CHAPTER VII
THE PETROL ENGINE
By R. J. Mecredy, Editor of the 'Motor News'
Every motor-car owner, whether he can afford to keep a mechanic or not, should make a point of studying and thoroughly understanding his engine. It is not merely that this will save him trouble and emancipate him from the tyranny of the skilled mechanic, but it will very materially increase his pleasure in the pastime, for the study of the engine affords almost as keen enjoyment as the actual driving.
The man who is uninitiated is likely to regard with despair the prospect of ever being able to understand the apparently complex machinery which propels his car. In reality it is exceedingly simple. Very little study will enable him thoroughly to grasp its principles, and after that the rest is merely a matter of common sense. When he has once learned how the engine works, and wherein it is likely to fail, he will quickly diagnose troubles which would otherwise prove insurmountable.
Of course, if one can afford it, it is desirable to keep a skilled mechanic, but it is an enormous advantage to feel that one is independent of his services, and cannot be 'taken in,' as is the ignorant novice. A mechanic, however, is by no means necessary—an ordinary handy man can quickly be taught to clean and lubricate, to keep the working parts thoroughly adjusted, and even to diagnose the ordinary roadside troubles which are bound to occur. From this it will be seen that it is almost essential for every motorist to know something of his car; and the purpose of this _0138.jpg)
Sections of Daimler Co.'s 4-cylindered Motor
chapter is to give the novice a complete insight into the various parts of a petrol engine, and their respective functions.
What is a Petrol Engine? — 'Petrol Engine' is a slang term for an engine driven by a series of explosions of a mixture of the vapour of a light spirit of petroleum with air.
'Gas engines' are similarly driven by explosions of a mixture of coal gas and air.
Both are known as 'internal combustion engines.'
In order to explain the system, there is here taken as an example a single-cylindered engine of the Daimler type.
_0139.jpg)
Fig. 1 represents a drawing of such a motor if it be cut in half; fig. 2 the same motor cut in half the other way through.
p is a piston which accurately fits in a cylinder, and is free to pass up and down the interior of same. The top of the piston travels between the dotted lines at the top, t, and the dotted line at the bottom, b. The piston p is connected by the connecting rod c to the crank cr by means of which it turns the flywheel f.
Compared with the propulsion of the front wheel of the old high-wheeled bicycle, the connecting-rod (c) represents the rider's leg, the crank (cr) the crank of the bicycle, and the flywheel (f) the large wheel of the bicycle.
The force which drives the piston downward, and so operates the fly-wheel f, is generated by the explosion of a mixture of gas and air in the combustion chamber t. This mixture reaches the combustion chamber through the induction pipe i p, and the induction or inlet valve i. It is fired by an electric spark occurring in the combustion chamber, or a red-hot platinum tube protruding into same, and the exploded charge is ejected through the exhaust valve e, as will be hereafter explained.
THE SUCTION STROKE
Let it be supposed that the fly-wheel has been set rapidly revolving, that the piston has been up at the top at t, and has just descended to the bottom of its stroke (b). In doing this it sucks down the valve i (called the inlet or induction valve), which is otherwise held closed by a spring, and thus draws through the valve from the induction pipe (ip) a mixture of vapour of petrol and air.
When the piston is at the bottom (b) the cylinder is fully charged with this explosive mixture.
The suction having stopped, the inlet valve is closed by its spring, and the cylinder is then air- or rather gas-tight.
THE COMPRESSION STROKE
The momentum of the fly-wheel then thrusts the piston up to the top (t) again, and in doing so, as there is no escape, the explosive mixture which had previously filled all the space in the cylinder between its head (h) and b is compressed into the very small space remaining between h and t.
This is what is known as compression. The explosive mixture has to be thus compressed before it is fired.
THE EXPLOSIVE STROKE
At this point the explosive mixture is fired either by means of an electric spark or by a heated tube. The systems of firing are dealt with in the chapter on Ignition (Chapter VIII.).
It is sufficient at present to note that the highly compressed explosive mixture is fired, and as there is no outlet for the suddenly expanded gases (for the force. of the explosion only tends to close tighter the inlet valve i and the outlet or exhaust valve e, which are referred to later), the whole force of the explosion goes to thrust down the piston from t to b. It is this thrust which gives the fly-wheel its momentum, its swing; it is this thrust, in fact, which makes the car move.
THE EXHAUST STROKE
At this point, when the piston is down at the bottom, at b, another valve, the exhaust valve (e) is opened (by an arrangement which is explained hereafter), and is kept open during the whole of this up-stroke from b to t, the consequence being that the exploded mixture is thrust out through this exhaust valve, which closes immediately the piston gets to the top again (t).
A COMPLETE CYCLE
This is the whole operation:—
Fig. 3, Diagram A. A spot is shown upon the fly-wheel before the beginning of the operation.
Fig. 3, Diagram B, shows that during the suction stroke the fly-wheel has made half a revolution.
Fig. 3, Diagram C, shows that during the compression stroke a further half-revolution is made and the spot has returned to its starting-point.
Fig. 3, Diagram D, shows that during the explosive stroke a further half-revolution of the fly-wheel is made.
_0142.jpg)
Fig. 3.—A complete Cycle
Fig. 3, Diagram E, shows that during the exhaust stroke a fourth half-revolution is made. So for every explosion there are two complete revolutions of the fly-wheel.
INDUCTION VALVES
In Fig. 1 it will be seen that the interior of the cylinder is separated from the induction pipe i p by an inlet valve marked i.
Fig. 4 (a) shows the induction valve in its place in the wall of the cylinder, and closed so that no mixture can pass.
Fig. 4 (b) shows a section of the induction valve when the valve is open leaving a free passage for the mixture in the direction of the arrows.
The spring above the valve is of such a strength that it keeps the valve closed, except when the power of suction is exerted, when it opens and the explosive mixture is admitted from the induction pipe through the aperture thus made.
EXHAUST VALVE
In Fig. 1 it will be noted that the exploded gases are expelled by the rising piston from the cylinder through the exhaust valve e and exhaust pipe e p.
Fig. 5 (a) shows the exhaust valve in its position in the wall of the cylinder and closed against the escape of the exhaust gases.
_0143.jpg)
Fig. 4.—Induction Valve
Fig. 5 (b) shows a section of the valve when it is open, leaving a free escape for the exhaust gases in the direction of the arrow.
It will be noted that the valve is kept in its seat by means of a spring, and it remains in this position throughout the suction, compression and explosion strokes. During the exhaust stroke, however, when the explosive gases are expelled from the cylinder through the exhaust valve into the exhaust pipe, the exhaust valve is held open by a mechanical contrivance.
THE MECHANICAL LIFT OF THE EXHAUST VALVE
The simplest form of exhaust-valve mechanism is to be found in engines of the De Dion type, as illustrated in fig. 6.
_0144.jpg)
Fig. 5.—Exhaust Valve
a is the end of the crank-shaft (fly-wheel shaft) on to which the small gear wheel b is fixed. This wheel engages with another wheel c, which revolves on the shaft d. The gear wheel c is double the size of b, consequently c only revolves once for every two revolutions of the crank shaft. In the solid with wheel c is mounted an eccentric cam e, which raises the exhaust valve h against the pressure of the spiral spring g, thus allowing the exploded gases to escape from the cylinder. The mode of working is very simple. When the cam e revolves so that the projecting part comes on top, it pushes up the plunger j. j in turn pushes the spindle f, which carries on its top the exhaust valve h, and the latter is consequently removed off its seating, and permits the exhaust gases to escape. As e continues to revolve _0145.jpg)
Fig. 6.—Exhaust
Valve Lifter the protruding portion sinks from under j, and the spring c pushes the exhaust valve h to its position on its seating. Needless to say, the gear wheels b and c must be so set that the cam e will open the exhaust valve h at exactly the right moment.
The action of the exhaust valves in the two cylinder engines of the Daimler type is described further on under the heading Governors. The principle is exactly the same.
CARBURETTER
The mixture, the ignition of which causes the impulse which drives the engine, is formed from the vapour which rises from petrol or motor-car spirit when mixed in proper proportions with air in other words, when this air is carburetted. The chamber in which this mixing takes place is consequently called the carburetter. There are three types of carburetter, which we will now describe.
(1) The Surface Carburetter, as used on the De Dion Motor Cycle. This carburetter is so designed as to give a considerable surface of spirit, off which the vapour rises, and together with a certain quantity of air is sucked into the mixing chamber, which is commonly known as the twin tap. In this mixing chamber the proper proportions of air and gas are finally regulated so as to give a perfect mixture. Fig. 7 explains the construction of the carburetter.
h is a pipe taken off the exhaust which carries a portion of the exploded charge through, but not into the petrol, for the purpose of warming it, and so assisting in its better vaporisation.
j is the air inlet, and attached to its lower extremity is a plate l. The suction of the piston through the induction valve and supply pipe causes a rush of air down through j, which mixes with the vapour rising from the petrol, and is drawn upwards into the twin tap, or mixing chamber on the top of the carburetter. The plate l is to prevent the petrol
_0146.jpg)
Fig. 7. — The De Dion Carburetter
from splashing upwards, and to diffuse the air over the spirit, and its position can be regulated by raising or lowering the tube j, so that it will rest just above the surface of the petrol. As will be seen, the plate l does not touch the sides of the carburetter, and consequently the vapour forming by the mixture of gas and air can rise round its sides into the upper portion of carburetter c, in which the mixing process is continued. f is a float with wire attached to indicate the height of the petrol.
k is the twin tap in which the mixing is finally completed. g' is the lever regulating the amount of air admitted to the twin tap, and by means of which the quality of the mixture is controlled. It is generally called the 'quality lever,' as it determines the quality of the mixture.
g is the lever regulating the aperture in the bottom of the twin tap, by means of which the perfect mixture is admitted to
_0147.jpg)
Fig. 8.—The Twin Tap or Mixing Chamber
the combustion chamber of the engine through the pipe e—in other words, it is a throttle lever. It is generally known as the 'quantity lever,' as it regulates the quantity of mixture reaching the combustion chamber, and consequently the force of the explosion.
v is a safety chamber of wire gauze to prevent the flame from the combustion chamber reaching the carburetter.
The construction of the twin tap will be seen from fig. 8. It consists of two concentric cylinders; the outer cylinder is in one piece, but the inner one is separated into two short cylinders, r and r', which are manipulated inside the outer one by the levers o and o'.
A is the upper part of the carburetter, corresponding to c in fig. 7. b, is the aperture through which the crude vapour ascends into the twin tap, passing through the wire gauze c en route. d is an aperture on top of the twin tap, through which the ordinary air enters. By the suction of the engine this crude vapour and air are drawn into r', and, mixing as they go, enter r, and so into the pipe e, and thence are drawn into the combustion chamber of the engine. As already mentioned, the lever g' operates the cylinder r', through which circular holes are cut, to admit the crude vapour and air. When the lever is in a certain position nothing but crude vapour is admitted to r'. By operating it, however, the air inlet d is gradually opened, and at the same time the vapour inlet b is gradually shut, thus regulating the quality of the mixture, or in other words the relative proportions of air and crude vapour. Similarly the lever g, operating the cylinder r, either opens or closes the aperture f, thus regulating the quantity of mixture which passes from the mixing chamber to the combustion chamber.
Elsewhere in this volume the importance of having the mixture correct both in quality and quantity will be duly dealt with.
The sectional view of that portion of the tap which governs the quality of the mixture, shown in figs. 9 and 10, will perhaps more clearly explain the way it works. In fig. 9 it will be seen that the crude gas aperture b is almost wholly closed, while the air aperture d is almost fully open. In fig. 10 b is half open and d half shut. It can be seen how the tap can be operated so that b will be wholly closed and d wholly open, or vice versa, c is the gauze through which the crude gas and air pass.
(2) The Benz Carburetter.—The second type combines features of the first and third, inasmuch as the principle of carburation is the same as the first, i.e. surface type, and the method of supplying petrol to the carburetter is on the same principle as the third, float feed. The chief examples of this type are to be found in the 'Benz' cars. The apparatus consists of a cylindrical vessel having a small compartment in the bottom through which a portion of the exhaust gases passes. The heat so derived assists in vaporising the petrol. A tube reaching to a point just above the level of the spirit admits the
_0149a.jpg) Fig. 9 |
_0149b.jpg) Fig. 10 |
necessary air, which may be regulated by a cap covering the holes through which the air is drawn. Another pipe projecting into the vessel conveys the carburetted air to the combustion chamber of the cylinder, into which it is drawn by the suction of the piston. The petrol supply pipe enters the side of the carburetter, and is bent downwards. On to the inside of the carburetter a lever is hinged, carrying a cap which closes the inlet pipe, when the opposite end of the lever is raised by a float contained within the carburetter. When the level of the petrol sinks, the float sinks with it, releasing the pressure from the end of the lever, and so admitting a fresh supply of petrol.
(3) The Spray Carburetter.—The carburetter fitted to the Daimler two-cylinder engine is a good representative of this class. It is depicted in fig. II. The general principle is as follows: The petrol enters the float chamber e through a pipe g. It is then drawn by the suction of the engine along a circular passage, and through the jet h, and impinges against the sloping sides of the carburetter. At the same time air is drawn though the air cylinder d and into the jet chamber through
_0150.jpg)
Fig. II.—The Daimler Carburetter
the aperture i. Here, rushing upward, it mixes with the atomised petrol, and the two are thoroughly mingled in the carburetter, and are thence drawn along the passages l and m to the induction valves, which in turn give admission to the combustion chamber.
The supply of petrol is governed as follows: As it ascends through pipe g into the float chamber e it raises the float c until the petrol has reached a point in the float chamber almost as high as the top of the jet. At this stage the upward movement of the float c actuates two arms b, b, which are attached to the float spindle a, and which depresses this float spindle until the conical end of it blocks up the aperture through which the petrol ascends. No more petrol can thus reach the float chamber until, by the depression of the float, this valve spindle is allowed to rise. By this means the petrol is always kept at a constant level in the float chamber. At k there is a tap fitted by means of which a further supply of air can be admitted to the carburetter to suit the condition of the atmosphere or the varying demands of tube or electric ignition. By screwing f home into the socket p the supply of petrol is wholly stopped. There are many varieties of the spray carburetter, such, for example, as the well-known Longuemare and the carburetter used on the De Dion car (as distinguished from the De Dion cycle), but in all the general principle is the same. In most of them a hand-worked lever regulates the quality of the mixture.
Two systems of feeding the spray carburetter are used, known respectively as pressure and gravity. In the case of the first named, the petrol tank is situated in the body of the frame at a lower level than the carburetter. Air is pumped into this tank, the pressure afterwards being kept up by the exhaust gases. The pressure of this air on the surface of the petrol forces it upward through a pipe into the carburetter, and where tube ignition is used, into the burners.
With the gravity-fed carburetter the tank is fitted in the body of the car, either under the front seat, between the front and back seats, or under the bonnet at a higher level than the carburetter, and the petrol finds its way into the carburetter by force of gravity.
Both systems have their adherents, but gravity is rapidly ousting pressure. The former certainly can be claimed to be the most simple and effective, but the latter is perhaps the safest, because any leakage of petrol can immediately be stopped by turning off the pressure cock.
SILENCER
The exhaust pipe from the engine which conducts off the exhaust gases after they have done their work in the cylinder is connected to a peculiarly constructed chamber, called a Silencer, attached to the frame of the car. The object of the silencer is to deaden the noise of the escaping gases by:
1. Breaking up the body of gas into a number of fine streams.
2. Allowing the gases to expand and cool.
3. Checking the velocity without putting back pressure on the engine.
4. Reducing the pressure of the gases till they are as nearly as possible the same as the atmosphere. To do this, the chamber is divided up into a series of compartments, and the gases in their passage from one to the other have to pass through baffle plates drilled with a number of fine holes, the combined area of which must be considerably in excess of the area of the exhaust pipe, to allow of a free passage for the expanding gases. The flow is thus broken up and subdivided into a number of fine streams of cool gas at or near atmospheric pressure, which cause little or no noise on their escape into the air. It is the sudden expansion of the gases at a high pressure which causes the noise.
Figs. 12 and 13 depict two types of silencer which are very largely used. Fig. 12 shows a sectional view of a silencer composed of three concentric cylinders, a, b, and c. a is composed of a tube or inverted cylinder of sheet steel; b is the second tube similarly constructed; while c is an extension of the exhaust pipe from the engine. Two chambers, d and e, are thus formed. The exhaust gases from the engine enter c, and passing through a number of holes at the end of the pipe at f, expand in the chamber d. Passing from the chamber d through the holes at g, the gases enter the chamber e, where a further expansion takes place. Finally the exhaust is ejected to the atmosphere through the holes at h. The construction of the silencer can easily be followed from the illustration.
Fig. 13 depicts the second type of silencer, which almost explains itself, i is a cylindrical steel body fixed to the end
_0153a.jpg)
Fig. 12.—Silencer
plates g and h. This body contains the baffle plates a, b, c, d, e, and f. The exhaust gases are seen entering the silencer through the exhaust pipe. The direction which they take through the baffle plates is shown by the arrows, and it
_0153b.jpg)
Fig. 13.—Silencer
will be seen that the pressure is reduced in each succeeding compartment in the cylinder. The bolt j, passing through the centre, serves to hold the silencer together and to resist the pressure of the gases.
SYSTEMS OF GOVERNING
To secure the greatest efficiency, durability, and power of the engine, it is necessary, as can be easily understood, to be able to control the speed—in other words, to regulate the number of revolutions of the fly-wheel per minute—and there are various devices for accomplishing this object. The range of speed of different engines varies very considerably. With the small single-cylinder engines it is necessary that the speed should be very great in order to secure sufficient power, and also to reduce vibration, while with the two- and four-cylinder engines this is not necessary, and consequently they are much more durable. As a rule the speed varies from about 750 revolutions per minute for the latter class to 2,000 for the small engines fitted to motor-cycles.
There are various successful methods of governing, which we shall now proceed to enumerate and describe.
(1) Timing of ignition.
(2) Exhaust-valve lifter.
(3) Exhaust-valve closer.
(4) Regulating lift of induction valve.
(5) Mechanically governing exhaust valves.
(6) Throttle.
(7) Governing both exhaust and throttle.
(8) Governing by variable induction and throttle valve.
(1) By Advancing or Retarding the Sparking.—This is the method adopted for single-cylinder engines, and consists in altering the time at which the spark occurs in the combustion chamber by the manipulation of a small lever. It will be easily understood that if the full force of the explosion occurs in the combustion chamber at the moment when the piston is at the highest point, that its effect will be greatest. This is due to the fact that the compression is then at its maximum, and the force of the expanding gas has a longer time to act on the piston. The compression and duration of pressure on the piston can be reduced by altering the timing of the spark, so that it occurs after the piston has begun to descend. This system is the simplest, but its effective use depends very largely on the skill and experience of the driver. Where this system is adopted, a throttle is also used, which enables the operator to regulate the quantity of mixture, and thus alter the power of the explosion. For example, when the maximum power is required, the throttle is open to the fullest, and the sparking advanced to the utmost that the engine will take.
(2) Exhaust-Valve Lifter.—This operates by preventing the exhaust valve from closing after the exhaust gases have escaped. When the exhaust valve is held fully up, no explosive charge is taken into the cylinder. If, however, the exhaust valve is held up to a very slight degree, a reduced charge will be admitted, and the force of the explosion consequently minimised.
In the case of cycles, an exhaust valve lifter is frequently fitted, in order to enable the rider to effect an easy start.
(3) Exhaust-Valve Closer.—This system is adopted on the De Dion voiturette, and consists in regulating the lift of the exhaust valve, but does not prevent the valve from closing in the usual course. It is also used in conjunction with the sparking advance. When the maximum power is required, the exhaust valve is not interfered with, and opens to its fullest compass. When less power is required, the exhaust valve is prevented from opening to that extent. Consequently the exhaust gases are not fully expelled, and as they partly occupy the space in the combustion chamber, a full charge of mixture cannot be admitted through the induction valve, and the explosion is weakened.
(4) By Regulating the Lift of the Induction Valve.—By operating a lever, the induction valve is prevented from opening to its fullest extent, and consequently the largest possible charge is not admitted to the combustion chamber.
(5) By mechanically Governing the Exhaust Valves.—This is effected by preventing the cam from raising the exhaust valve after an explosion has taken place. This system is only adopted on two- and four-cylinder engines, and is the one in most general use. The Daimler form is perhaps the most typical, and consequently we will proceed to describe it by means of diagrams. Briefly put, it consists in temporarily preventing one or more exhaust valves from opening, and consequently for the time being a fresh charge cannot be admitted into the combustion chamber, as the latter is charged to its fullest capacity with the exhaust gases. As already explained, the ordinary exhaust valve is opened by means of a plunger actuated by a cam on the two-to-one shaft (see fig. 6).
Figs. 14 and 15 show a side view of the mechanism which operates the Daimler exhaust valve. Taking fig. 14 first:—k corresponds to the stem of the exhaust valve marked f in fig. 6, while the shoulder f is the plunger, which, acting upwards, pushes the valve open and so allows the exhaust gases to escape. r is a roller free to revolve on the spindle 2. b is the two-to-one shaft marked d in fig. 6, and l is the cam on this shaft corresponding to cam e in fig. 6. It is shown in dotted lines because it is not visible when looked at from this aspect. It will be seen, however, on shaft b in fig. 15.
Now, as shown in the illustration, the exhaust valve is closed, but when the cam l revolves another half-turn, the projection on it bears against roller r, and so pushes upwards the arm d, which is hinged on the spindle i. Needless to say, the shoulder f is thereby raised, and in turn pushes upwards the lifting rod or digger k, thus opening the exhaust valve.
We will now turn to fig. 15. This shows how the various systems of levers in fig. 14 operate so as to prevent one or more of the exhaust valves from opening, and so make the engine cut out on one or more cylinders by preventing the ingress of a fresh charge of mixture to the combustion chamber. It will be seen that the eccentric portion of the cam i, is now uppermost, and is bearing against the roller r, so as to elevate the arm d with its shoulder f. In fig. 14 the end of the spindle k, which is commonly known as the digger, rests on shoulder f, but in fig. 15 it will be observed that k has been pushed outward by the arm j, so that the point of the digger misses f. Consequently, instead of the valve being raised, it remains shut.
We shall now explain how this is accomplished. The arm h, commonly known as the hammer, is the direct medium through which this change is effected. It will be seen in fig. 14
_0157a.jpg) Fig. 14 |
_0157b.jpg) Fig. 15 |
that it rests on a perfectly circular collar, e, whereas in fig. 15 it has mounted on to a cam, g, which is fixed eccentrically on to the shaft b. This cam g is so mounted on shaft b, that when the protruding portion of cam l is pushing up the roller r, the hammer h rests on cam g, at the maximum distance from the centre of shaft b. As a result, the hammer h is slightly depressed. Now h forms portion of the bracket c, and, consequently, when i is depressed it moves the bracket c slightly forward from the perpendicular, as seen in fig. 15. This, in turn, pushes forward the spindle k, so that the digger at the end of it misses the shoulder f, as shown in the diagram. A careful study of figs. 14 and 15 side by side, and the angles of the various arms and brackets, will show distinctly how their position is altered to accomplish the end in view. Fig. 17 shows a perspective view of the mechanism which will further help the reader.
_0158.jpg)
Fig. 16
Fig. 16 will assist the reader to understand the way in which the hammer climbs from cam to cam. It represents the end view of the two cams and the collar, with the hammer resting on the largest one, g. It will be noticed that their circumferences coincide at one point, thus enabling H to slip from one to the other with facility. k, the circular collar, is mounted truly on shaft b, whereas f and g are mounted upon it eccentrically.
We have now to describe the mechanism that operates the hammer h, and to show more clearly the cams on which it acts.
The mechanism which operates the hammer h is commonly known as the governor, and is clearly shown in fig. 17. This mechanism is attached to shaft b. In figs. 14 and 15 the end of this shaft is shown, whereas in fig. 17 a perspective view is given.
The principle of the governor will be more easily grasped
_0159.jpg)
Fig. 17
by making the following simple experiment. Attach a weight to a length of string, and, holding the end of the string, revolve the hand slowly in a circle. The centrifugal force will cause the weight to fly outwards, describing a circle, and if the hand is revolved rapidly, the weight will fly still further out until the string forms a right angle with the perpendicular. Now a in fig. 17 occupies the position of the hand. It is a gear wheel attached to shaft b (the two-to-one shaft) and is operated by a smaller gear wheel on the engine shaft, j, j are two weights pivoted at one of the extremities of the gear wheel, and which of course are free to fly outwards when the gear is rapidly revolving.
Now we would ask our readers for a moment to look at the diagram irrespective of the dotted lines, as this represents the position of affairs when the gear wheel a is at rest. It will be observed that there are two arms k, k, which carry the weights and are pivoted at i. c c is a sleeve which is free to slide upon the shaft b, and on this sleeve c c are two cams, f, g, and a collar e, the latter being a true circle, and the other two eccentrics. h represents the end of the hammer as shown in figs. 14, 15, and 16. Now, so long as h rests upon the circular collar e, the exhaust valves open in regular sequence (see fig. 14), and neither cylinder is cut out.
We shall proceed to describe how the hammer h is influenced to slip on to the eccentric cams f and g, and so make one or both cylinders cut out.
We would now call special attention to the dotted lines in the diagram. The weights j, j are connected by springs regulated to the proper tension, but which, for clearness sake, are not shown in the diagram. When the speed of spur wheel a reaches the maximum pace at which the engine has been set, the weights j, j fly out until they assume the position j' j' as shown in the diagram. The ends of the arms k, k are thus moved forward until they assume the positions shown in k', k', and in moving push against d, and consequently move the entire sleeve c c forward until d assumes the position d', and the collar and cams e, f, and g, positions e', f', and g'. The hammer h, not being free to move laterally, climbs in succession from collar e to cam k, and from cam f to cam g, until it occupies the position h', shown in the dotted lines. This position is also illustrated in fig. 16, but of course seen from a different aspect.
When the hammer h rests in this position on g, the arm j, fig. 15, is pushed so far forward that the diggers of the two cylinders both miss the shoulders f, f, and both exhaust valves remain closed, the speed of the engine immediately begins to slacken, and weights j', j', fig. 17, begin to return to their position of rest at j. Thereupon the hammer h' slips back on to the cam f', and allows one exhaust valve to open, and one cylinder to come into action. If this is sufficient to run the car and engine at their maximum speed, as for example when descending a slight incline, the hammer remains there and only one cylinder works. If it is not sufficient, the weights j', j' still further approximate to j, and the hammer slips back to the original position on the circular collar e, whereupon both exhaust valves open in turn, and both cylinders work at their full capacity.
l, shown further along shaft b, is the cam similarly lettered in figs. 14 and 15, which bears against roller r, and lifts arm d, which in turn raises the exhaust valve.
In figs. 14 and 15 we have only been able to show the way the cut-out motion works in connection with one cylinder. Fig. 18 depicts both, the lettering remaining the same.
As shown in this figure, h rests on the circular collar, and consequently neither cylinder will cut out. When h shifts on to the next cam, however, it is slightly depressed, as already explained, and thereby the arm j is pushed forward, and the digger at the end of k misses the shoulder f, whereupon the inner cylinder cuts out.
Seeing that the bracket c is in one piece, and is actuated by h, our readers will naturally wonder how it is that the further cylinder does not cut out at the same time. If they will examine the arm j1 in the second cylinder, they will observe that it is divided into two parts, and that the outer part, j2, telescopes into the inner. Consequently k1 cannot be pushed forward until the two shoulders of j1 and j2 come together. Now, when h mounts the third and largest cam, it is depressed to such an extent that the bracket c is moved forward until these two shoulders come in contact, and consequently both cylinders then cut out.
_0162.jpg)
Fig. 18
There is one other bit of mechanism in connection with the governor which to prevent confusion we do not illustrate, and that is the accelerator. It consists of a lever which presses against the sleeve c c, fig. 17, and so opposes the action of the arm k towards pushing the sleeve c c outwards. Now, the end of this lever is attached by means of a short coil spring, and a chain, to a foot or hand applied lever in the body of the car, by operating which the driver can increase or reduce the pressure on c c, and can consequently regulate the speed of the engine to any degree within the maximum and minimum limits, and so alter the speed of the car without changing the gears. For example, when travelling on a level road at top speed, if the driver wishes to slow down to pass a vehicle, he does not necessarily change his gear, but operates the accelerator, so as to alter the speed of the engine. The car will then slow until the pressure on sleeve c c is once more increased by means of the lever. Of course, in climbing steep hills, the engine, as a rule, will require its maximum power, and the accelerator must be operated to bring the greatest pressure possible on c c.
(6) Governing by Throttle.—This system has come into considerable vogue recently, and is now used on Panhard, Napier, Daimler, and other cars. It consists in controlling the speed of the engine by regulating the quantity of mixture admitted to the combustion chamber. This is not done by means of a hand lever, as described in (1) and (2), but is automatically worked by a governor, similar to that described in fig. 17, but simpler. The system being the same, it is hardly necessary to describe it at length.
The action of the governor arms operates a lever which in turn works a throttle valve situated in the supply pipe, close to the carburetter. This valve takes the form of a hollow plunger in which there are perforations, corresponding to similar apertures in the plunger slide. When the engine is working at its fullest power at maximum speed these apertures correspond, and consequently the largest amount of mixture passes to the combustion chamber. When, however, the work lightens and the engine begins to race, the governor comes into operation, and the holes in the plunger and plunger slide eclipse each other to a greater or lesser degree, according as a greater or lesser quantity of explosive mixture is required.
The system is more economical of petrol, and gives smoother running than governing the exhaust valves.
(7) Governing both by Exhaust and Throttle.—This system is a combination of (5) and (6). The exhaust valves are governed as already described, but in addition a hand-worked throttle valve is fitted, which regulates the quantity of mixture reaching the combustion chamber. The valve generally takes the form of a plug which turns inside the supply pipe and cuts off the supply according as more or less is required. By means of this throttle petrol can be economised, noise reduced, and the smoothness of running increased.
MOTORS WITH MORE THAN ONE CYLINDER
Engines with two cylinders have the advantage of two impulses for every two revolutions of the fly-wheel, whereas a single-cylindered engine only has one impulse for every two revolutions of the fly-wheel. The timing of the firing stroke depends upon the angle at which the cranks on the crankshaft are set. This angle may be either 180 or 360 degrees. In the first, where the cranks are set at 180, though there are two firing strokes for every two revolutions of the fly-wheel, the two cylinders fire immediately after each other, so that during one revolution there are two impulses and in the next no impulse. If the cranks are set at 360, an impulse occurs every revolution, but an engine with cranks set at this angle must be balanced to counteract the extra vibration due to all the parts moving in the same direction at the same time. The diagrams will show the movements of two double-cylindered engines, with cranks set at 180 and 360.
A four-cylindered engine gives four impulses for every two revolutions of the fly-wheel, and runs even more smoothly than the tvvo-cylindered one.
In fig. 19 the sequence of events in the two cylinders, whose pistons are connected to cranks set at an angle of 180 degrees, is shown. In the diagram the circle represents the path of a crank-pin, and the piston is shown in its relative position. When at the top it is about to begin a downward movement, and an opposite movement when shown, at the bottom. It will be noticed that when the piston of the
_0165.jpg)
Fig. 19
first cylinder is on its first downward stroke (1) the piston of the second cylinder is on its upward stroke. Taking the sequence of the first cylinder from a to b (1) a charge is drawn in; from b to c (2) it is compressed in the combustion chamber; from c to d (3) the charge is ignited and a working stroke obtained; while from d to e (4) the burnt gases are expelled. In the second cylinder it will be noticed that the sequence is directly opposite to that in the first, and in order to make this clear without particular reference to the diagram, the cycle is laid out as follows:
| First Cylinder | Second Cylinder |
| Suction | Exhaust |
| Compression | Suction |
| Firing | Compression |
| Exhaust | Firing |
From the above it will be seen that one working stroke immediately follows the other, so that an even impulse is
_0166.jpg)
Fig. 20
obtained once in every two revolutions of the crank shaft, the other revolution being non-effective.
The second diagram, fig. 20, explains the cycle of events in the cylinder when the cranks are set at an angle of 360 degrees. It will be noticed that the two pistons i, i, are both about to commence a downward stroke, and that they consequently work together instead of opposite to each other, as in the previous diagram. After the explanation of the first diagram it is unnecessary to go into the details of this figure, as the lettering and numbering correspond. The sequence in this case is:
| First Cylinder | Second Cylinder |
| Suction | Firing |
| Compression | Exhaust |
| Firing | Suction |
| Exhaust | Compression |
In this case half a revolution occurs between the two working strokes, or, as previously mentioned, a working stroke is obtained every revolution.
WATER CIRCULATION
In the large motors, water is utilised to cool the engine, and it is essential that this water should be made to circulate so that the boiling water in the jacket will be displaced by cooler water, and the former cooled in radiators, before it is again used. There are two systems, viz. natural circulation and forced circulation, which we shall now proceed to describe.
In natural circulation the fact that cold water is heavier than hot water is availed of. A head of water is obtained by fitting a tank above the level of the water-jacketed cylinder, and as the water in the jacket is heated by the explosions, the colder water from the tank flows in, forcing the heated water in the tank to take its place, and thus an automatic circulation is set up. The connecting pipes must be so arranged that they offer every facility for the free circulation of the water, the cold leaving through a pipe at the bottom of the tank and entering at the lowest point of the cylinder, while the hot leaves the top of the cylinder and enters the tank at the top. The circulation, though automatic and certain, is slow, and for this reason requires a larger body of water to produce a given cooling effect than is the case with forced circulation. Still, the certainty and simplicity of this method have distinct advantages.
In forced circulation a pump, either rotary or semi-rotary, is used, the direction of the flow being such that the water passes from pump to cylinder, thence to radiator, on to tank, and then through pump again, thus completing its circuit. The water in this way gets the maximum cooling effect from the radiator, and the body of water in the tank is kept cool. On account of the high speed of an oil engine and the comparatively small amount of power required to circulate the water, centrifugal pumps are becoming almost universal. As there are no valves to get out of order, and high speed is obtainable without extra gearing, this type of pump is very suitable. Semi-rotary pumps are also used, but necessitate lower speeds, and consequently extra parts to effect this. In the centrifugal pump the water is kept in motion by a fan-wheel working in an enclosed space; there being only just clearance for the fan, the centrifugal force thus obtained is utilised to project the water into the outlet pipe and up to the highest level of the system. In the semi-rotary pump a lifting force is obtained by means of the see-saw motion of a plunger with two valves working alternately. Another type of high-speed pump consists of two small gear wheels in mesh with one another in an enclosed space, with just sufficient space for their free revolution. The water is lifted and forced upwards by the intermeshing teeth acting in the enclosed as pistons.
RADIATORS
To enable the same water to be used continuously with little loss from evaporation, it must be cooled by some means; or in other words, the heat from the explosion in the cylinder absorbed by the water must be dissipated before that water can be used again, and the more perfect this dissipation is, the less body of water is required to carry on the car. To dissipate heat quickly, it is necessary to provide a large surface for the cooling medium to act on. As the air is the most convenient cooling medium, a continuous coil or battery of piping is placed on the front of the car, where it is most exposed to the current of air produced by the car in travelling. Through this coil the heated water is forced by the pump, the heat being carried off from the surface of the pipes by the air. In order to increase the radiation from the pipes, the area of the surface exposed to the air is increased by fitting flanges of thin metal in intimate connection with the pipes, or by originally forming the pipes with these flanges in the solid. Woven wire-work is also used, soldered to the pipes, with the same object.
THE CRANK CHAMBER
The crank chamber, or base chamber, as it is usually termed, forms the base of the cylinder. Its use for lubricating purposes is very important. About half a pint of oil is kept at the bottom of this chamber, into which the crank dips at every revolution, thereby splashing up oil which lubricates the wristpin, gudgeon-pin, crank bearings, crank-shaft bearings, the sides of the cylinder and the piston-rings. The lubrication of the latter is, of course, assisted by special oilers, which will be referred to later.
THE PISTON
The piston used in motor-cars is generally known as the trunk type. It is composed of an iron casting which is made a good sliding fit in the cylinder; around its upper end three or four square-bottomed grooves are cut, and in these the piston-rings fit. The rings are made of cast iron, and the bore being eccentric to its outer diameter, there is a certain amount of spring in them, and so a gentle pressure is kept against the cylinder, preventing any of the expanding gases passing the piston. The piston is made to balance the crank.
Needless to say, the lubrication of the piston rings is of very vital importance, for on that depends the free working of the piston in cylinder. In single-cylinder engines, they require frequent attention, and paraffin should be squirted down the compression tap at regular intervals. Occasionally, too, the cylinder head should be taken off, and the rings cleaned with a tooth-brush and paraffin. In double-cylinder engines, this constant attention is not required, for in addition to the splash system of lubrication, there are pipes running to the sides of the cylinders, through which oil drops constantly, and so keeps them lubricated. The speed of the engine, too. being so much less, there is not such danger of the oil being used up rapidly. Therefore it is as a rule sufficient to squirt paraffin every few days on to the top of the piston. Failing paraffin petrol is almost as effective.
RELATIVE POSITIONS OF INDUCTION AND EXHAUST VALVES
It was the general practice until a short time ago, a practice which is still in some cases continued, to place the exhaust valve directly under the induction valve. By this arrangement a simpler casting for the cylinder head is possible, as it only necessitates one opening for both valves. This system has now been improved upon by leaving the induction valves in their former position and making a second opening in the head for the exhaust valves. In the Daimler type of engine this alteration had a distinct advantage, for it enabled the exhaust gases to be got rid of at once (instead of, as previously, having to travel from one side to the other of the cylinder head), thus keeping the head much cooler and the induction valves free from the sudden and wide variations of temperature through the exhaust gases passing under them. This second opening in the head is also utilised to take the sparking plugs when electric ignition is used, as in this position they are constantly subjected to the scouring action of the exhaust gases which help to keep them clean and prevent any accumulation of carbonised oil to act as a short circuit.
APPLIANCES FOR STARTING THE MOTOR
The almost universal method of starting the motor is by means of a handle whereby the piston is operated and the charge drawn into the combustion chamber. In the case of cars with two or more cylinders various self-starters have also been introduced, which, on touching a button, explode the charge which remains in one or other of the combustion chambers, and so start the engine. These appliances, however, are only effective for a few hours after the engine has been running, as the charge escapes gradually.
VARIOUS TYPES OF ENGINES
There are various types of petrol engines on the market, but the main principles remain the same in all. The vertical engine is the most popular; then comes the horizontal, and in other cases engines worked at varying angles. Once, however, the motorist has thoroughly grasped the principle of the petrol engine, there is little difficulty in understanding these varieties. The same series of operations take place in the small single-cylinder engine of the motor-bicycle as in each of the four cylinders of the 60 h.-p. racing car.