THE PETROL ENGINE

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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

London
E. & F. N. SPON, Limited, 57 HAYMARKET

New York
SPON & CHAMBERLAIN, 123 LIBERTY STREET

1915

CONTENTS

LIST OF ILLUSTRATIONS

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

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.

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.

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.

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.

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:—

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, A₁, A₂, A₃, A₄, 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 L₁ is the inlet water pipe leading from the radiator (not shown) to the water pump, and L₂ is the delivery pipe from the pump to the respective cylinder jackets, L₃ 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].

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.

Fig. 8.—Sectional Drawing of a T-headed Water-Jacketed Cylinder. Valves on opposite sides of Cylinder.

Fig. 9.—Outside View of a Water-Jacketed Cylinder.

Figs. [8]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; V₁, V₂ are the valve seats; G₁, G₂ are the valve stem guides; H₁, H₂ are caps which may be removed when the valves are being put in or taken out; f₁, f₂, f₃, f₄, f₅ are called flanges. The flange f₁ 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 f₁. The flanges, f₂, f₃ are for the inlet and outlet water pipe attachments, and f₄, f₅ 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.

Stud
Fig. 10.

Bolt
Fig. 11.

Setscrew
Fig. 12.

Fig. 13.—Motor-Cycle Engine with
an L-headed Air-Cooled Cylinder.
Valves both on same side of Cylinder.

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.

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 S₁, S₂. 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.

Fig. 16.—Method of fixing the Gudgeon
Pin which allows for Adjustment
after Wear.

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.

Fig. 17.—Three forms of Piston Head.

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.

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.

Fig. 20.—Crankpin and Crankwebs.

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.

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 B₁, B₂ and the crankshaft proper at C. The portions of the crankshaft C which work in the bearings D₁, D₂ 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.

Fig. 23.—Sketch of a Balanced Crank.

Fig. 24.—Sketch showing
the unbalanced portion of
the Crank by Shaded Lines.

Fig. 25.—Two-throw Crankshaft balanced by
drilling Holes in Flywheel and Fan Pulley.

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.

Fig. 27.—A Flywheel
built up from Steel Forgings.

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.

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.

Fig. 30.—Sectional Drawing of the Cylinder of a Sleeve Valve Engine, with Inlet Ports uncovered.

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 S₁, S₂, 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 S₂ 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 S₂ 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.

Fig. 32.—Sketch showing Slot
in a Poppet Valve Head for
Grinding-in purposes.

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.

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.

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.

Fig. 36. and 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.

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].

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.

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.

Fig. 41.—Petrol Jet,
specially arranged for
Atomising the Petrol.

Fig. 42.—Compensated Petrol Jet.
A is the Main Jet and B the
Compensating Jet supplied
hrough 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.

Fig. 43.—Automatic Spring
controlled Extra-air Valve.

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.

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 B₁, B₂ 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 C₁, C₂ 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.

Fig. 45.—Sectional Drawing
of a Sparking Plug.

Fig. 46.—A Sparking Plug.

Fig. 47.—Outside View of a High Tension Magneto.

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 F₁, F₂ 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 T₁, T₂; 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.

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 M₁, M₂, M₃, M₄, 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 L₁, L₂, L₃, L₄. 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 T₁, T₂ 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.

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.

Fig. 51.—Ignition Coil Case.

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 G₁, G₂. 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.

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.

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, F₂ 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.

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 A₂ and A₃ represent two of the main engine bearings which support the crankshaft; C₁, C₂, C₃ are three of the crankpins; F₁, F₂, F₃ are oil troughs placed under the crankpins; D₂, D₃ 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 D₂, D₃, etc., and the other feeds oil troughs by means of branches such as G₂. 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.

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.

Fig. 57.—Forced Lubrication System.

Fig. 58. and 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.

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.