the two pulls
virtually act at the ends of a bar whose length is the longitudinal
distance between the vertical centre lines of the two cylinders.
Thus these two inertia forces tend to rotate the whole engine first
in a clockwise direction and then in a counter-clockwise direction,
according to which piston is moving up or down. The only way to balance
these forces under these conditions is to extend the crankshaft
longitudinally and place another pair of cylinders and cranks in line
with the first, but so arranged that the inertia forces tend to turn
the engine in the opposite direction to the first pair. This gives
us the well known four-cylinder arrangement so much in evidence at
the present time, the arrangement of cranks being shown in Fig. 21. A
six-cylinder engine gives _perfect balance_ if all the parts are of
equal weight, and the cranks at 120 degrees to each other in opposed
pairs.
Again, a single-cylinder engine gives one power stroke in every two
revolutions of the shaft, a two-cylinder gives a power stroke in
every revolution, a four-cylinder gives two power strokes, and a
six-cylinder gives three power strokes in every revolution of the
shaft. Hence a six-cylinder engine is very _flexible_ (i.e., can
accommodate itself easily to varying loads), is perfectly balanced,
and can be made both powerful and economical. One objection to the use
of engines with multiple cylinders (exceeding, say, _four_ in number)
is that the crankshaft is more liable to vibrate and cause very harsh
running at high speeds on account of the fact that the _periodicity
of the power impulses_ imparted to the shaft approaches the natural
period of vibration of the shaft. This effect arises from _torsional_
oscillations and is distinct from the _periodicity due to inertia
forces_ which acts in the vertical plane. A four-cylinder engine is
nearly as good as a six-cylinder of equal power, and is of course much
cheaper in first cost, takes up less room, and weighs less. A good
four-cylinder engine will often prove more economical in running costs
than a six-cylinder, as it will probably be running a greater length of
time at or near its full output, and the work done on the idle strokes
of the cycle will be less owing to the smaller number of cylinders.
Another feature to consider is the _arrangement_ of the cylinder
castings. A monobloc casting (cylinders all in one casting) gives a
very short engine and reduces the length of the crankshaft, but in the
event of one cylinder bore being damaged the advantage lies with the
separate cylinder construction.
_The Question of the Valves._—The question as to which is the better
engine, the sleeve valve or the poppet valve, cannot be said to have
been definitely decided yet. The great feature of the poppet valve used
to be its very quick opening and closing, but nowadays engines turn
over so fast that very strong springs are needed to close the valves
in a reasonable time. One complete revolution of the engine means that
the crank has turned through 360 _degrees_, and the inlet valve is open
while the crank turns through 190 degrees (on the average), but during
part of this time it is _being_ lifted or _opened_, and during an
equal period it is _being closed_. The question then is, “How long does
it remain _fully open_?” The answer is—not more than ten degrees at
the most! To keep the inlet valve open longer than this would require
excessively stiff springs and throw a great strain on the valve gear.
Now this is where the sleeve valve managed to get a look in—as one
might say. With _two_ sleeves moving in opposite directions, or one
sleeve receiving a special form of motion, we can open and close the
ports and keep them fully open for just as long period or even longer
than the poppet valve. If it were not for the fact that sleeve valves
are heavy and not so easy to keep gas-tight as poppet valves, it is
perfectly obvious that the poppet valve would have disappeared or taken
second place long before this.
Another great advantage of the sleeve valve is that by making large
ports we can easily secure larger valve openings than are possible,
for practical reasons, with a poppet valve. It is now claimed also
that the interior of the cylinders keeps free from carbon deposit much
longer with sleeve valves than with poppet valves, this carbon deposit
being due chiefly to the use of too rich a mixture which causes the
combustion to be imperfect and results in the deposit of solid carbon
on the walls and sides of the combustion chamber. =Carbon deposit= is
also caused by using unsuitable lubricating oil, but it principally
arises from the use of too rich a mixture for the purposes of securing
quick acceleration. _Perfect_ combustion is only secured by the use
of a relatively _weak_ mixture, which would prevent the maximum power
being developed and give rather a feeble acceleration. Modern engines
have to be very carefully designed to reduce this nuisance of the
carbon deposit to a minimum, and also with a view to its speedy and
efficient removal when it does take place. Detachable cylinder heads
have been introduced principally to allow of rapid removal of carbon
deposit from pistons and valves and the combustion chamber. If the
carbon deposit is allowed to accumulate, _pinking_ or sharp knocking
commences, due to pre-ignition of the charge by red-hot particles
of carbon. This results in loss of power, and is first noticed by
inability to climb steep hills that were formerly negotiated with
ease. Mention must also be made of the great claim for _silence_ of
running of sleeve valve engines, and this is thoroughly justified with
_high-class engines_ of the sleeve valve type, provided they are in
the hands of skilled drivers. In unskilled hands one finds that the
poppet valve is safer, and will stand more knocking about without much
increase in noise resulting. Rotary valves have fallen into disuse on
account of the difficulty of keeping them gas-tight. There is nothing
to choose between poppet and sleeve valves on the score of economy in
running.
=Economy and Durability.=—A good modern petrol engine of reasonable
size—say over 3 in. bore—will give one brake horse-power for an hour
from the consumption of two-thirds of a pint of petrol. This means
that an engine giving 12½ horse-power on the brake would use a gallon
of petrol every hour. But economy in petrol consumption is not the
only desirable feature of a petrol engine. There must be economy in
lubricating oil and in cost of replacements or repairs. Nowadays the
tendency with high grade steel alloys and other modern metals of high
strength and durability is to cut everything down to its minimum size
with a view to reducing _the cost of production_. This often leads to
many serious troubles in running on the road. In choosing an engine
one should carefully examine such points as provision for wear and
adjustment, strength and rigidity, and whether the engine impresses one
with a sense of its _durability_ and also its general _accessibility_.
CHAPTER X
TWO-STROKE ENGINES
In the two-stroke type of petrol engine the cycle of operations is
completed in two working strokes of the piston instead of the four
required by the “Otto” cycle; there is thus one explosion or power
stroke in every revolution of the crankshaft. Theoretically this
represents a great advance over the “Otto” cycle, but difficulties
and complications arise in the practical carrying out of the cycle.
The cycle on which it is desired to operate the engine is: _1st
stroke_—Compression; _2nd stroke_—Explosion. The charge would be
introduced on the compression stroke and exhausted towards the end of
the explosion stroke.
Now the charge of gas required by the engine consists of a mixture of
petrol vapour and air, and it must either be sucked in or pushed in
under _pressure_. In the “Otto” cycle the charge is _sucked_ in, and in
the two-stroke cycle it is delivered to the cylinder under _pressure_;
hence in the two-stroke cycle some form of pump is required which will
suck in the charge of air and gas, compress it a small amount, and
deliver it to the working cylinder at a pressure of 5 or 6 lb. per
square inch above atmospheric pressure. This is where the complications
commence; if we fit a separate pump for each cylinder, which is
what would generally be done, or if we made one pump serve for two
cylinders, we have to provide pump cylinders, pistons, rods and valves,
and therefore there is practically no gain over the four-stroke engine.
Hence it is that inventors all try to avoid the use of a separate
charging pump and turn their attention to the production of an engine
in which one or more of the existing portions is made to serve as
a pump for charging the working cylinder or cylinders with gas. A
favourite and fairly successful device is to make the crankchamber
gas-tight and use it as the cylinder of the pump, the underside of the
engine piston then forming the pump piston which draws the charge from
the carburettor into the crankchamber on its upstroke and compresses
it on its downstroke, delivering it to the working cylinder through
the inlet port as soon as the piston has uncovered it by its downward
movement.
_FIG. 63.—TWO PORT TYPE OF TWO-STROKE ENGINE WITH CRANKCASE
COMPRESSION._
There is _no exhaust valve_, as the piston uncovers the exhaust ports
a little before the inlet ports are opened. To prevent the new charge
escaping directly across the top of the piston from the inlet ports to
the exhaust ports, a deflector is fitted on the top of the piston equal
in height to the height of the exhaust opening and situated immediately
in front of and facing the inlet ports.
A two-stroke engine of the type referred to is shown diagrammatically
in Fig. 63. E is the gas-tight crankchamber, upon which the
water-cooled cylinder A is mounted in the usual manner and fixed by
studs or bolts. The piston P carries the deflector H, which is equal
in height to the height of the exhaust opening G. The piston rings
are prevented from turning by pins so arranged that the joint of the
rings does not pass across the ports. The connecting rod D is of
usual form, and also the crankshaft C. The carburettor, or induction
pipe leading from the carburettor, would be attached to the flange L,
and the automatic valve F controls the admission of gaseous mixture
from the carburettor to the crankchamber. The inlet ports N are often
only half the height of the exhaust ports. On the upstroke of the
piston a partial vacuum will be formed in the air-tight crankchamber,
which will allow the atmospheric pressure to force open the valve F
against the pressure of the spring and enable the air to flow into the
crankchamber through the carburettor and induction pipe, carrying the
charge of petrol vapour with it. We must note, however, that no vacuum
can be formed until the port N has been covered up by the piston, so
that a portion of the stroke is _idle_. On the downward stroke of the
piston the charge in the crankchamber is compressed, and as soon as
the piston uncovers the ports N the charge from the crankchamber flows
up into the working cylinder, displacing the burnt gases as it comes
into the cylinder. Exactly what happens next it is difficult to say;
the probability is that this new charge rises in the cylinder a short
distance (but not a sufficient amount to displace all the dead gases
from the top end of the cylinder) and that some of it gets squeezed
out of the exhaust opening as the piston rises and before it has had
time to cover the exhaust ports. Thus, owing to the idle portion of the
stroke during admission to the crankchamber and to the low compression
pressure adopted in the crankchamber, the pumping portion of the
engine has what is termed a very low _volumetric efficiency_.
It can be proved that this type of engine which endeavours to draw
sufficient gas to fill its working cylinder into the crankchamber by
means of a piston having only the _same_ diameter as the diameter of
the working cylinder itself, and which cannot avoid some idle movement
during the operation together with further loss from the exhaust
opening, is incapable of retaining more than a little over one-half a
cylinder full of fresh combustible gas at the instant when compression
commences; the remainder of the contents must be dead exhaust gas.
Thus, even allowing for the double number of power impulses resulting
from the use of the two-stroke cycle, it is difficult to see how this
form of engine could ever give more than about one and a quarter times
the power of a four-stroke engine having the same bore and stroke
even when the many difficulties experienced in the practical working
of two-stroke engines have been overcome. To use a high compression
pressure in the crankchamber would increase the volumetric efficiency,
but would result in lost work during the pumping process, besides
being undesirable at the delivery stage of the process; it is much
better for the transfer of the gases to take place as gently as
possible. If too high a delivery pressure is used the fresh gas will
enter in a sharp gust and get badly contaminated by mixture with the
foul exhaust products instead of gently displacing them in bulk. The
use of an automatic valve is very desirable for the gas inlet to the
crankchamber, but unfortunately it limits the speed of the engine
and also its _flexibility_ or ability to pull well at all speeds. An
engine with an automatic valve runs best at that speed for which the
tension of the spring is most suitable. If the spring is weak the
speed will be low. Tightening the tension on the spring will allow the
engine to speed up, but will prevent it running well at low speeds.
At high speeds and with correspondingly high tension the valve does
not give enough opening, and therefore limits the power of the engine.
It will, therefore, readily be seen that when a two-stroke engine
with automatic inlet valves is pitted against a four-stroke engine
with mechanically-operated inlet valves, the comparison is unfair
to the two-stroke cycle. With the position and arrangement of ports
shown in the drawings, one must have a deflector on the piston head
to prevent excessive loss of fresh gas through the exhaust opening.
After the engine has been running for some time at a high speed this
deflector becomes very hot, and as a general rule the cooling effect
of the incoming gases is not sufficient to prevent it attaining a _red
heat_ on the compression stroke, thus igniting the charge before the
piston reaches the top of the stroke. This defect, which is called
_pre-ignition_, causes the engine to _knock_, and results in a loss
of power; it may be partly overcome by admitting lubricating oil with
the charge, the oil then serving to cool the deflector as the charge
enters the cylinder. At high engine speeds there is great risk of
the hot exhaust gases in the working cylinder setting fire to the
incoming charge in the inlet ports, thus causing _backfiring_ into the
crankchamber. To avoid all possibility of backfire, some form of _air
scavenging_ must be adopted, but the general arrangement of this form
of two-stroke engine does not lend itself to such an addition—it would
merely reduce still further the quantity of gas reaching the cylinder.
A difficulty that is peculiar to multi-cylinder engines of the
two-stroke type arises from the use of open exhaust ports. The several
cylinders generally discharge their exhaust gases into a common
exhaust pipe or box, so that if one cylinder happens to be _missing_
fire the exhaust from another cylinder may set fire to the wasted
charge—this is usually referred to as _flashing-back_ from the exhaust
and results in irregular and spasmodic knocking. It will be clear from
the foregoing that this cycle of operations, which is so attractive
from the theoretical point of view, is not by any means so encouraging
from the practical standpoint, as many inventors have discovered.
The difficulties and failures of the early inventors which were so
discouraging for them have only encouraged their successors and spurred
them on to further efforts. After a time the attempt to produce a
_simple_ two-stroke engine was abandoned generally, and inventors
turned their attention to improved forms of two-stroke engines, some of
which were very costly and complicated, and none of which have survived
for motor-car purposes.
The writer of this volume became interested in the problem of the
two-stroke in connexion with one of these inventions for an improved
engine, and at a later stage patented and designed an improved engine
of the two-stroke _air scavenging_ variety, which by that time had
become a recognized type of two-stroke engine. This engine was
constructed and exhibited at one of the motor shows held in London
some years ago. A vast amount of experimental and research work was
carried out on it by the writer, but the work had to be abandoned when
incomplete owing to the Syndicate which financed the venture having
exhausted its resources. The promoters of the Syndicate were anxious to
produce an engine that would give _double_ the power of a four-stroke
engine, but their early attempts were not at all successful. One
of their four-cylinder engines, which would have been rated at 35
horse-power on the four-stroke cycle, only gave 12 brake horse-power
when tested by the writer. The engine designed by the writer, which
we may call the Kean two-stroke engine, would have been rated at 25
horse-power on the four-stroke cycle, and gave approximately 35 brake
horse-power. Although this result was excellent, so much advance had
been made in the four-stroke engine that it did not quite come up to
the best results obtained on that system, and hence we were unable
to show any marked advantage to be gained from its adoption. My
experiments clearly pointed out the road to further success, but owing
to the partial failure of my attempt to beat the four-stroke engine we
could not influence sufficient capital to reorganize and reconstruct
the Syndicate. My engine had not been designed to secure a high speed
of rotation but rather for strength and durability, but it exceeded
my expectations by turning up to 1,500 revolutions per minute. The
four-stroke had, however, got well ahead of me by that time, and 2,000
was becoming common for it, hence I was heavily handicapped in the race
for horse-power.
[Illustration: FIG. 64.—DIAGRAMMATIC SKETCH SHOWING HOW THE DUPLEX TYPE
OF TWO-STROKE ENGINE OPERATES WITH AIR SCAVENGING.]
A description of my engine will probably prove of interest. To
understand the principle of the engine we must turn to the diagrammatic
sectional view of Fig. 64. Instead of using the crankchamber of the
engine as a gas pump, this type of engine has a _duplex_ piston, and
the pump chamber is formed by an annular extension of the main engine
cylinder. At first sight one would say this resulted in a very high
engine, but as a matter of fact the increase in height is not more than
about 25 per cent. in the cylinders, and there is no difference in the
crankchamber height to that of a four-stroke engine. The outstanding
feature of the invention is the provision of a pump piston of larger
effective diameter than the main piston and the arrangement of transfer
pipes by which one pump feeds its neighbour’s power cylinder, and
_vice versa_. These are the basis of the invention, and were being
used a long time before the writer had even heard of this type of
engine, but it was left for him to seize upon their capabilities and
correctly proportion the area of the annulus with respect to the main
engine piston. A careful study of the two-stroke problem revealed the
inherent defect of the _low volumetric efficiency_ and the tremendous
possibilities of having an unlimited volume for the pump chamber
by simply increasing the area of the lower or annular piston. Then
followed the writer’s attempt to tackle the outstanding practical
difficulties enumerated above. The engines already employed _air
scavenging_, but could not really use it effectively until proper
proportions had been fixed upon for the respective pipes, valves, and
ports. The cycle of operations is as follows:—On the downstroke of No.
1 piston the annular portion draws a charge of gas from the carburettor
into the annular chamber D_{1} (Fig. 64) through the inlet valve B_{1}
and at the same time pure air is drawn into the transfer pipe by the
valve A_{2}. On the upstroke the charges of air and gas are compressed
into the transfer pipe, and as soon as the piston P_{2} uncovers the
inlet ports the air and gas enter the working cylinder. In my engine
I used a relatively high compression pressure for the transfer of the
charge and curved the inlet ports up towards the head of the cylinder
as shown. The head of cylinder I made curved, and the exhaust ports
were carefully rounded and curved also. The deflector on the head of
the piston I inclined, to curl the gases back against the wall of the
cylinder, and I reduced the height of the deflector to that of the
_inlet_ port (instead of the exhaust port). My ultimate aim was to
abolish the deflector entirely by suitably shaping the inlet ports, and
I estimated that the path of the gases would be in the direction of the
arrows. The object of raising the compression pressure in the lower
cylinder was twofold. First of all I aimed at an increase of volumetric
efficiency there, and secondly I hoped to propel the scavenging air and
the new charge right up to the head of the cylinder and so clear out
all the dead gases. Then by suitably curving the head of the cylinder
I expected to compel the scavenging air to keep going ahead of the
gaseous mixture and curl round and down, then following the exhaust
gases out of the exhaust port.
My efforts in this direction were very unfortunately frustrated to a
large extent by the fact that the cylinders of my engine had already
been cast before I fully realized the _tremendous_ importance of
_curving the cylinder head_ and giving a very _steep inclination_ to
the inlet ports. We did our best to rectify matters in the machining
and finishing stages, but any engineer will understand the limitations
now imposed upon us. It was impossible to get new cylinders cast
owing to lack of time and funds, as we were intending to exhibit the
completed engine. Thus I cannot say that my ideas were ever given a
really satisfactory test; the inlet ports _were_ curved and inclined
and the cylinder head _was_ rounded off, but not to such an extent
that I can feel certain no further improvement could ever be made
in those directions. Other improvements which I introduced were an
improved automatic inlet valve for the gases, which was fitted inside
the induction pipe and whose spring tension could be adjusted while
the engine was running without letting any air leak into the induction
pipe; also an improved air scavenging valve, which could be set to
give the full amount of air to the engine and yet be controlled from
the dashboard of the car to give any desired quantity of scavenging
air from _no air_ up to _full air_. _Very_ large inlet valves were
fitted, but when indicator diagrams were eventually obtained from the
engine they showed that they were not nearly large enough and that the
carburettor opening was too restricted, thus cutting down the power
(and very likely the speed) of the engine by probably over 25 per cent.
High tension magneto ignition was fitted and thermo-syphon cooling.
Arrangements were made to carry 80 lb. of water in the system, so that
the engine never showed any tendency to boil even when the car had
been running for long periods on the low gear. A pump was afterwards
fitted, but it did not effect the cooling of the water any better than
the natural circulation, which was quite satisfactory. The range of
speed was from 150 revolutions per minute up to 1,500 revolutions per
minute; the lower figure is very good indeed, and can be attributed to
the large number of impulses obtained due to the two-stroke cycle. At
the highest speed the crankshaft received 6,000 impulses per minute,
or equivalent to a four-stroke engine running at 3,000 revolutions per
minute. The effective pressure in the cylinder was, however, only just
over 40 pounds per square inch, due to the throttling at inlet already
explained. In a four-stroke engine we would expect just double that
figure. The extraordinary thing about this was that, under heavy load,
when the speed was brought down to about 300 revolutions per minute,
the effective pressure had risen to nearly 200 lb. per square inch, but
this appears to be due to imperfect scavenging (or cleansing) of the
cylinder under these conditions.
The question of silencing the exhaust from the engine had caused me
some difficulty in the earlier experiments, so that I now tackled this
problem and designed a special form of silencer in which the gases
were first expanded to remove their pressure and then afterwards their
velocity was taken up without shock. This answered so well that a
_cut-out_ made no difference whatever, and on taking diagrams with the
optical indicator I discovered that the exhausting process was divided
into equal periods of slight pressure and slight vacuum with an average
of zero pressure (just atmospheric). We have seen in the earlier part
of this chapter how the fitting of automatic inlet valves is liable to
hamper the engine and reduce its flexibility, and this impressed me
very much with the earlier engines so that at one time I adopted dual
springs for the inlet valves. These springs were mounted one above the
other, the lower one being much stiffer than the upper one. The idea of
the invention was that the weak springs would serve for slow running
and all loads up to _say_ half the lift of the valve, and then the
stiffer springs would secure correct action at high speeds. Further
than this, I had them all coupled on a bar which was controlled from
the driver’s seat, and by means of which I could cut out the weaker
springs or reduce their effect at will. It certainly answered well
in the older engines, but my new engine, shown in Fig. 65, was so
satisfactory that I abandoned the idea. About five different systems of
lubrication were experimented with and many lubricating oils. Finally,
forced lubrication was employed for all the bearings and a drip
sight-feed for the pistons.
[Illustration: FIG. 65.—GENERAL ARRANGEMENT OF THE “KEAN” FOUR-CYLINDER
HIGH SPEED HIGH COMPRESSION DUPLEX TWO-STROKE ENGINE EMPLOYING AIR
SCAVENGING. IN THIS ENGINE THERE IS _NO_ CRANKCHAMBER COMPRESSION.]
Much trouble was caused at one time in the new engine by _knocking_ of
various kinds, and many hours were spent in locating these troubles
and curing them. The first kind of knocking was most violent and
almost made one hold one’s breath in anticipation of seeing parts of
the engine go skywards. This turned out to be partial seizure of a
piston owing to a hard spot in the cylinder. After curing this, general
knocking from all cylinders began, and was found to result from worn
gudgeon pins. These had been mild steel and case-hardened; they were
discarded for _Ubas_ steel of slightly larger diameter, and this
trouble disappeared. Then _pre-ignition_ was discovered. When the
magneto was switched off the engine slowed down and nearly stopped,
then began to run on again, knocking and hammering in a most diabolical
manner. All cylinders were taken off again, all parts ground up, and
corners well rounded off, but still it continued. At first it seemed
to be due to the deflectors, but on several very careful examinations
(which of course meant dismantling the whole engine every time and
removing the cylinders) no trace of overheating or burning could be
found on these or anywhere else in the interior of the cylinder.
Then the trouble was traced to the electrodes of the sparking plugs.
This was followed by two or three weeks’ continuous experiments on
fitting different types of plugs, and the same type of plug was tried
in four different positions inside the cylinder. Then the device of
fitting the plugs to an adapter and so keeping them at the top of a
small hole instead of projecting into the cylinders was tried. They
still showed signs of overheating, and strange to say no loss of power
or flexibility was noticeable. Finally, I fitted a water tank on the
dashboard and allowed the engine to suck water into the induction pipe
while it drew its mixture from the carburettor in the usual manner. I
had previously fitted separate drip-feed of water to the air scavenging
valves with a view to effecting cooling of the engine, but abandoned it
owing to lack of results. Very soon I discovered that for every gallon
of petrol the engine consumed I could let it take nearly half a gallon
of water into the induction pipe. The engine ran much quieter and very
smoothly, and for a time I thought I had succeeded, although the water
gave me trouble in restarting if I happened to stop the engine while
it was in use. It meant that the water had to be shut off some minutes
before the engine was going to be stopped. The day after I thought
I had effected a cure for the pre-ignition _intermittent knocking_
began, and there was also general knocking _always_ for a second or two
when _accelerating quickly under load_. After much loss of time and
the expenditure of a large sum of money on experiments, I persuaded
the Syndicate to let me take some diagrams from the engine with an
optical indicator, and eventually after nine months they consented,
but they would not agree to my taking the engine out of the chassis
and putting it on the bench for a proper _power test_. Therefore my
diagrams were taken while the engine was in the garage in its chassis,
and the load was applied by the propellor shaft brake, the shaft itself
being withdrawn. Anyone who has attempted even in a well-equipped
laboratory and with the aid of a proper brake to take diagrams from
a petrol engine when the indicator is driven by a _flexible shaft_,
will understand and appreciate my work in securing thirty photographic
records under as many conditions of load and speed. After carefully
analyzing my diagrams, I came to the conclusion that the intermittent
knocking was undoubtedly _flashing back_ from the exhaust, and
the acceleration knocking was due to a _cushion of hot gas_ which
accumulated in the head end of the cylinder at times when the engine
speed was low and the load on the engine was heavy.
Having explained these things to the Syndicate and pointed out the
need for still larger valves, they set about attempting to raise fresh
capital for the final attempt at success. They were not successful, and
up to the present nothing more has been done. The Syndicate was wound
up, the members drifted apart, and the patents were allowed to lapse.
The engine and chassis were eventually sold, and are still doing good
service somewhere in the North of England. Meantime the writer has
not rested, but has steadily formulated his ideas for the improvement
of the engine, which have resulted in the securing of a fresh patent
early this year. In the new engine the charge enters at the head end
of the cylinder, there is a special transverse combustion chamber, and
many improvements are introduced in the scavenging and flow of gases;
also there is no deflector at all on the piston head. Funds have not
yet been secured to enable an experimental engine to be constructed,
but it is to be hoped they will be forthcoming, for the benefit of the
motor-car industry generally, as the future undoubtedly lies with the
two-stroke.
During the whole of this time the writer was engaged as Chief Assistant
in the Engineering Department of Leeds University, being in charge of
the experimental work of the students in the laboratories there. Many
of the drawings were made by students in their vacation, and the writer
is greatly indebted to his friend, Professor John Goodman, for so
kindly allowing him the necessary freedom during vacation times when
there is often much miscellaneous work that requires attention.
Before closing this chapter one may add a few words on carburation and
ignition for two-stroke engines. A four-cylinder two-stroke engine
should have cranks at right angles to secure the maximum torque on the
shaft. Looked at in end view the cranks form the four arms of a cross
and thus four impulses are given every revolution, but as the ordinary
magneto only gives two sparks in every revolution it must be driven at
_twice_ the crankshaft speed. This puts a great strain on the machine
at top speed, and also on the insulation of the windings and the plugs,
so that the plugs require constant attention. Magneto troubles were
found to be eliminated by the use of the special _racing_ pattern
magneto supplied by some manufacturers and the choice of high grade
sparking plugs.
Carburation troubles were not so easily dealt with. A multi-cylinder
two-stroke engine should undoubtedly have a multiple jet carburettor
and some form of hand-controlled _extra-air_ inlet valve on the
induction pipe; also the mixing chamber of the carburettor should be
water-jacketed by _hot_ water. It was also found necessary to fit a hot
water-jacket round a portion of the induction pipe, as the demand for
petrol vapour was so great and the rate of evaporation so high that
frost readily formed on the induction pipe unless the weather was very
warm. The two-stroke engine requires its petrol much faster than the
four-stroke, so that the float of the carburettor should be delicately
balanced and the height of the petrol in the jet should be quite level
with the top of the orifice, although this often leads to flooding.
Reviewing the description of what we have designated the Kean
two-stroke engine, we may sum up the results of these experiments by
saying that the engine _could_ have developed considerably more power
than it did had diagrams been taken from it in the first instance and
the severe throttling in the carburettor and automatic inlet valves
been discovered; moreover, the flashing back from the exhaust would
have been located much sooner and probably cured by a re-arrangement
of the exhaust manifold. If the exhaust manifold had been arranged so
that there was a separate branch for at least each pair of cylinders,
it would very likely have been stopped, or at any rate greatly reduced.
But what could not have been altered was the _acceleration knocking_.
It must not be imagined because I have been very frank in the criticism
of my own work that the engine was a failure; it was a great _success_,
but not sufficiently successful to represent an improvement on the best
four-stroke practice. The car ran well, was very reliable and efficient
in petrol consumption; the engine was quiet and extremely flexible; but
it had one very objectionable feature in that every time you pressed
the accelerator pedal down sharply, either to put on a spurt for the
purpose of passing slower traffic or in _rushing_ a short gradient, a
peculiar knocking or hammering arose from the engine cylinders—this
is what I describe as acceleration knocking and must not be confused
with the knocking or hammering of a four-stroke engine when labouring
on a gradient. My engine would be _full of life_ all the time it was
knocking like this, and gradually as the speed increased the noise
would ease-off, even though no change of gear had been made.
The diagrams proved to me that this knocking was due to _pre-ignition_
caused by a cushion of hot gases remaining in the top of the working
cylinder, and in my opinion no alteration of the ports or cylinder head
would have influenced this defect to any marked extent; therefore I
should never attempt again to feed the new charge in at the _bottom_
end of the cylinder of a two-stroke engine if I wished to obtain the
maximum amount of power from it. It seems to me that other people
must also have been impressed with similar misgivings, for in one or
two types of engine using crankchamber compression we see a special
attempt made to overcome it, although the method adopted leads to a
rather undesirable arrangement of the engine mechanism. In the type of
engine I refer to the charge may be drawn into the crankchamber in the
usual manner, if desired, but the working cylinder is a casting with
two bores having two separate pistons and a common combustion chamber.
The charge enters above one piston while the crank is on its bottom
dead-centre and is exhausted from the space above the other piston
simultaneously, and the path of the gases is from the inlet port up
to the top of No. (1) bore, then down to No. (2) bore, and out of the
exhaust. This ensures that there shall be no cushion of hot exhaust
gases left in the combustion chamber (or top end of the cylinder).
These engines have given quite good results, and would be much more
extensively used but for the fact that there is double compression
to overcome in starting, and their running torque, due to the number
of impulses given to the crankshaft, is no better than a four-stroke
engine. Fig. 66 shows the arrangement of the cylinders and the path
of the gases. A_{1} and A_{2} are the twin pistons working in the
water-jacketed cylinder casting B, and having the common combustion
chamber C. The connecting rods may drive separate cranks in opposite
directions or both be coupled together and work a single crank. It will
be seen that in this type of engine the piston does not require any
deflector.
The simple two-stroke engine described at the beginning of this chapter
is often constructed in such a form that no automatic inlet valve is
required on the crankchamber.
In this case the induction pipe is connected to a third set of ports
just below and a little to one side of the inlet ports to the working
cylinder, and these are uncovered by the piston towards the completion
of its upstroke, thus allowing the carburetted air to enter the
crankchamber. Such an arrangement constitutes a _three-port_ two-stroke
engine, which is of course _less efficient_ than a two-port engine
with automatic valve, but has the great merit that it is entirely
_valveless_, and therefore extremely simple and cheap to manufacture.
It is much used for motor boat work, both in this country and in
America, on account of its relatively low speed of rotation.
[Illustration: FIG. 66.—TWIN-CYLINDER TWO-STROKE ENGINE WITH
CRANKCHAMBER COMPRESSION.]
CHAPTER XI
HORSE POWER AND THE INDICATOR DIAGRAM
A book on “The Petrol Engine” would hardly be complete without some
reference to horse-power and the indicator diagram. The following
definitions must be carefully studied.
=Work.=—A force is said to do mechanical work when it overcomes a
resistance in its own _line of action_. The line of action of a force
is a line indicating the direction in which the force acts. Engineers
_measure_ work in foot-pound units. The product obtained when we
multiply the magnitude of the force or resistance (in pounds) by the
distance through which it has acted or been overcome (expressed in
feet) gives the quantity of work done in foot-pounds.
_Example_:—A force of 50 lbs. is exerted in overcoming a resistance
through a distance of 12 feet. Find the work done.
Work done = Force (in lbs.) × Distance (in ft.)
= 50 × 12 = 600 ft. lbs.
=Power.=—The _rate_ at which work is done is a measure of the power
exerted. One horse-power is exerted when 33,000 foot-pounds of work are
done in one minute. The work done per minute (in ft. lbs.) divided by
33,000 gives the horse-power expended.
_Example_:—To propel a motor-car along a level road at a speed of 30
miles an hour requires a tractive effort or pull of 70 lbs. if the
vehicle weighs one ton. Find the horse-power required, at the road
surface.
Work done per minute in ft. lbs.
Horse-power = ———————————————————————————————
33,000
Force (in lbs.) × Distance through
which it acts per minute (in ft).
= ——————————————————————————————————
33,000
70 × 30 × 5280
——————————————
60 7 × 264
= —————————————— = ———-——— = 5·6
33,000 330
_Example_:—If the car in the preceding example had to climb a gradient
which rose one foot for every four feet traversed by the car, find the
additional horse-power needed to keep up a speed of 30 miles an hour
while climbing the gradient.
Here we have to raise a weight of 1 ton vertically upwards through a
height equal to one-fourth of the road surface covered, every minute.
Additional Horse-power required
(30 × 5280 1)
2240 (lbs.) × (————————- × -) ft. per min.
( 60 4)
= ———————————————————————————————————————————
33,000
2240 × 660
= —————————— = 44·8
33,000
Total Horse-power to climb the gradient of 1 in 4 at 30 miles an hour =
5·6 + 44·8 = =50·4=
=Brake Horse-Power.=—The length of the circumference or boundary line
of a circle is 6·28 times the length of the radius of the circle or
3·14 times the length of its diameter. Hence, if an engine exerts a
pull of P lbs. at the end of a brake arm of length R feet when it is
maintaining a speed of N revolutions per minute (we may imagine the
brake to be fitted round the rim of the flywheel), we can calculate the
=brake horse-power= thus:—
Brake Horse-Power or B.H.P. = Work done on the brake
per minute in ft. lbs./33,000
Hence B.H.P =
(Pull at the end of the brake arm (in lbs.)) × (6·28 times the radius
of the arm (in feet)) × (the number of revolutions made by the engine
(in one minute))/33,000
= {P × 6·28 × R × N}/33,000
[Illustration: FIG. 67.—PETROL ENGINE BRAKE.]
_Example_:—An engine being tested by a brake applied to the flywheel as
shown in the sketch (Fig. 67) exerts a pull of 50 lbs. at a speed of
2,000 revolutions per minute. If the length of brake arm is 30 inches,
calculate the brake horse-power developed.
Work done per minute = 50 × 6·28 × 30/12 × 2000 ft. lbs.
B.H.P. = {50 × 6·28 × 30/12 × 2000}/33,000 = 47·5
=Rated Horse-Power.=—For taxation purposes the Treasury makes use
of a formula for the rating of petrol engines according to their
probable horse-power. This formula is based on a certain speed of the
piston which was regarded as a limiting value some years ago (when
the formula was first proposed) and on the attainment of a certain
effective pressure in the cylinder.
Horse-power from the Treasury formula = 0·4 d^{2}n.
Where d = diameter of cylinder in inches,
n = number of cylinders.
With modern engines much greater horse-power is obtained, and a near
approximation to the true output is obtained by using what is now known
as the Joint Committee’s formula.
Brake Horse-Power = 0·46 n (d + s) (d - 1·18)
Where d = diameter of cylinder in inches.
s = length of piston’s stroke in inches.
This formula is only to be used in an attempt to predict the _probable_
maximum horse-power which any engine will give. It must not be confused
with the ordinary brake horse-power formula.
_Example_:—Find the probable maximum horse-power of an engine having
four cylinders each 3 in. bore and a piston stroke of 4 in. What would
be its horse-power for taxation purposes?
_By Joint Committee’s formula_—
B.H.P. = 0·46 × 4 (3 + 4)(3 - 1·18) = 1·84 × 7 × 1·82 = 23·35
_By Treasury formula_—
B.H.P = 0·4 × 3^{2} × 4 = 0·4 × 9 × 4 = 14·4
=Indicated Horse-Power.=—The horse-power which an indicator would show
as being developed inside the cylinder of a petrol engine, above the
piston, would be called the _indicated_ horse-power, and should always
work out a greater number than the brake horse-power or power available
at the engine flywheel, because some of the power liberated from the
combustion of the petrol within the cylinder is lost in friction of the
piston and bearings.
The Indicated Horse-Power or I.H.P. = P_{e} × A × L x N_{e}/33,000.
Where P_{e} = mean effective pressure from the diagram,
in lbs. per sq. inch.
A = area of piston in square inches = 0·7854
(diameter of cylinder)^{2}
L = length of stroke of piston, in feet.
N_{e} = number of power impulses per minute
delivered to the crankshaft.
Since a four-stroke engine gives one power impulse to the crankshaft
in every two revolutions, it follows that N_{e} is equal to _half_ the
number of revolutions per minute for a single-cylinder engine of that
type, and _twice_ the number of revolutions for a four-cylinder engine.
A four-cylinder two-stroke engine might be arranged to give either _two
or four_ impulses per revolution of the crankshaft—depending upon the
arrangement of the cranks.
_Example_:—A four-cylinder four-stroke engine runs at a speed of 2,000
revolutions per minute and the mean-effective pressure in the cylinders
is 75 lbs. per square inch. Calculate the indicated horse-power if the
cylinders are 4 in. × 4 in.
I.H.P = P_{e} × A × L × N_{e}/33,000
= 75 × 0·7854 × 4^{2} × (4/12) × 4000/33,000
= {75 × 12·56 × 4000}/99,000 = 38
=The Indicator Diagram.=—At the commencement of this chapter we
explained that the work done by a force was measured by multiplying
the number representing the magnitude of the force (in pounds) by the
distance through which it had acted (measured in feet). This product
gave us the quantity of work done in foot-pound units. Thus “_quantity
of work done_” is really the product of two numbers, just as the area
of a rectangular floor space is measured by length times breadth. In
symbols we write W = F × S where F is the magnitude of the force or
resistance in pounds and S the distance through which it has acted, in
feet. It is interesting to contemplate this symbolical expression W =
F × S together with the expression Area = Length × Breadth, because
it gives us a new idea for measuring work. Imagine a diagram of the
kind shown in Fig. 68, in which the curved line AB has been obtained
by _plotting_ values of F and S for any imaginary case. The diagram
is supposed to represent pictorially how the particular force under
consideration has varied in magnitude as it has traversed a space
represented, to some scale, by the length DC. It is clearly seen that
the force has been _decreasing_ in an _irregular_ manner from some
large value represented by the height DA to a small value represented
by the height CB. We now proceed to show that the shaded area ABCD
measures the total amount of work done by this force.
[Illustration: FIG. 68.—FORCE-SPACE OR _WORK_ DIAGRAM.]
Considering for a moment just the small strip _efdc_ of the diagram we
see that it is easy to find a rectangle _abcd_ equal in area to it.
Now the _height_ of this rectangle will be the _average value_ of the
force while it traversed the space _cd_, and hence the area of the
rectangle _abcd_ gives the =work done= by the force in passing from _c_
to _d_. Similarly by dividing up the whole diagram we would obtain a
number of little rectangles each equal in area to the magnitude of the
work done from point to point. Thus the whole area ABCD gives the whole
work done. To measure the work done in an engine cylinder we must use
some form of _indicator_. An indicator is an instrument which traces
out a diagram on which _abscissæ_ (or horizontal distances) represent
displacements of the piston and _ordinates_ (or vertical distances)
represent the pressures acting on the piston.
[Illustration: FIG. 69.—PETROL ENGINE INDICATOR DIAGRAM. FOUR-STROKE
CYCLE.]
Ordinary steam engine indicators with pencil motion and paper drum
are not suitable for use with fast running petrol engines. The moving
parts of these indicators are too heavy and their springs too sluggish
in action to keep correct time with these high speed engines. Again,
there is too much friction between the pencil and the paper drum, as
well as in the lever joints. Therefore special indicators have to be
used, in which the diagram is traced out by a beam of light reflected
from a mirror on to a ground glass screen or photographic plate. One
corner of the mirror is tilted in time with the movement of the engine
piston by means of a special reducing mechanism, and another corner of
the mirror is tilted in a direction at right angles to the first by
means of a very short thin rod kept in contact with a metal diaphragm
subjected to the pressure of the gases in the engine cylinder. A beam
of light is thrown on to the mirror from a lamp, and after reflection
traces out the diagram on the screen or plate. Such an instrument would
generally be described as a _manograph_. An indicator diagram from a
four-stroke engine is shown in Fig. 69. The line ABC represents the
suction stroke of the piston during which the pressure of the gases in
the cylinder falls a little below that of the atmosphere. Atmospheric
pressure is shown by the height of the line LL above the base, or line
of zero pressure (perfect vacuum). The inlet valve can be opened at B
and closed at D after the crank has turned the bottom dead-centre and
begun the compression stroke. The line CDE represents the compression
stroke of the engine, during which the gases are compressed and their
pressure rises. The height of the point E above the line LL gives the
_compression pressure_ to the scale of the diagram. Ignition occurs at
E, and results in an instantaneous rise of pressure to F due to the
explosion, which is, however, quickly followed by expansion to G. The
exhaust valve opens at G, the gases are _released_, and the pressure
falls still further to point H. The line HA represents the exhaust
stroke of the piston, and the exhaust valve would be closed after
the crank had passed its upper dead-centre and commenced the suction
stroke. The distance marked (x) on the diagram measures the _clearance_
volume (or volume of the space above the piston containing the valves
and referred to as the combustion chamber) to the same scale that the
length of the diagram measures the volumetric displacement of the
piston. The volume traced out by the piston during any working stroke
is measured by multiplying the area
of the piston in square { centimetres} by the length
{ inches }
of the stroke in { centimetres} the product giving us
{inches }
the capacity of the cylinder in cubic { centimetres}. The
{ inches }
area of the diagram HEFG gives the work done during one cycle of
operations, and the area of the small diagram ABCD gives the work
lost in taking in and expelling the charge. The small area should be
subtracted from the large one to get the useful work done per cycle of
operations. The area of the diagram HEFG may readily be obtained by
finding its vertical height at a number of equidistant points, and from
these measurements ascertaining the average or _mean_ height of the
diagram. The average height of the diagram (in inches) multiplied by
its length (also in inches) gives the area in square inches.
[Illustration: FIG. 70.—INDICATOR DIAGRAM FROM A TWO-STROKE ENGINE.]
The average or mean height of the diagram also gives what we term the
_mean effective pressure_ acting on the piston, and constitutes the
P_{e} of the indicated horse-power formula above. The area ABCD is
always small and generally neglected with four-stroke engines. There
are _two separate diagrams_ for a two-stroke engine. The diagram for
the working cylinder is A_{1}B_{1}C_{1}D_{1} in Fig. 70, and that for
the crankchamber is E_{1}F_{1}G_{1}H_{1}. The effective work done
per cycle is measured by the difference in the area of these two
diagrams. The piston uncovers the exhaust port at B_{1} and closes
it again at C_{1}; it uncovers the inlet port at F_{1} and covers it
again at G_{1}. From F_{1} to G_{1} the charge is being delivered
from the crankchamber to the working cylinder. The area of the loop
E_{1}F_{1}G_{1}H_{1} is larger than the corresponding portion of the
four-stroke diagram and should not be neglected.
CHAPTER XII
LIQUID FUELS
Important factors in the choice of a liquid fuel for use in portable
internal combustion engines are: (1) low cost; (2) ease and safety
of transportation or storage; (3) high volatility, i.e., readily
convertible into vapour; (4) non-corrosive action on metals; (5) high
heat efficiency; (6) ability to give satisfactory results in existing
types of internal combustion engine.
=Petrol= is a liquid fuel composed of carbon (C) and hydrogen (H) in
chemical combination. The principal method of producing petrol is by
distillation of crude petroleum. The best mixture to use in a petrol
engine is one composed of 2 cubic feet of petrol vapour to every 98
cubic feet of air. Petrol does not require any heat to vaporize it
under ordinary atmospheric conditions. _Pre-ignition_ of the charge is
liable to occur if the compression pressure exceeds 100 lbs. per square
inch. It does not corrode or deteriorate metal parts, but leaves a
black carbon deposit if not properly burned. Its volatility is high and
its specific gravity is low, being about 0·71. An average figure for
the calorific value of petrol would be 20,000 B. Th. U. per lb. Petrol
is very expensive and also needs care in handling. Private motorists
are not allowed to _store_ petrol or benzol.
=Benzol= is a liquid fuel containing more carbon (C) and less hydrogen
(H) than petrol. The principal method of obtaining benzol is by
distillation of coal tar. The strength of the mixture should be such
that a little more air is supplied in proportion to the quantity of
fuel used than is required for petrol. Generally, it may be said that
when an engine has been running on petrol and is changed over to benzol
the size of the carburettor jet orifice should be slightly reduced
and the weight of the float increased—no other changes need be made
anywhere. Benzol is very volatile and also highly dangerous to handle,
on account of its low flash-point. It often contains impurities which
attack the metal parts of the engine and gum up the valves. It is more
liable to deposit carbon than petrol. Benzol attacks rubber, and paint
on coachwork. It is as expensive as petrol at the present time. The
specific gravity of benzol may be taken as 0·88 and its calorific value
as 19,000 B. Th. U. per lb. It may be compressed above 100 lbs. per
square inch without pre-igniting.
=Alcohol= is a liquid fuel composed of carbon (C), hydrogen (H), and
oxygen (O). The principal method of obtaining alcohol is from the
fermentation of vegetable matter, such as potatoes, beetroot, etc.
About 6 cubic feet of vaporized alcohol to every 94 cubic feet of air
should be used. The volatility of alcohol is very poor compared with
petrol or benzol, and it generally contains some water in suspension.
It will stand double the compression pressure of petrol without
pre-igniting. Alcohol is not so liable to deposit carbon as petrol or
benzol, but is very liable to cause rust. It is not obtainable as a
fuel in Great Britain at present, owing to the high duty on it. Engines
for use with alcohol ought really to be specially constructed for the
purpose. Its calorific value is only 12,000 B. Th. U. per lb., and its
specific gravity is 0·82. Alcohol requires to be heated before it will
vaporize, this heat generally being obtained from the exhaust gases
after the engine has been first started up. Alcohol is fairly safe to
handle or store.
=Paraffin= is obtained during the distillation of petrol from crude
petroleum, and consists of carbon (C) and hydrogen (H) in _almost_ the
same proportions as petrol. Its volatility is low, and it requires
heat to vaporize it. The heat required for vaporization is usually
obtained from the exhaust gases after the engine has been got running.
In starting up a lamp must be used for heating the vaporizer of the
carburettor. Paraffin will stand a little higher compression than
petrol before pre-igniting. The specific gravity of paraffin may be
taken as 0·80 and its calorific value as 18,000 B. Th. U. per lb. It is
much cheaper than either petrol or benzol, being only about one-third
of the cost. The chief objections to its use are its smell and the
greasy character of the stain left by it on coachwork or clothes;
also the difficulty of having to heat the vaporizing chamber of the
carburettor. It is much safer to handle and store than either petrol
or benzol, and requires about the same proportion of air to form an
explosive mixture as that given for petrol. The range of variation of
strength in the mixture which is permissible with paraffin is much less
than with either petrol, benzol, or alcohol. Alcohol has the greatest
range of variation in mixture strength. Paraffin is also very liable to
deposit carbon, owing to the small range of variation permissible in
the strength of the mixture.
=Thermal Efficiency.=—In the foregoing notes we have used certain terms
which have not previously been explained, and therefore it is necessary
to give one or two definitions.
The _Specific Gravity_ of a fuel is the ratio of the weight of one
gallon of the fuel to the weight of one gallon of water. As a gallon of
water weighs 10 lbs., it will be evident from the above notes that a
gallon of petrol only weighs 7·1 lbs., whereas a gallon of benzol will
weigh 8·8 lbs. (approx.), hence it is not surprising to learn that more
_mileage per gallon_ is obtained with benzol than with petrol, even
though the calorific value of benzol, per lb., is less than that of
petrol. Sometimes the specific gravity is referred to as the _density_
of the fuel, but this is only correct when grammes and centimetres
are being used. The density of any fuel is the weight of 1 cubic foot
expressed in pounds or, in general terms, the mass of unit volume of
the fuel. The _density_ of petrol in English units would be about 44
lbs. per cubic foot.
One _British Thermal Unit_ is the quantity of heat required to raise
the temperature of 1 lb. of water by 1 degree (Fahrenheit scale) when
the temperature of the water is about 60°F.
The _Calorific Value_ of any fuel (reckoned on the British system of
units) is the amount of heat (expressed in British Thermal Units) which
will be given out by 1 lb. of the fuel when it is completely burned.
The liquid fuels we have to deal with are hydrocarbon compounds, and
when completely burned the whole of the carbon is burned to carbon
dioxide (CO_{2}) and the hydrogen to steam (H_{2}O), leaving no
residue. By means of a _calorimeter_ we can experimentally determine
the calorific value of any fuel.
It has long been known that work can be turned into heat, and the
petrol engine is a good example of the reverse process which consists
in turning heat into work. In a steam engine and boiler plant the heat
of the fuel is liberated under the boiler, and then a portion of it
gets transferred to the water in the boiler and forms steam, which
is then taken to the engine and does work in the cylinder, the whole
being a wasteful process. The petrol engine is an _internal combustion_
engine, or one in which the fuel is burnt inside the engine cylinder
itself and converted directly into work. From every British Thermal
Unit of heat liberated by the combustion of the fuel in the cylinder
we should be able to get 778 foot-pounds of work if the _thermal_
(or heat) _efficiency_ of the engine was 100 per cent. The thermal
efficiency (η) of any engine may be defined as the ratio which the heat
equivalent of the work done per minute by the engine bears to the heat
which would be liberated by the complete combustion of the quantity of
fuel admitted to the cylinder per minute. Thus—
(Horse-power of the Engine × 33,000)/778
η = ———————————————————————————————————————————————————————————
(Number of pounds of fuel consumed per minute) × (Calorific
Value of the fuel)
_Example_:—An engine developing 30 horse-power uses 0·50 lb. of benzol
per minute. What is its thermal efficiency? The calorific value of
benzol may be taken as 19,000 B. Th. U. per lb.
η = (30 × 33,000/778)/(0·50 × 19,000) = 0·134, or 13·4 per cent.
APPENDIX
ENGINE TROUBLES
Many of the troubles that are likely to arise have already been
referred to in previous chapters, but the following additional notes
may be found useful.
=1. Engine refuses to start.=
Care must be taken to observe exactly what happens, and one cannot do
better than ask oneself mentally some of the following questions.
(_a_) _Is the ignition “on”?_
If a magneto is fitted the earth connexion should be open, but if a
coil and accumulator are fitted the earth connexion should be closed.
(_b_) _Is the petrol reaching the carburettor jet?_
Before removing the jet for the purpose of examining and cleaning it,
it would be advisable to ascertain whether the petrol was reaching the
float chamber. Provided there is a reasonable amount of petrol in the
tank and the tap is turned on, there must be a stoppage either in the
petrol filter, the petrol pipe, or the bottom portion of the float
chamber. Examine the filter and float chamber before disconnecting any
pipes.
(_c_) _Is there a good compression in all the cylinders?_
If there does not appear to be any compression in any of the cylinders,
it is probable that the carburettor throttle is closed and no air or
gas can enter the cylinders. If there is a good compression in some
cylinders and a poor one or none at all in others then—
(1) One or more of the valves may be held off its seat by dirt, by
distortion, or by some derangement of the valve gear. Examine the
valve gear externally, turning the engine slowly to watch its action.
Afterwards remove valve caps and inspect valves if necessary.
(2) One or more of the sparking plugs or valve caps may be short of
its washer. In this case the blow will be heard as the engine is
turned round by hand.
(3) A piston may be cracked or broken or a cylinder cracked.
(4) A cylinder may have got badly worn and the rings on the piston
jammed so that they no longer keep it gas-tight.
(_d_) _Is the engine very stiff to turn over?_
Stiffness is due as a rule to lack of oil on the cylinder walls, caused
by absence of oil in crankchamber or the film of oil on the cylinder
walls having been washed off when _priming_ the engine with petrol in
attempting to start it. If a connecting rod is bent, or the crankshaft
distorted or a piston ring broken, stiffness will also be noted. Very
often by removing the valve caps and pouring a teaspoonful of oil or
paraffin into each cylinder the engine may be freed by vigorously
turning the starting handle by hand until the cylinders and pistons are
well lubricated.
(_e_) _Is there any sign of an attempt to fire the charge_ such as an
occasional puff of smoke from the exhaust or inlet, or an occasional
jerk round of the engine as you turn the starting handle, or an
occasional “bang” in the exhaust box?
If the ignition is “_on_” and the carburettor jet clear, the
compression good and the engine quite free, yet there is no sign of a
“fire” from any of the cylinders, it is possible that air is leaking
into the induction pipe through a faulty joint or any one of the
following ignition troubles may have occurred:—
(_f_) _Defective sparking plug or plugs._ This may arise from water or
oil or dirt between the plug points; or from faulty insulation in the
body of the plug. To test whether the plugs are at fault an easy method
is to take a screwdriver with a wooden handle and place the metal
blade on the terminal of the plug, letting the point come about one
thirty-second of an inch from the metal of the cylinder or any of the
pipes; when the engine is turned by hand the spark will be seen to pass
across this improvised gap if the magneto and leads are in order.
(_g_) _Defective electrical connexions._
The high tension cables may be broken, or disconnected, or
short-circuited. The _earth_ wire may be short-circuited (i.e., in
electrical contact with some other wire or metal fitting). There may be
a short-circuit in the ignition switch.
(_h_) _Defective magneto or coil._
The low tension contact breaker lever may be jammed so that the make
and break is inoperative, or one of the carbon brushes may have got
broken. Occasionally one finds the magnets of the machine have lost
their power; or there is some electrical defect in the armature or
condenser. The battery may have become exhausted. The trembler blade
may be stuck up. Water may have found its way on to the high tension
electrode or into the safety spark gap.
=2. Engine starts up fairly well, runs a little, and then stops.=
Take care to notice the manner in which the engine runs and stops. Note
whether it runs regularly or irregularly and for how long a time.
If the engine runs _regularly_ with all cylinders firing, then probably
the exhaust is choked or the petrol supply fails. Failure of the petrol
supply may be due to the use of too small a jet in the carburettor,
too low a level in the float chamber, or to partial stoppage in the
pipe line. Another cause of this trouble of intermittent running would
sometimes be loss of battery power when using coil ignition, i.e.,
batteries want recharging.
If the engine runs _irregularly_ the trouble is probably due to too
much oil in the cylinders causing the plugs to _misfire_, the presence
of water or dirt in the petrol, a defective valve, a broken carbon
brush, or poor electrical contact somewhere in the magneto, the low
tension contact breaker (coil), or high tension distributor (coil).
To ascertain whether the engine is firing regularly on all cylinders,
or to detect which cylinder is _misfiring_, the best procedure is to
open the compression taps in turn while the engine is running and in
each case speed up the engine while you have the tap open. Cylinders
which are firing _well_ give a sharp _cracking_ noise, those which
are not firing merely give a _hissing_ noise. If no compression taps
are provided, each plug must be short-circuited to the frame in turn
by the screwdriver method given above. The short-circuiting process
causes a reduction in engine speed except on that plug which is already
not firing. The method is not so good as the compression tap process,
because the plugs often get oiled up during the short-circuiting
process and the difficulty is accentuated.
=3. Timing the Ignition.=
My colleague, Mr. Oliver Mitchell, has pointed out to me that it is
often impossible to tell directly when the piston is exactly at the
top of its stroke, and he recommends a study of the accompanying Valve
Setting Diagram (Figure 71). From this it will be seen that it is
sufficiently near to bring the engine first of all to such a position
that the exhaust valve has _just closed_; then make a chalk mark on
the flywheel and give the engine one complete turn round; the piston
will then be in the firing position if the flywheel is turned a shade
backwards. Another method would be to _retard_ the ignition fully and
time it so that the spark occurred one complete revolution after the
_inlet_ valve had _just commenced to open_. When either valve is closed
its tappet can be felt to be _free_, the amount of freedom depending
upon the clearance between the tappet head and valve stem.
[Illustration: FIG. 71.—DIAGRAM OF VALVE SETTING.]
INDEX
A
Acceleration, quick, 78
— under load, 92
Accessibility, 39, 50
Accumulator, 61
Adapter, 92
Adjustable tappet head, 29, 30
Adjusting screws, 19
Advanced ignition, 55, 56, 59
Aeroplane engine cylinder, 10, 16
Air, 1, 41, 42, 46, 87, 89, 114
— carburetted, 42, 97
— lock, 72
— pump, 50
— — hand, 50
— scavenging, 16, 35, 84, 85, 87, 88, 89
— to petrol ratio, 42
— valve, automatic extra, 43, 49
— velocity, 46, 70
— volume of, relative to petrol vapour, 42, 108
— weight of, relative to petrol vapour, 42
Alcohol, properties of, 109
Anti-freezing solution, 72
Armature of magneto, 52, 54
— relative speeds of engine and magneto, 54, 55
Atmospheric pressure, 3, 90
Atomization of petrol, 48, 49
Automatic extra-air valve, 43, 49
— inlet valve, 82, 84, 88
B
Backfire, 62, 84
Balance, perfect, 76
— want of, 75, 76
— weight, 25
Balancing the crankshaft, 25, 26, 76
Barrel of cylinder, 12
Base, oil, 8, 40, 65
Battery, 116
Bearings, main, 24, 64
Benzol, properties of, 45, 108
Blade, trembler, 57
Boiling of water in jackets, 72
Boxes, core, 11
Brake, for petrol engine, 100
Brake horse-power, 99
British thermal unit, 111
Brush, carbon, 53, 55, 116
Built-up cylinder, 16
— flywheel, 27
Buoyancy, 45
Burning, rate of, of mixture, 1, 57
— of deflector, 91
Bursting of flywheel rim, 27
Bush, phosphor bronze, 23
C
Cables, high tension, 61
Calorific values, 111
Cam, 30, 33
Camshafts, 29, 33, 40
Cams, stationary, for magneto, 53
Capacity of cylinder, 106
Caps, valve, 13, 31
Carbon brush, 53, 55, 116
— deposit, 63, 78
Carburation, 42, 94
Carburetted air, 42, 97
Carburettor, 5, 8, 42, 82
— jet type, 42
— multiple jet, 47, 94
— points of a good, 49
— recent improvements in, 47
— spray type, 42
— surface type, 42
— wick type, 42
Castings, cylinder, 11
Cast-in-pairs, cylinders, 16
Centrifugal force, 27
Chaindrive, silent, 38
Charge, 3, 80
Charging pump, 80
Chassis, 40, 71
Choke tube, 46, 47
Chrome steel, 20, 36
Circulating water pump, 8, 71
Circulation, forced, 71
— local, 73
— pump, 71
— thermo-syphon, 69
Clearance between piston and
cylinder walls, 17
— of valve tappet, 29
— space, 4, 105, 107
Coil ignition, 57, 59, 61
— — system, wiring diagram for, 61
— non-trembler, 59
— supplementary, for starting, 58
— trembler pattern, 57, 61
Collector ring, 55
Combustion, 78
— chamber, 4, 96
Compensating jet, 48
Compression, meaning of, 3
— pressure, 82, 83, 105
— stroke, 6, 80
Cone clutch, 27
Connecting rod, 4, 21, 96
— — forces acting on, 20
— — phosphor bronze, 20, 22
— — steel, 23
Consumption of fuel, 79
— of oil, 67
Contact breaker, low tension, 53
— — wipe form of, 58
— screws, platinum tipped, 53, 57
Cooling, 69
Core-boxes, 11
Core, iron, of ignition coil, 57
Cost of production, 22, 64, 79
Cotter, 29
Crank chamber, 8, 39
— — gas-tight, 81, 82, 96
— cheeks or webs, 24
— motor-cycle, 23
— pin, 24
— radius, 4
— shaft, 23, 28, 77, 89
Crank shaft, balancing the, 25, 76
— — vibration of, 76
— — whipping of, 76
Cushion of hot gas, 93, 96
Cut-out, 90
Cycle, four-stroke, 5
— motor, 10, 11
— Otto, 5
— two-stroke, 80
Cylinder, 5, 9
— aeroplane engine, 15, 16
— air-cooled, 11, 15
— barrel, 12
— built-up, 16
— castings, 11
— head, detachable, 32, 78
— jackets, 12, 15, 69
— L-headed, 16
— pump, 87, 88, 107
— revolving, 10
— T-headed, 15
— water-cooled, 8, 32, 82, 96
— working, 81, 87, 107
Cylinders, cast _en bloc_, 16, 77
— cast in pairs, 16
— cast separately, 16
— choosing the number of, 75
— firing order of, 60
D
Dead centre, 4, 116
— gases, 83
Defective coil, 114, 116
— ignition system, 114
— magneto, 114, 116
— sparking plug, 113, 114
— valve, 113, 114, 116
Deflector, 81, 87, 93, 96
— overheating of, 84, 91
Density, 45, 111
Deposit of carbon, 63, 78
Description of a typical petrol engine, 8
Detachable cylinder head, 32, 78
Devices, ignition, 51
Diagram, indicator, 92, 102
— for four-stroke engine, 105
— for two-stroke engine, 106
— for valve setting, 116
— work, 103
Difficulties in starting, 72, 75, 92, 96, 113
Discharge, spark, 51, 54, 58
Distributor, high tension, 52, 53, 58
Double sleeve engine, 32, 78
Down stroke, 4, 81, 82
Drawings, working, 11, 93
Drip feed, 64, 90, 92
Drive, silent chain, 38
Dual ignition, 58
— springs, 90
Ducts for oil, 41
Duplex piston, 86
Durability, 49, 79
E
Earthing wire, 60
Eccentric, 33, 36
— rod, 33, 36
— sheaves, 33, 36
— straps, 33, 36
Economy and durability, 79
Efficiency, thermal, 110, 111
— volumetric, 83, 86, 88
Electric spark, 51, 54, 58
Electrodes of sparking plug, 51, 91, 92
Engine, four cylinder, 9, 76, 77, 94
— internal combustion, 111
— motor car, 9
— multi-cylinder, 77
— points of a good, 75
— single cylinder, 75
— six cylinder, 76, 77
— troubles, 113
— two cylinder, 76
— two-stroke, 80, 95
Evaporation, 1
Exhaust pipe, 6
— ports, 81, 88
— — open, 81, 88, 96
— smoky, 66
— stroke, 7, 35
— system, overheating of the, 57
— valves, 5, 10, 37, 81
— valves, timing of the, 35, 37, 78, 116
Explosion stroke, 7, 21, 26, 35
Explosive mixtures, 1, 108, 109, 110
Extra-air valve, 43, 49, 94
F
Fan, 70
— pulley, 8, 70
Feed, drip, 64, 90, 92
Fibre, tappet head, 30
— wheel, 38
Film of oil, 65
Filter, petrol, 43
Fins, heat radiating, 70
Firing order of the cylinders, 60
— stroke, 60
Fixed ignition, 57
Flanges, 12, 13
Flash point of lubricating oils, 63, 67, 68
Flashing back from exhaust, 84, 92, 93, 95
Flexibility, 77, 83, 92
Float chamber, 43, 44, 94
Flooding, 45, 94
Fluctuation of engine speed, 27
Flywheel, 8, 26
— built-up, 27
— function of, 26
— rim, bursting of, 27
— single stamping of steel, 28
Force, 98
— centrifugal, 27
Forced circulation, 71
— lubrication, 65, 90
Forces, inertia, 75
— space diagram, 103
Four cylinder engine, 9, 76, 77, 94
— stroke cycle, 5
Frequency of vibrations, 76
Fuel, consumption of, 79
Fuels, liquid, 108
Furring of tubes and jacket spaces, 73
G
Gap of sparking plug, 51, 54
Gas, compressing the, 3
— pump, 80
— tight crankchamber, 81, 82, 96
Gearing, overhead, 16
Gears, helical, 38
—spur, 37, 38
General overheating, 72, 73
—principles, 1
Gilled tube, 70, 74
Gills, 70
Gravity feed, 50
Grease in cylinder jackets, 72
Grinding in a valve, 31
Gudgeon pin, 17, 18
— — wear on, 19, 90
Guides, valve stem, 8, 12, 13, 30
H
Hand air pump, 50
Handle, starting, 71
Head, adjustable tappet, 30
— of water, 69, 71
Heat efficiency, 112
— energy of the petrol mixture, 9, 17, 111
— radiating fins, 11, 70
— surplus, 9, 17
Helical teeth, 38
High tension cables, 61
— — distributor, 52, 54, 58
— — magneto, 8, 51, 52, 54, 89
— — terminals, 51, 57
Holes, cotter, 29
— oil, 63
Honeycomb radiator, 70, 74
Horse-power, 98
— brake, 99
— indicated, 101
— Joint Committee’s formula, 101
— of car, 99
— Treasury formula, 101
Hot-water jacket, 43, 47, 94
Hydrogen, 108, 109
I
Idle stroke, 26, 82
Ignition, advanced, 55, 56, 59
— coil, 57, 59, 61
— devices, 51
— dual, 58
— fixed, 57
— knock, 57, 84
— normal, 55
— retarded, 55, 59, 62
— synchronized, 59, 61
— timing the, 60, 62, 116
— two-point, 59
Improved system of splash lubrication, 64
Improvements in carburettors, recent, 47
Indicated horse-power, 101
Indicator diagram, 92, 102
— — for four-stroke engine, 105
— — for two-stroke engine, 106
Indicator, optical, 92
Induction pipe, 5, 8, 43, 82
Inertia forces, 75
Inflammable vapour, 63
Injection of water to cylinder, 92
Inlet ports, 81, 88
— valve, air, automatic, 43, 49, 88
— — automatic, for gaseous mixture, 82, 88
— — mechanically operated, 5, 30, 31
— — timing the, 34, 37, 78, 116
— water pipe, 8, 70, 73
Inspection openings, 39
Insulators, 51
Internal water cooling, 92
J
Jacket, cylinder, 12, 15, 69
— hot water, 43, 47, 94
— space, too large, 73
— water, overheating of, 72
— — temperature of, 74
— — weight of, 74, 89
Jet, atomizing, 48
— compensating, 48
— petrol, 44, 46
Jet-in-tube carburettor, 42
Jigs, 24
Jockey pulley, 38
Joints, water-tight, 11, 15
Journals, 24
K
Kean’s two-stroke engine, 85, 94
Knock ignition, 57, 73, 84
Knocking, acceleration, 92, 93, 95
— intermittent, 84, 92, 93
— spasmodic, 84, 91
L
=L=-headed cylinders, 16
Lag in opening and closing valves, 34
— time, 56
Lead of valve, 35
Leakage of gas past the piston, 18, 114
Leaky pistons, 114
Liquid fuels, 108
— petrol, 108
Local circulation, 73
— overheating, 72
Lock, air, 72
Lock nuts, 29
Low tension contact breaker, 52
— — terminal, 57, 58
— torque, 75
Lubrication, forced, 65, 90
— improved system of splash, 64
— splash system of, 63
M
Magneto armature, relative speeds of, 54, 55, 94
— for four cylinder engine, 52, 54, 60
— for six cylinder engine, 54
— high tension, 8, 39, 40
— ignition system, wiring diagram for, 60
— racing pattern, 94
— two-stroke engine, 94
Make and break, 53, 58, 62
Manograph, 104
Materials, packing, 15
Mean effective pressure, 89, 107
Mechanically operated valve, 30, 84
Metal segments of distributor, 55
Mild steel, 23
Mileage per gallon, 110
Misfiring, 84, 116
Mixing chamber, 43, 47
Mixture, explosive, 1, 108, 109, 110
— heat energy of the, 9, 17, 111
— strength of the 1, 42, 108, 109, 110
— too weak a, 46
— weakening the, 49, 73
Momentum, 35, 36
Monobloc casting, 77
Motor car engine, 9
— cycle, 10, 11
— — crank, 23
Moulds, 11
Multiple jet carburettor, 47, 94
Mushroom type valves, 5, 8, 30
N
Natural circulation, 69
Needle valve, 44
Nickel steel, 20, 36
Noise from valves, 30, 39
Non-trembler coil, 59
Normal ignition, 55
O
Oil base, 8, 40, 65
— consumption of, 67
— ducts, 41
— film, 65
— holes, 63
— pressure, 65, 67
— price of, 67
— properties of, 63
— pump, 8, 39, 40, 65, 67
— — speed of, 67
— troughs, 65
Open exhaust port, 81, 88, 96
Optical indicator, 92
Orifice, 45, 73
Oscillations, torsional, of the crankshaft, 77
Otto cycle, 5
Outlet water pipe, 8, 70, 71, 73
Overflow pipe, 72
Overhead gearing, 16
Overheating of deflector, 84, 91
— of the exhaust system, 57
— general, 72, 73
— local, 72
Overloading the engine, 55, 89
P
Packing materials, 15
— rings, 17, 18, 81, 87, 97
Paraffin, properties of, 109
Partial seizure, 19, 33, 63, 64, 90
— vacuum, 6, 46, 82, 90
Passages, valve, 10
Patterns, 11
Perfect balance, 76, 77
— combustion, 78
Periodicity, 76, 77
Petrol, consumption of, 79
— engine, description of a typical, 8
— filter, 43
— jet, 44, 46
— mixture, heat energy of, 9, 17, 111
— pipe, 43
— properties of, 108
— supply, failure of, 113
— tank, 43
— tap, 43
— vapour, 1, 42, 43, 80
— — ratio of air to, 42
Phosphor bronze bush, 23
— — connecting rod, 22
Pinking, 79
Pipe, exhaust, 6, 8
— induction, 5, 8, 43, 82
— inlet water, 8, 70, 71, 73
— outlet water, 8, 70, 71, 73
— overflow, 72
— transfer, 86, 87
— vent, 39
Piston, 4, 17, 32, 82, 96
— duplex, 86
— leaky, 73, 114
— ring, cast-iron, 18, 82
— — steel, 19
— seizure of the, 19, 33
— steel, 19
— weight of, 19
Pitch of the teeth on a gear wheel, 38
Plain tube radiator, 74
Platinum tipped contact screws, 53, 57
Plug, sparking, 5, 8, 51, 91, 92
Points of a good carburettor, 49
— — good engine, 75
Poppet valve, 29, 77, 79
Ports, exhaust, 33, 78, 81, 88
— inlet, 33, 78, 81, 88
— open exhaust, 81, 88, 96
Power, 98
— stroke, 7, 60, 80
Pre-ignition, 84, 90, 92, 95, 108, 109, 110
Pressure, atmospheric, 3, 90
— compression, 3, 80, 82, 83, 88, 105
— feed, 50
— mean-effective, 89, 107
— of oil, 65, 67
Prices of oils, 67
Primary winding, 53, 58
Production, cost of, 22, 64, 79
Properties of fuels, 108, 109, 110
— of oils, 63
Pulley, fan, 8, 70
— jockey, 38
Pump, air, 50
— charging, 80
— circulating, 8, 67, 71
— cylinder, 87, 88, 107
— gas, 80
— hand air, 50
— oil, 8, 39, 40, 65, 67
— piston, 86
— water, 8, 39, 71
Push rod, 29
Q
Quick acceleration, 78, 92
— make and break, 53, 59, 62
R
Racing pattern magneto, 94
Radiator, gilled tube, 69
— honeycomb, 70, 74
— size of, 71, 72, 74, 89
Rate of burning, 1, 57
Rated horse-power, 101
Ratio of air to petrol vapour, 42
Recent improvements in carburettors, 47
Relative speeds of engine and magneto, 54, 94
Release of gases, 105
Relief valves, 67
Residue, 63
Retarded ignition, 55, 57, 59, 62
Revolving cylinders, 10
Ring, packing, 17, 18, 81, 87, 97
— steel piston, 19
Roller, 29, 58
Rotary valves, 79
S
Sagging of camshaft, 36
Scavenging, 16, 35, 84, 85, 87, 88,89, 93
Scoops, 65
Screws, adjusting, for gudgeon pin, 18
— platinum tipped, for contact breaker, 53, 57
Seat of valve, 13, 29
Segments, metal, of distributor, 55
Seizure of piston, 19, 33, 63, 64, 90
Shafts, eccentric, 33
Sheaves, eccentric, 33
Short-circuiting terminal, 55
Silence in running, 79, 92, 95
Silencer, 6, 89
Silent chain drive, 38
Simple two-stroke engine, 81, 85
Single cylinder engine, 75, 76
— sleeve valve engine, 32
— throw crankshaft, 23
Six cylinder engine, 76
— — — magneto for, 54
Sleeve valves, 32, 77, 78
Sleeves, double, 32, 78
Slots, 32
Smoky exhaust, 66
Spark, electric, 51, 54, 58
Sparking plug, 5, 8, 13, 51, 91, 92
Spasmodic knocking, 84, 91
Specific gravity of fuels, 108, 109, 110
Speed, fluctuation of engine, 27
— of oil pump, 67
Speeds of magneto armature and
distributor, 54, 55, 94
Splash system of lubrication, 63
— — — improved, 64
Split pin, 19
Spray type of carburettor, 42
Springs, dual, for automatic valve, 90
— valve, 8, 29, 90
Sprocket wheels, 38
Spur gears, 38
Stampings for connecting rods, 22
Starting difficulties, 72, 75, 92, 96
— handle, 8, 71
Stationary cams for magneto, 53
Steel, chrome nickel and chrome vanadium, 20, 36
— flywheel, built-up, 27
— — single stamping, 28
— mild, 23
— pistons, 19
— — rings, 19
— tungsten, for valves, 30
— Ubas, 90
Storage of petrol, 108
Strap, eccentric, 34
Strength and character of spark, 58
— of mixture, 1, 42, 108, 109, 110
Stroke, meaning of, 3
— suction, 6, 21, 35, 60, 80
Suction, meaning of, 2
Supplementary coil, for starting, 58
Surface carburettor, 42
Synchronized ignition, 59, 61
T
Tank, petrol, 43
Tap, compression, 13
— petrol, 43
Tappet head, 29, 30
— valve, 8, 29
Teeth, helical, 38
— pitch of, 38
Temperature of jacket water, 74
Terminals, high tension, 51, 57
— low tension, 57, 58
T-headed cylinder, 15
Thermal efficiency, 110, 111
— unit, British, 111
Thermo-syphon circulation, 69, 89
Three-port two-stroke engine, 97
Throttle valve, 44, 47
Time lag, 56
Timing the ignition, 60, 62, 116
— the inlet valve, 34, 37, 78, 116
— the exhaust valve, 35, 37, 78, 116
— wheels, 8, 37
Too rich a mixture, 46, 78
Too weak a mixture, 46, 73, 78
Torque, low, 75
Torsional oscillations of crankshaft, 77
Transfer pipe, 86, 87
Treasury rating for horse-power, 85, 101
Trembler blade, 57
— coil, 57, 61
Troughs, oil, 65
Tube, amount required for radiator, 74
— gilled, 74
— plain, 74
Tungsten-steel valves, 30
Twin piston two-stroke engine, 96
Two cylinder engine, 76
Two-point ignition, 59
Two-port two-stroke engine, 81, 85, 95
Two-stroke cycle, 80, 83, 87
— engine, Kean, 85, 94
— — simple, 81, 85
— — twin piston, 96
— — two-port, 81, 85, 95
— — three-port, 97
— — valveless, 97
U
Ubas steel, 90
Unbalanced mass, 25
Underframe, 40
Up-stroke, 4, 81
V
Vacuum, partial, 6, 46, 49, 82, 90
Valve, automatic inlet, 82, 84, 88
— — extra air, 43, 49, 88
— caps, 13, 31
— defective, 113, 114, 116
— exhaust, 5, 37, 81
— extra air, 43, 49, 94
— guides, 8
— head, 29
— inlet, 3, 5
— mechanically operated, 5, 30, 84
— mushroom type, 5, 8, 30
— needle, 44
— passages, 10
— relief, 67
— seat, 13, 29
— springs, 8, 29, 90
— stem guides, 8, 12, 13, 29
— tappet, 8, 29
— throttle, 44, 47
— timing the exhaust, 35, 37, 78, 116
— — the inlet, 34, 37, 78, 116
— to grind in, 31
— tungsten steel, 30
Valves, noise from, 30, 39
— poppet, 29, 77, 79
— rotary, 79
— sleeve, 32, 77, 79
Valveless two-stroke engine, 97
Valve-setting diagram, 116
Vanadium steel, 20, 36
Vaporization, 1, 46, 48, 49, 94
Vapour, inflammable, 63
— petrol, 1, 33, 42, 80
Velocity of air, 46, 70
Vent pipes, 39
Vibration of crankshaft, 76
Viscosity, 63
Volatility, 108
Volume displaced by piston, 105, 106, 107
— of clearance space, 4, 105, 107
Volumetric efficiency, 83, 86, 88
Vulcanite fibre tappet head, 30
W
Washer, 29
Waste heat, 9, 17
Water boiling in jackets, 72
— circulating pump, 8, 67, 71
— cooled cylinder, 8, 32, 82, 96
— cooling, internal, 92
— head of, 69, 71
— hot, for jacket, 43, 47, 94
— injection, 92
— jacket, 10, 12, 70, 72, 73, 96
— pipe, inlet, 8, 69, 70, 71, 73
— — outlet, 8, 69, 70, 71, 73
— pump, 8, 39, 40, 72
Water-tight joint, 11, 15
Weakening the mixture, 49, 73
Wear on gudgeon pins, 19, 90
Webs, crank, 24
Weight of piston, 19
— of water in radiator, 74, 89
Weights, balance, 25
Wheels, fibre, for timing gear, 38
— sprocket, for chain, 38
— timing, 8, 34
Whipping of crankshaft, 76
Wick type of carburettor, 42
Winding, primary or low tension, 53, 58, 61
— secondary or high tension, 53, 58
Wipe form of contact breaker, 58
Wire, earthing, 60
Wiring diagram for coil ignition system, 61
— — for magneto ignition system, 60
Work, 98
— diagram, 103
Working cylinder, 81, 87, 107
— drawings, 11, 93
Wrist pin, 18
(PR. 1315.)
Butler & Tanner Frome and London
* * * * *
Transcriber's Notes
Obvious typographical errors have been silently corrected. Variations
in hyphenation have been standardised but all other spelling and
punctuation remains unchanged.
Italics are represented thus _italic_, bold thus =bold= and underlining
thus +underline+.
In several of the equations the final answer was presented in bold
text. To avoid confusion with the mathematics this has not been
indicated.
*** End of this LibraryBlog Digital Book "The Petrol Engine - A Text-book dealing with the Principles of Design and - Construction, with a Special Chapter on the Two-stroke - Engine" ***
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