MOTOR-CAR PRINCIPLES

MOTOR-CAR
PRINCIPLES

THE GASOLINE AUTOMOBILE

BY

ROGER B. WHITMAN

ILLUSTRATED

NEW AND ENLARGED EDITION

D. APPLETON AND COMPANY
NEW YORK, LONDON  MCMX

Copyright, 1907, 1909, by

D. APPLETON AND COMPANY

INTRODUCTION

The development of the gasoline automobile at home and abroad has produced a great variety of designs, good, bad, and indifferent, but the advancement of the industry has weeded out the unsatisfactory and improved the good until with few exceptions the leading makes show a striking similarity in all but details. The advantages of certain forms of construction have been recognized, and their adoption by the large majority of makers has produced what may be called a standard type.

The object of this book is to explain the principles that underlie automobile construction and operation, and to illustrate the movements and mechanical combinations adopted in present-day practice. It is not the intention to explain the exact details of construction of the different cars, and the illustrations have been prepared with the sole object of making the principles clear, for with an understanding of these there should be no difficulty in comprehending any particular application of them.

The lubrication table on pages 244 and 245, which was prepared by Mr. T. D. Hanauer, is reproduced through the courtesy of the Scientific American.

The advantages of magneto ignition for internal combustion engines are so obvious that designers and inventors have directed their attention to the perfection of apparatus that will improve present methods. The number of systems proposed for the purpose is very large in comparison with the number in actual and practical use, and as in a work of this size it would be impossible to describe the many methods for the application of the magneto that are on the market, attention has been given only to those that are in actual, everyday use. The absence of a practical treatise on the principles, application, and care of low and high tension magnetos is the reason for the addition of the appendix to this work.

R. B. W.

CONTENTS

Lubrication Table [244-245]

Testing Chart Between [326] and [327]

Index [327-339]

LIST OF ILLUSTRATIONS

ILLUSTRATIONS IN APPENDIX

MOTOR-CAR PRINCIPLES

CHAPTER I
GASOLINE ENGINE PRINCIPLES

The action of a steam, gasoline, or hot-air engine depends on the principle that when air or other gas is heated it expands, and that if it is confined in a space that will not permit it to expand, in striving to do so it creates pressure against all parts of the chamber in which it is contained. The more a gas is heated, the more it will expand if it is free to do so, and if not free, the greater will be the pressure that it will exert in striving to expand. Pressure may thus be generated by heat, and following along similar lines, heat may be produced by pressure, for when the pressure of a gas is increased by compressing it, or forcing it to occupy a smaller space, the gas will become heated. The reverse is also true, that when a gas is cooled, its volume is reduced, which reduces the pressure that it exerts; similarly, reducing the pressure by permitting the gas to expand reduces its temperature.

To state these principles in another form, to create pressure in a gas it must either be heated or compressed into a smaller space, and to reduce its pressure it must either be cooled or permitted to expand.

The action of a locomotive, the most familiar type of steam engine, is no mystery, and the production of steam in the boiler, its passage to the cylinder, and the application of its steady pressure against first one side of the piston and then the other, resulting in the turning of the driving wheels, are well understood. Water being converted into steam in the boiler, pressure is created because of the tendency of the steam to expand, but the only place in which it may expand is the cylinder, where in so doing it moves the piston.

Fig. 1.—Engine Actions.

A gasoline engine is similar to a steam engine in that its piston is moved by the pressure exerted by a heated and expanding gas; it is different in that the pressure is produced inside of the cylinder by the combustion of an inflammable mixture of gasoline vapor, instead of being generated in a boiler away from the cylinder. The heat of the combustion creates great pressure, and as the piston is the only part that can give before it, it is moved from one end of the cylinder to the other, this motion being utilized in the turning of the crank shaft. The combustion, which is so rapid that the generally accepted term for it is explosion, can occur only after the mixture has been drawn into the cylinder, and so prepared that it ignites quickly and burns completely, with the object of obtaining the greatest possible heat from it in the shortest possible time. In order that one explosion may be followed by another, the burned and useless products of combustion must be expelled to make place for a fresh charge of the inflammable mixture.

These successive events, forming a cycle, must be performed as long as the engine runs, and the constantly changing pressure in the cylinder due to the movement of the piston allows a fresh charge to enter, prepares it, and expels the products of combustion after the pressure that they have exerted has been utilized.

While in the great majority of steam engines the steam acts first on one side of the piston and then on the other, in an automobile gasoline engine the pressure is exerted on only one side, the combustion of the mixture taking place between the piston and the closed end, or head, of the cylinder. The other end of the cylinder is open, and the piston slides between the ends, its movement from one end to the other, called a stroke, corresponding to a half revolution of the crank shaft.

Gasoline engines are divided into two classes, according to the number of strokes of the piston that are necessary to accomplish the cycle; in the most usual type, four strokes are necessary, the class being called the four-stroke-cycle, or four-cycle, in distinction to the two-stroke-cycle, or two-cycle, in which but two strokes are necessary.

Of the five events that compose the cycle, three (the inlet, during which the fresh mixture enters the cylinder, its compression or preparation, and the exhaust of the burned gases) are performed by the piston; during the power event the piston is moved by the pressure resulting from the combustion, while the combustion event is due to an outside source. In the four-cycle type of engine, which is in almost universal use for automobiles, the events are considered with reference to the movement made by the piston during which they are performed, and may be called the inlet, compression-combustion, power, and exhaust strokes. In order that the engine may continue to run, it is obvious that the events must be performed in the correct order, and that the failure of one will affect all the others.

During the inlet stroke, a charge of fresh mixture enters the cylinder as the piston makes an outward stroke from the closed toward the open end. When the piston makes the following inward stroke, the mixture is compressed and combustion occurs, the pressure from which drives the piston outward on the power stroke. This is followed by another inward stroke, which pushes the burned gases out of the cylinder. It will be seen that power is developed during only one stroke of the four, the other three being required in the preparation for the following power stroke. The movement of the piston over these three dead strokes is secured by attaching to the crank shaft a heavy fly wheel, the momentum of which, acquired during the power stroke, keeps the crank shaft revolving and the piston in motion while the events are performed.

Fig. 2.—Gasoline Engine Cycle.

The space between the piston and cylinder head in which the combustion occurs is called the combustion space, and the inlet and exhaust valves open into it, the first being that by which the fresh mixture enters, and the second that by which the products of combustion escape. The device for igniting the mixture projects into the combustion space, and the means of ignition in universal use for automobile engines is an electric spark.

INLET STROKE

During the stroke (Fig. 2), the piston is moved outward by the crank shaft, which is revolved either by hand or by the momentum of the fly wheel. This movement increases the size of the combustion space, thereby reducing the pressure in it, and the higher pressure of the atmosphere outside of the cylinder will force fresh mixture into the combustion space, the inlet valve being open to admit it. If the piston moves slowly, the mixture will be able to enter fast enough to keep the pressure in the combustion space equal to that outside, but at the high speed at which a gasoline engine is run the piston will reach the end of its stroke before a complete charge has had time to enter, so that the pressure in the combustion space will still be below that of the atmosphere. If the inlet valve closed at this point so that no more mixture could enter, the combustion of the partial charge would result in a lower pressure than would be possible with a full charge; the inlet valve should therefore remain open until the piston reaches the point of its next inward stroke at which the pressure in the cylinder equals that outside.

COMPRESSION-COMBUSTION STROKE

The compression and the combustion of the charge occur during the next inward stroke of the piston.

The period between the bringing together of the liquid gasoline and air and its admission to the cylinder is too brief to secure perfect combination, and the mixture that results is not satisfactory. A portion of the air will not have been able to come into contact with the gasoline, and much of the liquid will not have been vaporized; what passes into the cylinder consists of pure air, liquid gasoline, and a more or less perfect mixture of the two. The combustion of this would be slow and incomplete, resulting in loss of power and waste of fuel. In order to render the mixture more perfect, advantage is taken of the heat that is produced by compression; the inward stroke of the piston raises the temperature of the mixture by compressing it, the heat rendering the gasoline more volatile, and the compression forcing it into combination with the air. Even this does not result in the formation of a perfect mixture, for the period is too short to effect it. The failure of an engine to deliver full power may often be traced to this condition, for the air and gasoline vapor, instead of being thoroughly combined and mixed, will be in layers, so to speak, and the combustion will be slow and uneven. Future development of the internal combustion engine will no doubt eradicate this, to the increase of efficiency and economy.

The charge of inflammable mixture can produce a certain amount of heat, and the more rapidly and completely this heat is obtained, the greater and more sudden will be the rise in pressure. The pressure will be greater when the mixture is contained in a small space than when in a large, and as the combustion space is smallest when the piston is at its inmost point, the greatest pressure will be obtained if combustion is complete at this point. If the combustion of the mixture were instantaneous, it should be ignited at this point; but even though very rapid, it nevertheless burns slowly enough to make it necessary to ignite it sufficiently before the end of the stroke to have the combustion complete as the piston comes into position to move outward. The instant at which the mixture must be ignited in order to produce this result depends on the speed of the piston, for the interval between the ignition of a good mixture and its complete combustion does not vary to any great extent. When the piston is moving slowly, the mixture may be ignited toward the end of the compression stroke, for there will be sufficient time for complete combustion by the time the stroke is ended; but when moving at high speed, ignition must occur much earlier in the stroke, as otherwise the piston will have completed the compression stroke and begun to move outward on the power stroke before the mixture is entirely burned. The instant at which ignition occurs also depends on the mixture that is used, for its quality and proper combination make a difference in the rapidity with which it burns. The better the quality of the mixture, the faster and more completely it will burn, and ignition may occur later in the stroke than would be possible with a mixture of poor quality. As the mixture is ignited by the passing of an electric spark in the combustion space, the difference in the instant at which it occurs may be secured by permitting the spark to pass earlier or later, and this is under the control of the driver.

When ignition occurs early in the compression stroke, the spark is said to be advanced, in distinction to a retarded spark, which passes when the compression stroke is more nearly complete.

If the spark is advanced too much, combustion will be complete before the piston has reached the end of the compression stroke, and it will be necessary to force it to the end of the stroke against the pressure by the momentum of the fly wheel, in order that it may get into position to move outward on the power stroke. In such a case, the momentum may not be sufficient to overcome the pressure, and the piston will be brought to a stop. A retarded spark results in the combustion of the mixture being completed after the piston has begun to move outward on the power stroke, and the pressure will then be reduced because it is exerted in a larger space, the piston consequently being moved with less force; if the spark is still further retarded, the combustion will not be complete by the time the exhaust begins, and the heat from only a portion of the mixture will be utilized, because it will still be burning as it is forced out of the cylinder.

The position at which the spark occurs is one of the means by which the speed of the engine is controlled, for the low pressure that results from a retarded spark moves the piston at low speed, while the greater pressure from an advanced spark drives the piston outward with more force and higher velocity.

While high compression of the charge improves its quality, and results in combustion being more rapid and complete, it has limits, and if carried too far the heat generated by the compression will be sufficient to ignite the mixture. This would have a bad effect on the operation of the engine, for the pressure would then be produced at the wrong point of the stroke, retarding instead of assisting the revolution of the crank shaft. Modern practice has shown that in engines that are maintained at a proper temperature the best results are obtained by compressing the mixture to from sixty to eighty pounds to the square inch; there are instances in which a higher compression is obtained, but the liability to ignite the mixture prematurely makes it undesirable.

POWER STROKE

The increasing size of the combustion chamber as the piston moves outward on the power stroke permits the gases to expand, and in doing so the temperature will fall, the pressure decreasing in consequence. A further decrease in pressure is caused by the hot gases being in contact with the metal cylinder and piston, which absorb heat. The more slowly the engine runs, the longer the gases will be in contact with the cylinder walls, and the more opportunity there will be for loss of heat from this cause; at higher speeds, there will be less time for heat to be absorbed by the cylinder walls, and more will be utilized in expanding the gases and producing work.

Even at the outmost position of the piston, the combustion space will not be large enough to permit the gases to expand until their pressure has dropped to that of the atmosphere, so that they will still be exerting pressure. By opening the exhaust valve, the gases will have an outlet for expansion, and will begin to rush out. While the pressure might be utilized against the piston to the end of the power stroke, it has been found that better results are obtained by opening the exhaust valve before the piston reaches the end of the power stroke. There is then a higher pressure forcing the gases out than there would be later in the stroke, and the greater quantity of gases that escapes leaves less to be expelled during the exhaust stroke.

EXHAUST STROKE

The inward movement of the piston pushes out of the open exhaust valve the gases that have not escaped through their desire to expand. The exhaust valve remains open for the entire stroke, but when the engine is running at high speed the piston moves so rapidly that the gases cannot escape fast enough to prevent their being slightly compressed. When the piston is at its inmost point, the gases are still flowing through the valve because of this slight compression, and if the valve closed, a portion would be retained. The best results come from the closing of the exhaust valve not at the end of the exhaust stroke, but a short time after the piston has begun to move outward again, during which period the compression forces the gases out. The exhaust valve closes at the point when the slight compression has been reduced to the pressure of the atmosphere by the escape of the gases and the enlargement of the combustion space.

If the piston completely filled the combustion space when at its inmost point, all of the burned gases would be expelled, but the necessity for leaving a space in which combustion may take place renders this impossible, and a small portion of the burned gases therefore remains in the cylinder. The space between the cylinder head and the piston when at its inmost point, called the clearance, should be as small as possible, in order that the amount of these gases remaining in the cylinder may not be sufficient to contaminate the fresh charge and weaken the pressure of its combustion.

The passages through which the burned gases are led away from the cylinder must be large and free from obstructions, for if a free flow is not permitted back pressure will be set up, which will prevent the largest possible amount of gases from escaping, and leave a greater portion to contaminate the fresh charge.

The power that a gasoline engine is capable of developing depends on the size of the cylinder, the pressure acting on the piston, and the speed at which it operates. A steam engine, which obtains its pressure from a boiler, can do work as soon as the steam is turned into the cylinder, but a gasoline engine must be running before it can be called on to deliver power. Because of the cycle of events on which its operation depends, the piston must be forced to perform the inlet and compression strokes before pressure can be developed, and it is necessary to revolve the crank shaft by outside means until a charge of mixture has been taken into the cylinder, compressed, and ignited, when the engine begins to work by the pressure from the combustion, and takes up its cycle. Not until this has been done can it be called on to do work.

A steam engine can be made to deliver more power than it is built for by increasing the pressure acting against its piston, and the full pressure of the boiler can be utilized when extra work is necessary. The power developed by a gasoline engine being greatly dependent on its speed, and there being no reserve by which greater power can be developed in emergencies, it is necessary for an engine of this type to be perfectly adapted to the work that is desired of it. At excessive speeds the piston acquires great momentum, which must be overcome at each end of a stroke by the crank shaft, and while a speed above normal may be attained, it results in the quick destruction of the bearings and the severe straining of the engine. The best results in efficiency and long life accompany the running of the engine at the slowest speed possible for the development of the required power.

CHAPTER II
ENGINE PARTS

The sudden and powerful outward movements of the piston under the pressure from the combustion are transmitted to a crank shaft, which must be of great strength in order to resist the heavy strains under which it operates. It is made of the best steel available for the purpose, and has as many cranks as the engine has cylinders. The cranks are generally made in one piece with the shaft for the sake of strength, and for stiffness there are as many bearings as possible. The number of bearings for the crank shaft of an engine with four or more cylinders depends on the arrangement of the cylinders. If the cylinders are evenly spaced, there will be room for a bearing between each pair of cranks, so that a four-cylinder engine will have five bearings, one at each end, the other three being between the cranks. If the cylinders are in pairs, there will not be room between the cranks of a pair for a bearing, the only space for it being between the pairs; a four-cylinder engine built in this way will thus have but three bearings, one at each end and one in the center. Crank shafts are described by their bearings as three, five, etc., point crank shafts.

Fig. 3.—Gasoline Engine in Section.

Fig. 4.—A, Two-throw crank shaft; B, four-throw crank shaft, 180°.

The relative positions of the cranks of a crank shaft are expressed in degrees of a circle; if, for instance, the cranks project from opposite sides of the shaft so that they are a half revolution apart, it is called a 180-degree crank shaft.

The outer ends of the crank arms, which correspond to the cranks of a bicycle, support the crank pin, which may be likened to the pedal, and to this the large end of the connecting rod is attached, the small end being connected to the piston. The connecting rod must be of great strength, tough but not brittle, and is made of steel or bronze.

Fig. 5.—One-Throw Crank Shaft.

Fig. 6.—Connecting Rod.

The piston is a trifle smaller than the bore of the cylinder, and its length is usually greater than its diameter. It is hollow, with one end closed, the closed end being that against which the pressure is exerted. A wrist pin passes through it, and through the small end of the connecting rod, to enable the latter to swing from side to side in following the turning of the crank shaft. A tight joint is maintained between it and the cylinder walls by cast-iron piston rings, which are of square or rectangular cross-section, split so that they may spring open, and fitted into grooves cut around the piston. They are of such shape that their tendency to expand keeps them pressed against the cylinder walls, but being split, their elasticity prevents their binding; they fit the grooves snugly, and while they may move freely in them, they hold the pressure from escaping. The number of rings varies with the design of the engine, but the most usual arrangement is three to a piston, placed around the upper end.

Fig. 7.—A, Piston; B, piston in section.

Fig. 8.—Piston Rings.

The cylinder should be of the highest grade of cast iron, with the smoothest possible surface for the piston to slide against.

Fig. 9.—Conical Valve Seat.

Fig. 10.—Automatic Inlet Valve in Cage.

The valve openings, or seats, are circular, and are usually made slightly funnel shaped, the disks that cover them being slightly conical to fit. The large end of the funnel is toward the combustion space, so that when the disk is lifted from its seat it moves inward. Valves are held against their seats by coil springs that surround the valve stems, which are rods extending from the center of the disks, and there are two methods by which they are opened. In an automatic valve, the spring that holds the disk against its seat is weak, and the higher pressure outside of the cylinder during the suction stroke forces the disk away from its seat against the pressure of the spring. The valve remains open until the pressure in the combustion space is about equal to that outside, when the spring draws the disk back to its seat, to which it is held as long as the pressure inside is higher than that of the atmosphere. This arrangement is only possible for inlet valves, and is largely used, but exhaust valves, and often inlet valves as well, are mechanically operated; that is, they are opened and held open by a mechanism driven by the crank shaft, in the form of a cam. A cam can best be described as a “wheel with a hump on it,” or, in other words, it is a piece of metal mounted on a shaft, cylindrical in form except for one portion, which projects farther from the shaft than the rest. The cam revolves with the shaft, and the projection, called the nose, will displace anything resting against it. The illustration shows a cam in three positions of its revolution, with the end of a valve stem resting against it—the roller being attached to the stem to reduce the friction. The valve stem is held in guides, so that the only movement it may have is up and down; when the cam revolves, the nose lifts the stem and opens the valve, holds it open as long as the flat end of the cam is under the stem, and when the nose passes from under, the valve is drawn to its seat by the spring.

Fig. 11.—Cam Action.

The moment at which an automatic valve opens is governed partly by the tension of its spring; if it is too strong, greater pressure will be required to open it, and it will close sooner than if the tension is light. Accurate adjustment of this spring is necessary in order that the charge may enter the combustion space without delay, and continue to enter as long as possible. The opening and closing of mechanically operated valves depend on the shape of the cam, and not being affected by the more or less uncertain action of a spring, they are more positive in action.

The cam shaft on which the cam is mounted is driven by the crank shaft, but as the valve opens but once during two revolutions, the cam shaft revolves at half speed, making one revolution while the crank shaft makes two. This is done by means of gears.

If two gears running together, or in mesh, have the same number of teeth, they will make the same number of revolutions, but if one has twice as many teeth as the other, the smaller will revolve twice while the larger revolves once. As the cam shaft must revolve but once while the crank shaft revolves twice, its gear must have twice the number of teeth as the gear on the crank shaft. The cam shaft is also called the secondary, or half-time shaft, and the gears that drive it the two-to-one gears.

In some designs of engines, the nose of the cam bears directly against the valve stem, but it is more usual to place a valve-lifter rod, or push rod, between them, the cam acting on the rod and lifting it, and that in turn lifting the valve stem. When the nose of the cam is not acting on the stem or rod, there must be a small space between them, for if the stem or rod rests firmly against the cam at all times, the valve disk might be prevented from seating firmly. The space is left between the stem and lifter rod, the spring acting only on the stem.

Fig. 12.—Four Arrangements of Valves.

The valves may open into the cylinder in a variety of ways; both may be in one pocket, or one may be in a pocket and the other in the head, or each in a separate pocket, or both in the head. The first two illustrations show automatic inlet valves, and the third and fourth mechanically operated valves; when two mechanically operated valves are in the head, it is necessary to open them by means of rocker arms, for because of their position it would be impossible for the valve-lifter rod to act directly on their stems.

If after the explosion the burned gases were permitted to escape directly into the open air from the cylinder, the effect would be the same as the firing of a gun, and for the same reasons. The pressure in the cylinder being higher than that of the atmosphere, the sudden expansion of the gases would produce a report, and as this would be most undesirable for an automobile, provision is made by which the gases are cooled and permitted to expand gradually, so that when they reach the open air they are at its pressure, or nearly so. This is done in the muffler, or silencer, to which the exhaust pipe conducts the products of combustion. The muffler consists of a series of chambers of different sizes, one inside of the other; the gases pass from the smaller to the larger, expanding as they go, until from the largest they should escape without noise, having lost their heat and pressure.

While the pressure exerted during the power stroke depends on the heat of the gases, and it is necessary to have the engine hot in order that there may be as little loss of heat as possible, the temperature must not be permitted to rise to the point at which the lubricating oil would burn. Lubricating oil for gasoline engines is made to stand high heat, but if heated beyond its limit it will burn, and then, besides the loss of its property of lubrication, a deposit of carbon, hard or gummy, will form, fouling the combustion space or piston rings, and interfering with the operation of the engine. Overheating is prevented either by circulating water through channels surrounding the combustion space, or by directing a blast of air against it.

Fig. 13.—A, Force circulation water-cooling system; B, thermosiphon circulation water-cooling system. (Flow of water indicated by arrows.)

The channels, called water jackets, provided for the circulation of the water, are usually cast with the cylinder, or formed of sheet metal. Cool water enters at the bottom and escapes at the top, absorbing heat during its passage. Of the two systems of keeping the water in circulation, the most usual consists of a rotary pump, which forces the water through the jackets and then to a cooler, or radiator, which is so placed that it is exposed to the air currents set up as the car moves. In order to cool the water, the radiator must have a large surface exposed to the air, and the water must pass through it in small streams. The early types consisted of coils of small copper tubing, on which were strung disks of copper, the water flowing through the tubing, and the disks absorbing its heat and giving it up to the air, but these are being abandoned in favor of cellular or honeycomb radiators. These types, which are usually placed at the extreme front of the car, are made up of a great number of short lengths of small tubing, in any one of several shapes, placed side by side, and held together either by plates or by soldering their ends.

Fig. 14.—A, Centrifugal pump; B, vane pump; C, gear pump and cover.

Fig. 15.—Radiator Constructions. A, Spiral flange (water passes through the tube); B, cellular, and C, honeycomb (air passes through the tubes; water passes between the tubes).

Fig. 16.—Radiator and Fan.

The heated water enters at the top of the case in which the tubes are contained, and flows to the bottom, finding passages between the tubes, while the air passes through, being assisted by a fan driven by the engine.

The second system of keeping the water in circulation follows the principle that heated water tends to rise, its place being taken by the cooler water that tends to sink. This, called the thermosiphon or gravity system, requires all of the parts and connections to be large and completely filled with water. The water in the jacket rises as it absorbs heat from the cylinder walls, and flows out to the radiator, which it enters at the top. Its place in the jacket is taken by the cooled water from the bottom of the radiator, and this circulation continues, being more rapid as the difference in temperature between the heated and cooled water increases. It is naturally not so rapid as circulation that is forced by a pump, and more liable to become inoperative by the clogging of the pipes, jacket, or radiator.

Of the methods of increasing the surface of a cylinder in order to cool it by a blast of air, the most usual is to cast it with flanges that project from all parts of the combustion space. These become heated as the temperature of the cylinder walls increases, and the air that is blown against them carries off the heat. Other methods consist of setting pins or copper strips into the cylinder walls, of such form that the air current strikes a surface composed of points, by which the heat easily passes to the air. Another system consists of surrounding the combustion chamber with a jacket open at the bottom, air being blown into it from the top by a powerful blower.

Other things that are necessary for successful air cooling are large valves by which the hot gases may be quickly discharged when their period of usefulness is ended, and small cylinders rather than large, as the heat from small quantities of gases may be carried off more quickly than from large.

The lubrication of a gasoline engine must be carefully looked after, as on its thoroughness depends the continued delivery of power. The most usual method of lubricating the piston and cylinder walls is to keep the crank case filled with oil to such a point that the end of the connecting rod dips into it in turning. This spatters the oil to all parts of the crank case, and a portion is caught in a groove cut around the lower end of the piston. The inward movement of the piston spreads the oil on the cylinder walls, and it is distributed around the piston rings, so that they move easily in their grooves. As the oil is used up, it is replaced from a lubricator so that a constant level is maintained, and this operates either by gravity, or by a small force pump driven by the engine, or by the maintaining of pressure in the oil tank.

Mechanically operated or pressure lubricators supply oil to all parts of the engine, and as the quantity passed to each bearing is adjustable, a feed may be maintained that is exactly suited to the requirements.

The bearings of an automobile operate under such widely different conditions that one kind of lubricant will not be suitable for all. The maker of the car has tested the different oils, and it is advisable to follow his instructions on the brand and grade most suitable for each bearing, rather than to try experiments. The Lubrication Table on pages 244 and 245 gives the kind and quantity of lubricant most suitable to the work performed by each bearing.

CHAPTER III
ENGINE BALANCE

A drawback to the use of reciprocating engines is that the weight of the piston and connecting rod in sliding first one way and then the other produces great vibration, and that the crank shaft in bringing these parts to a stop at each end of the stroke is subjected to violent shocks that in time wear it loose in its bearings. With internal-combustion engines this vibration, and the shock on the crank shaft, are greatly increased by the intensity with which the pressure is exerted.

Engines with one cylinder may be balanced to some extent by the use of counterweights attached to the crank shaft, and by the use of so heavy a fly wheel that its momentum produces a comparatively steady movement; but a perfect absorption of the vibration would require the engine to be run at a constant speed, which is not possible with those used on automobiles.

In two-cylinder engines the vibration may be reduced by so arranging the parts that the pistons slide in opposite directions, the weight of one being balanced by that of the other. This plan is used in engines of the horizontal double-opposed type, which is considered to be the most satisfactory for low powers. The cylinders are horizontal, with their open ends toward each other, the crank shaft lying between them. The crank shaft is two-throw, 180°; that is, there are two pairs of crank arms, projecting from opposite sides of the shaft, so that they are a half revolution apart.

Two cylinder engines are also built with vertical cylinders, and are of two types, according to the construction of the crank shaft. In one, the crank shaft is 180°, and in the other both pairs of crank arms project from the same side of the shaft so that the crank pins are in line, this being called a 360° crank shaft.

In the 180° type one piston moves up as the other moves down, so that they balance, but it results in the power strokes occurring in both cylinders during one revolution of the crank shaft, with no power during the revolution that follows.

To understand the reason for this, the order in which the events of the cycle occur must be recalled, and it must be remembered that of the four strokes of the piston during which they are performed the two outward strokes are inlet and power, and the two inward strokes compression and exhaust. If the piston of a two-cylinder vertical engine (Fig. 17) is moving downward on the power stroke, piston No. 2 will be ascending, and the only events that can then be performed in its cylinder are compression or exhaust. If performing compression, it will move under power during the next stroke (the other half of the revolution), No. 1 then exhausting (Table No. 1). This brings the two power strokes in one revolution, and during the next revolution there will be no power stroke, for No. 1 will be performing suction and compression, and No. 2 exhaust and suction.

Fig. 17.—Engine Arrangements Showing Order of Firing.

If, as shown in the second table, No. 2 is moving upward on exhaust while No. 1 moves down under power, its previous stroke, the first half of the revolution, will have been the power stroke, and the same condition will exist of two power strokes occurring in the same revolution.

In either case the balance of the moving parts is offset by the irregular production of power, which produces bad results in the setting up of strains in the engine, and the uneven running of the car.

In two-cylinder vertical engines with 360° crank shaft (Fig. 17) the pistons move up and down together, which of course results in bad balance. In this arrangement, however, the applications of power may be evenly spaced, for as the piston in cylinder No. 1 moves downward on the power stroke, piston No. 2 moves in the same direction, and may make either the inlet or power stroke. If it makes the inlet stroke, it must move inward before it can again move outward on the power stroke, and there will be an interval of one stroke between the power strokes of the two cylinders. To have the power strokes in the two cylinders occur together, as suggested as the alternative, would obviously give bad results in the great strain imposed on the crank shaft, the weight of the fly wheel that would be necessary to carry two pistons through three dead strokes, and the jerky running of the car.

The defects of two-cylinder vertical engines with either design of crank shaft outweigh any possible advantage of that construction, and the horizontal double-opposed type, in evenly occurring power strokes, mechanical balance, and simplicity, is in almost universal use for cars of low power.

A two-cylinder engine does not require so heavy a fly wheel as a one-cylinder engine in proportion to the power delivered, because there is a power stroke every revolution instead of in alternate revolutions, and the parts must be carried over only one dead stroke instead of three. Similarly, the fly wheel of a four-cylinder engine may be still lighter, for in that type there are two power strokes in every revolution, with no dead strokes. At the same time, it is not possible to dispense with it entirely, for the pressure acting on the piston at the beginning of the power stroke falls rapidly as the piston moves before it, and the momentum by which the fly wheel tends to revolve at a constant speed steadies and smooths what would otherwise be a jerky motion.

The crank shaft of a four-cylinder engine is 180°, four-throw, with the two end cranks projecting in the opposite direction to that of the two inside cranks. This arrangement is used in preference to having the cranks project alternately; that is, the first and third to one side and the second and fourth to the other, because of economy in manufacturing, and because practice has shown that it results in less vibration in the running of the engine.

This construction of the crank shaft does not permit the power strokes to occur in rotation, cylinder No. 2 following No 1, and then Nos. 3 and 4, for it has been explained that when cranks are 180° apart the power strokes occur during one revolution, and that when 360° apart there is a dead stroke between the two power strokes. Cranks 1 and 2 are 180° apart, as are 3 and 4, and also 2 and 4, but 2 and 3 are 360°; that is, their crank pins are in line, which is also the case with 1 and 4. The power stroke in cylinder No. 2 may follow that in cylinder No. 1, but must be followed by that in cylinder No. 4, as that is 180° away from No. 2, and No. 3 comes last, being 180° from No. 4. The succession in which the power strokes occur, called the firing order, is thus 1, 2, 4, 3, and this is arranged for by the setting of the valves and the timing of the ignition.

This firing order was used for the first four-cylinder automobile engines, but it has lately been suggested that the firing order 1, 3, 4, 2 shows advantages in the reduction of strains and vibration, and is being adopted.

Three-cylinder engines are built with 120° crank shafts; that is, the cranks are one third of a revolution apart, instead of one half of a revolution, as is the case with 180° crank shafts. This gives three power strokes during two revolutions, and results in excellent balance. Six-cylinder engines have crank shafts of similar construction, and produce six power strokes during two revolutions, with the best balance that it is possible to obtain without an excessive number of cylinders. A power stroke occurs at every 120° that the crank shaft revolves, and as each power stroke endures for a half revolution, or 180°, it follows that one commences when the previous one is only two thirds complete. This gives a very steady application of power, for the crank shaft is at all times operated by the driving effect of the combustion strokes, and it is possible to run the engine smoothly at greatly varying speeds by control of the position at which the spark occurs in the combustion space, and the admission of a greater or less volume of the mixture during the inlet stroke.

The type of engine that will give satisfaction depends on the work that is demanded of it, the care that it will receive, and the knowledge of the operator. A one-cylinder engine, having a minimum of parts, may be successfully managed by a novice, but its vibration, and the fact that it delivers power for only one fourth of the time that it runs, make it unsatisfactory for any but light work. Cars of medium power are almost universally equipped with horizontal double-opposed engines, with excellent results, and they are not difficult to maintain. More powerful cars have four-cylinder vertical engines, which give a sufficiently constant pull for all practical purposes, with a correspondingly greater demand for care and attention. Six-cylinder engines are of doubtful advantage for general use, without the services of a chauffeur, for while they develop more constant power than engines with fewer cylinders, the complication of parts and the care necessary are increased.

CHAPTER IV
TWO-CYCLE ENGINE

The two-cycle type of gasoline engine differs from the four-cycle type described in the foregoing chapters in that the five events composing the cycle are performed during one revolution of the crank shaft, or two strokes of the piston, power being developed during every outward stroke of the piston instead of alternate outward strokes.

Fig. 18.—Two-Cycle Engine.

In order that this result may be attained, the construction of the engine is changed, and, as will be seen in Fig. 18, the crank case is utilized as a receiver for the mixture before it passes to the combustion space. The valves are replaced by ports, which are openings into the combustion space that are covered and uncovered by the piston as it slides in the cylinder. The inlet port is uncovered when the piston is at the inmost point of its stroke (Fig. 18, A), and then admits the mixture to the crank case; the by-pass port and the exhaust port are uncovered when the piston is at the outmost point of its stroke (Fig. 18, B), the former then permitting the mixture to pass from the crank case to the combustion space, and the latter is that through which the burned gases escape after combustion has taken place.

During an inward stroke, the pressure in the crank case is reduced as the piston slides away from it, and fresh mixture is forced into it by the higher atmospheric pressure as soon as the inlet port is uncovered. This port is covered when the piston makes an outward stroke, and the mixture, not being able to escape, is compressed. Its tendency to expand causes it to flow to the combustion space when the by-pass port is uncovered, and in entering it strikes a ledge on the piston so that it is deflected to the top of the combustion space instead of being able to shoot across the cylinder and out the open exhaust port. The inward stroke of the piston covers these two ports and compresses the mixture, ignition occurring in the regular manner. The pressure developed by the combustion drives the piston outward, and as soon as the exhaust port is uncovered (which is slightly before the uncovering of the by-pass port), the gases, which are still expanding, begin to escape, and are further expelled by the fresh charge that enters and drives them before it. Thus the five events of the cycle are performed during an inward and an outward stroke of the piston, the crank case end of the piston drawing a charge of fresh mixture into the crank case and forcing it into the combustion space, and the combustion chamber end compressing it and being acted on by the pressure from the combustion.

At slow speeds, two-cycle engines have advantages over the four-cycle type in the production of a power stroke at every revolution of the crank shaft, and the absence of valves and valve mechanism with their weight and possibility of giving trouble. This simplicity makes the two-cycle engine popular for motor boats, where they are run at slow and constant speed, but for higher and changing speeds these advantages are outweighed by disadvantages that show little sign of being overcome.

With the engine running at a thousand revolutions a minute, it can be understood that the ports will be open for only a brief period during each stroke, and that the faster the engine runs the shorter will be the period during which the gases may enter or leave the combustion space. The inefficiency of two-cycle engines as compared with engines of the four-cycle type is due entirely to the fact that the burned gases have not sufficient time in which to escape from the combustion space, nor the fresh charge time to enter. The fresh charge that does enter being incomplete, and being contaminated by the portion of the burned gases that has not been able to escape, result in the “choking up” of the engine, and in the production of lower power than the dimensions and weight of the engine should warrant.

CHAPTER V
CARBURETION AND GASOLINE FEEDS

Pure gasoline vapor will not burn, and in order to render it inflammable it must be combined with oxygen. The simplest manner of effecting this is to mix air with it, and when the correct proportions are obtained, the oxygen supplied by the air will be sufficient to result in the complete combustion of the gasoline vapor, without a surplus of either of the ingredients. This mixing is called carburetion, the air being said to be carbureted. A correct proportion of gasoline vapor and air results in rapid combustion; an excess of air makes combustion slower, and excess of gasoline vapor prevents the combustion from being complete, a residue of carbon remaining. The correct proportions of air and gasoline vapor are obtained by the use of a device called a carburetor, which is connected to the combustion chamber by the inlet pipe, and in such a manner that everything entering the combustion space by the inlet valve must first pass through it.

Liquid gasoline is led to the carburetor from the supply tank, and the air enters it when the pressure in the combustion space is reduced by the piston in making the inlet stroke. The speed with which the air flows through the carburetor depends on the extent to which the pressure is reduced, and the gasoline vapor that is required to form a mixture of the correct proportion must be maintained in accordance with it. While there are various classes of carburetors, practically all that are used for automobile engines are of the float-feed type; that is, the supply of gasoline is maintained by a float, just as water tanks are kept filled to a desired depth by a hollow metal ball that floats on the liquid and controls the valve by which the water enters.

Fig. 19.—Carburetor Principles.

The principal parts of a float-feed carburetor are the float chamber and the mixing chamber, the gasoline flowing from the supply tank to the float chamber, and from there to the mixing chamber, where it is combined with the air. The gasoline flows out of the float chamber through a small pipe, the end of which, called the spray nozzle, projects into the mixing chamber so that the current of air rushes past its tip (Fig. 19). When the inlet stroke is not being performed, and there is no air passing through the mixing chamber, the float in the float chamber keeps the gasoline at such a level that it stands just below the tip of the spray nozzle. In this condition the gasoline in both the float chamber and the spray nozzle is under atmospheric pressure; but when the pressure in the combustion space is reduced as the piston makes the inlet stroke, the pressure in the inlet pipe and mixing chamber is also reduced, and the gasoline will be forced out of the spray nozzle by the higher pressure in the float chamber. The passage in the tip of the spray nozzle is small, and the gasoline is broken up into fine drops as it spurts out, and in this condition is partly absorbed by the air and partly carried into the cylinder. By regulating the amount of gasoline that may pass out of the spray nozzle, it may be adjusted to the volume of air flowing through the mixing chamber, so that any desired proportion may be obtained.

If the engine were to be run at a constant speed, the relative pressures on the gasoline in the float chamber and spray nozzle, and the quantity of gasoline forced out of the spray nozzle, would remain in correct proportion to the volume of air; but as the engine of an automobile is run at greatly varying speeds, the pressure in the mixing chamber is not constant, but varies to correspond. If the piston makes fifty inlet strokes a minute, the reduction of the pressure in the mixing chamber is more gradual than would be the case with the piston making two hundred inlet strokes a minute. The more rapidly the pressure is reduced, the greater will be the effect of the unchanging atmospheric pressure in the float chamber, and the more gasoline will be forced out of the spray nozzle. This will result in the presence of too much gasoline in proportion to the air, giving a mixture that is too rich. It is therefore necessary to provide an arrangement by which the pressure in the mixing chamber will not be changed as the speed of the engine varies, and this is accomplished by the auxiliary air inlet, which is closed when the engine runs slowly, but opens to correspond with increasing speed (Fig. 19). The faster the engine runs, the more the auxiliary air inlet will open, to admit a correspondingly greater amount of air to prevent the pressure in the mixing chamber from being reduced below the point at which the required amount of gasoline is forced out of the spray nozzle.

The most rapid combination of the gasoline and air is secured by breaking the gasoline up into fine spray as it leaves the nozzle. In order to break the gasoline into fine particles, the tip of the spray nozzle is made with a fine opening, and often forms the seat for the gasoline adjusting valve, which is a needle-pointed rod that is screwed in or out to reduce or enlarge the opening; a further breaking up results from the placing of a metal cone, or the end of a rod, in such a position as to be struck by the gasoline as it flows out.

A carburetor with an auxiliary air inlet is provided with two points of adjustment, to control the flow of gasoline from the float chamber to the spray nozzle, and to govern the admission of the auxiliary air. The flow of gasoline must be just sufficient to carburet thoroughly the air passing through the mixing chamber when the engine runs at low speed, and the auxiliary air inlet must open to correspond with increasing speed, to admit sufficient air to keep the pressure reduced to proper proportions.

The auxiliary air inlet may be operated either by the reduced pressure that permits the atmospheric pressure to open a valve, or by the governor of the engine, which opens the inlet as the speed increases, and closes it on slowing down. The first of these two types, called an automatic carburetor, is in most general use for automobiles, and is sufficiently satisfactory, but is not as accurate as the mechanically controlled type, which acts independently of pressure, and exactly according to the speed of the engine.

Fig. 20.—Automatic Carburetors.

Carburetors may be divided into two types, according to design: those in which the float and mixing chambers are side by side, and those in which the mixing chamber passes through the center of the float chamber (Fig. 20). The former is along the lines of the first forms of float-feed carburetors, and the latter is of more recent design, its object being a more compact device, and one that is not affected by a change of level when the car is on a hill. Both have advantages and disadvantages, and the use of one as against the other is optional and a matter of opinion.

The carburetors illustrated are not of any particular makes, and are intended to show principles rather than construction.

In carburetors with a side mixing chamber the float is usually a metal box, the joints of which are as far as possible proof against leakage. Guides prevent it from having any but an up-and-down motion, and in thus moving it controls the gasoline inlet valve by a rod attached to it, or by a separate valve stem. In this carburetor a separate valve stem is used, and it is moved through the action of a rocker arm controlled by the float. This construction is usual when the gasoline enters the float chamber from the bottom, the flow being permitted or checked by a needle valve. When the level in the float chamber drops as the gasoline runs out of the spray nozzle, the float sinks, and lifts the valve point from its seat. This admits gasoline from the supply tank, and the float in rising on it depresses the valve point, shutting off the flow. The gasoline adjusting valve is in the passage between the float and mixing chambers. The main or initial air inlet is in the side, the air being drawn through the mixing chamber, and past the spray nozzle. The auxiliary air inlet is a simple valve, opening inward, and held against its seat by a coil spring, the tension of which is adjustable. The pressure in the mixing chamber being reduced in accordance with the increasing speed of the engine, the valve is opened more and more as the atmospheric pressure against the outer surface of the valve overcomes the tension of the spring.

In carburetors with central mixing chamber the float is ring- or horseshoe-shaped, and usually made of cork, well varnished to prevent the absorption of the gasoline. The air enters at the bottom, passing directly to the mixing chamber. The auxiliary air inlet is at the top, and is of the arrangement already described. The mixture passes out at the side, and may be controlled by a throttle, which may be a damper arrangement, as shown, or other device by which the quantity passing to the combustion space is at the will of the operator.

Fig. 21.—Mechanically Controlled Carburetor.

The float and mixing chambers of a mechanically operated carburetor are the same as in the side-float chamber type, the difference being in the control of the auxiliary air inlet (Fig. 21). This consists of a tube attached to the mixture outlet, within it sliding another tube moved by the governor as that expands or contracts with the speed of the engine. There are openings in the sides of both tubes, but when the sliding tube is at its inmost position these are not in line, and consequently are closed. When the governor acts with increased engine speed, the sliding tube is drawn out, and one or more openings come into line, air entering through them to the mixing chamber. The faster the engine runs, the larger become the openings, and in consequence the greater is the amount of air that they admit. The illustration shows the ball type of gasoline valve, the ball on the end of the valve stem being drawn against its seat as the float rises.

Fig. 22.—Float Feed Carburetor with Gravity Gasoline Feed. 1, Filling cap; 2, tank; 3, gasoline valve; 5, trap and strainer; 6, carburetor; 7, float chamber; 8, float; 9, float valve; 10, spray nozzle; 11, gasoline adjustment; 12, air adjustment; 13, throttle; 14, mixing chamber; 15, initial air inlet; 16, primer.

While these types are in practically universal use for automobile engines, there are other methods by which the proportions of the mixture may be maintained. In one form the gasoline drops on a funnel made of fine wire gauze, which is placed in the mixing chamber in such a manner that the air in entering passes through it. The liquid forms a film over the gauze, and is picked up by the air, as it is in a condition that permits it to evaporate rapidly. In surface carburetors air is forced through the gasoline tank, or through an absorbent material soaked with gasoline, and becomes thoroughly saturated. This mixture is then thinned with pure air until the desired proportion is obtained, when it passes to the combustion space. The objections to these forms arise from the clogging of the parts with the impurities present in gasoline, and while they give excellent results when new, they deteriorate rapidly and present such resistance to the flow of the air current that they become useless.

There are two methods of supplying the carburetor with gasoline. Of these the most usual is the gravity feed, in which the tank is placed at a higher level than the carburetor, so that the gasoline flows down to it. The tank is usually placed under the seat, and the piping so arranged that the carburetor is the lowest point of the system. This method of feeding is satisfactory if the tank can be placed sufficiently above the carburetor to have the flow unaffected by an ordinary hill, but if it is not so placed, a steep ascent may tilt the car to such an extent that the carburetor is above the level of the gasoline in the tank, in which case the flow of course ceases.

Fig. 23.—Pressure Feed Gasoline System. 1, Filling cap; 2, tank; 3, pressure pipe; 4 and 5, check and relief valves; 6, hand pump; 7, check valve; 8, pressure gauge; 9, feed pipe; 10, carburetor.

The pressure feed, which has been adopted on all high-grade cars, operates through the maintenance of pressure in the supply tank, the gasoline being forced out without regard to gravity. The tank is tight, so that the pressure cannot escape, and is connected by a long pipe of small diameter either with the combustion space of one of the cylinders or with the exhaust pipe, so that the pressure of the burned gases is maintained in it. As the pressure cannot exist until the engine is running, a hand air pump is usually provided, by which a sufficient pressure may be produced in the tank to force out enough gasoline for starting. It is necessary to use as long a pressure pipe as possible, in order to prevent the possibility of flame passing through it to the supply of fuel, a long pipe, exposed to the air, cooling the gases to such an extent that they cannot ignite the gasoline.

The pressure pipe is always fitted with a check and relief valve, which acts as a safety valve in preventing the pressure in the tank from reaching a point at which the joints might be strained, and also retains the pressure which would otherwise escape when the engine stops. In some cars an auxiliary gasoline tank is provided on the dash, being fed by pressure from the main tank, and from which gasoline flows to the carburetor by gravity. The short distance of this tank from the carburetor and its elevation prevent the possibility of the flow being stopped by any tilting of the car short of an upset.

Fig. 23A.—Types of Float Valves.

Because of the liability of the presence of water in the gasoline, as well as dirt and grit, the gasoline line should be fitted with a strainer, or trap. This may be in any position, but it is usual to have it close to the carburetor, if not built into it. The simplest strainer consists of a number of thicknesses of fine wire gauze, so arranged that it may be easily taken out for cleaning. This will separate the dirt from the gasoline, and water may be caught in a trap, which is a pocket where the water, being heavier than the gasoline, may settle and be drawn off.

CARBURETOR PARTS

Fig. 23B.—Types of Auxiliary Air Inlets.

Practically all the carburetors on the market are combinations of a few forms of float valves, auxiliary air inlets, and spray nozzles. In addition to the forms shown in Figs. 20 and 21, the most usual float valves may be seen in Fig. 23A. In the first two types shown in this diagram, the float valve stems are separate from the floats, and are sufficiently heavy to shut off the flow of gasoline by their weight. In the third type, the gasoline enters the float chamber from the top, and as the valve stem is attached to the float, the rising of the float results in the shutting off of the gasoline. The fourth type is in use on carburetors with central mixing chambers, the float being hinged to one wall of the float chamber. The loose valve stem is supported by the hinge, and rises to a seat in the valve when the gasoline is at the proper depth on the float chamber.

Fig. 23B illustrates the most usual forms of auxiliary air inlets. In the first type, the valve disk slides on the valve stem, and enlarges the size of the main air inlet. All of the air thus passes the spray nozzle. In the second type, the inlet for the auxiliary air is separate from the main air inlet, the two currents meeting in the mixing chamber, and the extra air diluting the rich mixture that is formed at the spray nozzle. This action is more correct in theory than that of the preceding type, and better practical results are obtained from it. These air valves are defective in opening and closing too abruptly, and in tending to vibrate rather than to remain open a fixed distance. The air inlet illustrated in the fourth diagram was designed to overcome these faults. When the engine is not operating, the air inlets are closed by a hollow piston that is held up by a spring. The upper part of the piston rod carries a metal disk that is attached by a flexible leather washer to the walls of an upper chamber. The portion of the chamber above the disk is tightly closed, except for a small hole in the cover that provides the only communication between the atmosphere and the air confined in the chamber. When the engine runs at speed, the atmospheric pressure against the upper side of the disk is greater than the pressure against the lower side, and the disk is therefore forced downward against the action of the spring. The movement of the disk moves the piston, and as this latter slides downward it uncovers the openings and admits air. The small size of the opening in the cover prevents air from entering or leaving the chamber above the disk rapidly, and the movement of the piston is therefore steady and free from jerks. The third diagram illustrates two positions of a mechanically operated auxiliary air inlet, controlled by a governor.

CHAPTER VI
IGNITION

The charge of explosive mixture in the combustion space is ignited, or set on fire, by an electric spark, and the apparatus for producing and controlling this spark is called the ignition system. It is with this part of the mechanism of an automobile that a novice has the greatest difficulty, for an electric current is usually regarded as being surrounded by an air of mystery. It does its work silently and without visible reason, and when it fails the average man is under the necessity of beginning at the beginning and examining all of the parts of the system because he has so little understanding of the why of it that he is unable to locate trouble in any but a rule-of-thumb method. The principles of electricity may be involved, but the production of a current, its handling, and the uses to which it may be put are not difficult to understand.

Speaking broadly, the parts of the ignition system are the source of current, the arrangement in the combustion space at which the spark is produced, the device by which the instant when the spark passes may be controlled, and the circuit by which these parts are connected. Before going into a description of these, however, it is necessary to understand something of the nature and action of an electric current.

Every generator of electricity has two terminals, or poles, and the flow of current from one to the other is due to what may be explained as a difference in pressure between them. This difference in pressure is similar to that existing when two tanks, one full of water and the other empty, are connected by a pipe. The water will flow from the full to the empty tank as long as there is a difference in level, which is the same thing as a difference in pressure, the flow ceasing when the water in one tank is level with that in the other. When a path is provided between the two poles of the generator, the current will flow from one to the other, always in the same direction, leaving by the positive pole and returning by the negative.

Because of this tendency to flow, the current may be made to perform work, for it will light a lamp, ring a bell, or do anything else within its power in order that it may flow from the positive pole of the generator to the negative. In this there is also a similarity to the two tanks, for if a water wheel is placed in the pipe connecting them, the flow of water from one to the other will operate it.

The path over which the current flows may be formed of any conductor of electricity, such as carbon or any metal; substances by which the current will not flow are called nonconductors or insulators, and those in most common use are rubber, china and glass, wood, wood fiber, mica, etc.

While all metals are conductors, some are better than others, the difference being in the resistance that they offer. A comparison illustrating resistance may be made between the friction presented to the flow of water by a small pipe and by a larger one, the water flowing more easily through the latter than the former. In flowing, the current must overcome the resistance of the conductor, and in so doing will lose part of its strength and will heat the conductor, there being more loss of current and greater heat as the resistance increases.

It is obvious that in order to obtain a current of the greatest strength the conductor by which it flows must present the least possible resistance, and for this reason copper is used almost universally to convey the current from one place to another. A copper wire will carry safely a current that would heat an iron wire of the same size to the melting point. The resistance of a small conductor is much greater than that of a large one, so that the size of a conductor must always be considered in relation to the current that is to be conveyed.

A current of electricity may be measured, just as water flowing through a pipe may be measured, and by the same measurements of pressure and volume. The pressure under which the current flows is measured in volts, and the quantity that passes in amperes; there is also a term for resistance, that being measured in ohms.

Because of the desire of the current to flow from the positive to the negative pole, the circuit must be so guarded that there may be no leakage, for if it can return to the generator without doing its work it will do so, taking any path that offers less resistance to its passage. Leakage is prevented by insulating the wires, which may be done by wrapping them with silk or cotton thread, or by coating them with rubber. The wire by which the current flows from the generator to its work is called the lead (pronounced leed), and the conductor by which it flows back, the return. The greatest care must be taken that there is no leakage from the lead wire in order that there may be enough current to perform the work; but when the current has done what was expected of it, any conductor that does not present too much resistance will serve to return it to its source.

While wire is sometimes used on ignition circuits for the return as well as the lead, the most usual method is to utilize the metal of the engine to return the current to its source. The engine being made of iron, it is a conductor, and while this metal is not so good a conductor as copper, yet there is so much of it in an engine that its resistance need not be considered. This method is called grounding the return, the term coming from telegraphy, in which the dampness of the earth forms one of the conductors.

A grounded circuit requires careful insulation of the lead wire, for as it is very likely to come into contact with the metal of the engine, a break in the insulation would permit the current to leak and to return to the generator without doing its work, this condition being called a short circuit. Grounding the circuit saves wire and reduces complication; simplicity is important in automobile work, and a reduction of the parts much to be desired.

In the foregoing the word flow has been used in describing the action of an electric current in a wire, and there are so many points of similarity with the flow of water in a pipe that the term conveys a better idea than would technical expressions. Electricity, however, has neither substance nor weight, and cannot be said to flow or to move in the strict sense of the words; but as flow is in general use, and is the most descriptive of the commonly understood words that are applied to it, it is made use of here.

A current of electricity may be generated either chemically or mechanically, and always at the expense of something else. A chemical generator produces a current at the expense of a metal, usually zinc, which is eaten by the chemicals in proportion to the amount of current delivered; a mechanical generator produces a current at the expense of the power that drives it.

Chemical generators are of two kinds, primary and secondary cells. While primary cells are of many forms, that called the dry cell is universally used for the ignition of automobile engines, being adapted to the purpose because there is nothing to spill, and may be used in any position. It consists of a zinc cup lined with some material of the nature of blotting paper, in which, but not touching it, is a stick of carbon, the space between being filled with absorbent material and broken bits of carbon moistened with the proper chemical solution. The top of the cell is sealed to prevent evaporation, and thumb nuts on the projecting end of the carbon stick and the zinc cup form the terminals to which the wires are attached. The carbon being the positive pole and the zinc the negative, the current flows in that direction whenever a circuit is provided.

The dry cell gives a current at a pressure of about one and a half volts when new, the voltage dropping as the cell is used until it is exhausted, when the trifling cost of a new cell should not be considered in replacing it.

A better source of current than the dry cell is the secondary or storage cell, which is a reservoir rather than a generator. When a current of electricity is passed to it, usually from a lighting circuit, a chemical change takes place, the action being called the charging of the cell. When the change is complete, the cell is disconnected, and will then deliver a current because the parts of the cell that underwent a chemical change tend to return to their previous condition. The current that it gives off is practically equal to that by which it was charged, and may be used steadily or intermittently until the cell is exhausted. It is made of prepared lead plates, which are placed in a hard-rubber or celluloid jar, with a cover to prevent the spilling of the solution with which the plates are covered.

The charging of storage cells must be done by an expert, for mishandling will ruin them, and it is far better to put this matter, as well as repairs, in the hands of a man who knows his business than to attempt it without the proper knowledge and appliances.

One point that the automobilist must watch, however, is the position and quality of the solution in the cells, called the electrolyte. The cells should always be full enough to have the plates well covered, and any loss by leakage or splashing should be made up, for the plates must not be exposed to the air.

Electrolyte is made by adding one part of chemically pure sulphuric acid to from three to four parts of distilled or rain water, adding the acid slowly and stirring constantly. The combination of acid and water generates heat, and care must be taken not to pour the water into the acid, for the generation of heat would then be so sudden that the mixture would boil and splash, burning whatever it touched. By means of an instrument called a hydrometer, the density of the mixture may be ascertained, and when the proportions are correct the scale should read 1,200. Hydrometers are made with the instrument inside of a glass tube, one end of which has a spout, the other being fitted with a rubber bulb. By inserting the spout in the vent hole of the cell cover, the tube may be filled with electrolyte, the hydrometer showing whether or not it is of the correct density.

The cells should be tested every little while, and if the density is not correct acid or water should be added to make it so. Never probe inside of the cells with anything but a glass or hard-rubber rod, for metal would cause a short circuit between the two plates it touched, and probably ruin the cell.

The terminals of a storage cell are marked in order to distinguish them, the symbols being the plus (+) sign for the positive pole and minus (-) for the negative, these being universally used in this way on all electrical work. In some makes of storage cells the positive pole in addition is painted red and the negative black.

A storage cell when fully charged gives a current at a pressure of about two and a half volts; the voltage drops as the cell is used, and when it gets down to 1. 8 volts it must be recharged. When a storage cell is discharged, it should be recharged immediately, for otherwise it will deteriorate.

One dry or storage cell does not give a sufficient current to supply the required spark, and several must therefore be used. The most usual method of connecting dry cells, and the only method of connecting storage cells, to form a battery, is to connect the negative pole of the first cell to the positive pole of the second, using a short length of insulated wire of sufficient size, the negative of the second to the positive of the third, and so on, until all of the cells necessary to supply the required current are used. This leaves the positive pole of the first cell and the negative pole of the last cell free to be connected into the circuit. The current obtained will have a pressure that is as much greater than the pressure of one cell as there are cells in the battery; for instance, if there are four cells in the battery, of one and a half volts each, the voltage of the battery will be six volts. This method is called connecting in series (Fig. 24).

Fig. 24.—Battery Connections.

When the voltage of one cell is sufficient, and it is desired to increase the volume, or amperage, of the current, the cells may be connected in parallel, which is done by connecting together all of the negative poles by one wire, and all of the positive poles by another, the two wires leading to the circuit. This increases the amperage as many times as there are cells.

If it is desired to connect the cells so that both the voltage and amperage are increased, the cells may be divided into two or more groups, with the same number of cells in each. The cells of each group are connected in series, and the free poles of the groups connected in parallel, which is called connecting in series-multiple (Fig. 24). This is a most satisfactory method of connecting the cells for an ignition circuit, for it gives a uniform current, and increases the life of the cells. The voltage of the current is equal to the voltage of one cell multiplied by the number of cells in one group, and the amperage that of one cell multiplied by the number of groups.

The action of a mechanical generator, which when driven produces a current of electricity, is due to magnetism, that being understood to be the power sometimes possessed by iron or steel to attract other pieces of iron or steel. Magnetism may be manifested either by permanent or electro-magnets. If a piece of steel is magnetized—that is, made a magnet—it continues to be a magnet, for steel retains that property, while iron does not. A magnet made of steel is called a permanent magnet.

If a piece of iron is wound with insulated wire through which an electric current passes, the iron will become a magnet, and will remain so as long as the current flows. This is called an electro-magnet.

The two ends of a magnet are the positive and negative poles, and the positive pole of one magnet will repel the positive pole and attract the negative pole of another magnet. Between the two poles of a magnet there is a constant flow of what are known as magnetic lines of force, which may be made visible by laying a piece of paper over the two poles and scattering iron filings on it. The filings will form into curves from one pole to the other, showing the presence of these lines.

If a wire is moved across these lines of force, so that it cuts them, a current of electricity will be set up in it, and if the wire is wound into a coil and moved across the lines, the current will be as much stronger as there are turns of wire cutting them. A mechanical generator consists of a coil of insulated wire, arranged so that as it revolves in the magnetic field each turn of wire cuts across the lines of magnetic force.

The magnet producing the lines of force that compose the magnetic field is called the field, and the revolving coil of wire the armature.

Mechanical generators for producing the current for an ignition circuit are of two types, magnetos and dynamos, the former being in much more general use than the latter. In a magneto the field is a permanent magnet, and in a dynamo it is an electro-magnet. The magnets forming the field of a magneto are bent into horseshoe shape, so that the poles are close together, and the armature revolves between them. The field of a dynamo is of similar shape, wound with insulated wire, the current that flows through it being generated by the dynamo itself; the field is strongly magnetized only when the dynamo is running.

The armature of either type is a piece of soft iron, with grooves cut in it lengthways, in which the wire is wound; the winding thus being contrary to that of thread on a spool. The current generated in the wire of the armature is taken off by means of brushes, and connection is made through binding posts set in the base.

As the care of a mechanical generator requires a knowledge of electrical engineering, it is better to return it to the makers in case of injury, rather than to attempt to repair it without the necessary experience. (See Appendix.)

Practically all ignition circuits are provided with two sources of current, so that in case of the failure of one the other will be available, and either may be connected into the circuit by means of a switch. The simplest form of switch is a piece of flat spring brass, pivoted at one end so that the other may be swung from side to side to make contact with one or the other of two brass knobs, or switch points. One pole of each generator, usually the negative, is grounded, and the other connected to one of the points. The flat spring, or switch blade, is connected to the ignition circuit, so that when it is in contact with one of the points the corresponding source is called on to supply the current. The blade may rest between the points, not touching either, in which case the circuit is broken, and no current flows.

CHAPTER VII
IGNITION—(Continued)

While there are various methods by which an electric spark may be produced, but two are in use for automobile work, one being the high-tension or jump-spark system, and the other the low-tension or make-and-break system. The difference between these is in the pressure of the current, which in the first is great enough to enable it to jump from one terminal to another a short distance away, forming a spark as it passes. The pressure of the current of the low-tension system is not sufficient to permit it to do this, but when two terminals of the system are brought into contact so that the current may flow, a spark will form as they are separated.

MAKE-AND-BREAK SYSTEM

In the make-and-break system, the igniter, which is the device in the combustion space at which the spark occurs, is made with two metal points, one of which is stationary and the other movable, the latter being acted on by a cam, through a tappet. As the cam revolves, the tappet is lifted and the movable point brought into contact with the stationary; when the nose of the cam passes from under the tappet, the movable point is snapped away from the stationary through the action of a spring (Fig. 25). These points are so connected into the circuit that when they are in contact the current flows, the circuit being broken when they separate. Shortly before the spark is desired the movable point is brought into contact with the other so that the circuit is completed and the current flows, and they are separated at the instant when the spark is required to ignite the mixture, this occurring as they separate and break the circuit.

Fig. 25.—Make-and-Break Ignition.

The current that is supplied by the battery is not capable of producing this spark, for it flows with large volume and small pressure—i.e., high amperage and low voltage—and it is necessary to transform it to a current of higher voltage and lower amperage, which is done by means of a primary induction coil. A coil of this description consists of a core made of a bundle of soft iron wires, around which is the primary winding, formed by winding several layers of insulated copper wire on it. The core becomes a magnet when a current of electricity flows through the winding, and ceases to be a magnet when the flow ceases, the magnetization occurring slowly on the completion of the circuit, and demagnetization instantly on the breaking of the circuit.

The influence of a magnet, as, for instance, its attraction for a piece of iron, is felt throughout a field the extent of which depends on the strength of the magnetization. When a loop of wire forming a closed circuit is placed in the magnetic field, a current of electricity will be set up in it whenever the strength of the field changes. If the strength of the field does not change, no current will be set up; but the greater the change in strength, whether from strong to weak or from weak to strong, the greater will be the strength of the current. This principle is applied in the case of a coil, for the wire that forms the winding is in the magnetic field of the core, and a current is set up in it whenever the strength of the field changes. The strongest current will be set up, or induced, when the strength of the field changes from no magnetism to a degree when the core is magnetized to its fullest capacity or the reverse, and when the change occurs in the least possible time. The current that is set up will last only during the time when the change in strength occurs, ceasing to exist when the strength of the field is uniformly strong or weak. The more rapid the change, the greater will be the pressure of the current, but the shorter the period of its existence.

It is this induced current that forms the spark in the combustion space, the battery current serving only to establish the condition that will produce it. When the two points of the igniter come into contact and complete the circuit, the flow of the battery current through the winding of the coil will gradually magnetize the core, and on the igniter points being separated, this flow through the winding will cease, the strength of the magnetic field dropping instantly from full intensity to practically nothing. This change induces a powerful current in the winding, which current in passing over the circuit causes a spark between the points of the igniter as they separate. The induced current is set up so instantaneously on the breaking of the battery circuit that the spark is usually considered as being that of the battery current, but, as stated, the battery current serves only to magnetize the core, the spark resulting from the current induced by its demagnetization on the breaking of the circuit.

A magneto is a machine that when driven generates an electric current, and is in almost universal use for this system of ignition. In many cars the system is so arranged that the driver has his choice between ignition by battery and coil, as described, and ignition by magneto, but the mechanical production of a current is so reliable that many automobile builders rely on the magneto alone. Magnetos of the type used for ignition are described in the appendix.

JUMP-SPARK SYSTEM

The jump-spark or high-tension system of ignition depends on the production of a current at such a pressure that it can jump from one point to another through high resistance, such as is presented by air at ordinary or low temperature, and especially when compressed, the spark occurring as it passes.

This high pressure is obtained through the use of a secondary induction coil, which consists of a core and primary winding similar to those of the primary induction coil already described, and in addition has a secondary winding, formed by a great length of insulated copper wire, which to save bulk and weight is very fine, wound over the primary winding. The change in the strength of the magnetic field produced by the core affects this secondary winding, just as it affects the primary winding; that is, a current is set up in it at every change in the strength of the magnetic field, the pressure of the current depending on the greater or less extent of the change, and the rapidity with which it occurs. The magnetization of the core takes place but slowly, for the particles of iron must absorb it one from the other, but the demagnetization occurs instantly; the current that is induced during the demagnetization is therefore the greater, and is the current that has sufficient pressure to jump across the gap.

The ignition system employing this coil consists of two distinct parts: the primary circuit, which magnetizes the core, and the secondary circuit, which leads to the combustion space the current that is induced in the secondary winding.

The primary circuit includes the battery or generator that supplies the current, the primary winding of the coil, a timer, and a vibrator.

THE TIMER

The spark must occur in the combustion space only at the instant when the mixture is in a condition to be ignited, and it is obvious that only then is the secondary current required. As the secondary current results from the flow of the primary current through the primary winding, a device must be included in the primary circuit to permit the current to flow and magnetize the core at this instant. This device is called a timer, or commutator, and is nothing more than a revolving switch operated by the engine, that completes the primary circuit at such times as the secondary current is required, and breaks the circuit when the secondary current has done its work in the production of the spark. Because ignition occurs but once in each cylinder during two revolutions of the crank shaft, the timer completes the circuit but once in that interval, and is placed on the half-time shaft that operates the exhaust valves. In its simplest form, a timer consists of a disk of hard rubber, wood fiber, or other insulating material in which is set a metal plate, mounted on the half-time shaft and revolving with it. A flat spring carried on a plate of hard insulating material bears against the edge of the disk, and during one revolution of the latter the metal plate set in it will be brought into contact with the spring (Fig. 26, first diagram). If one wire of the primary circuit is connected to the metal plate of the disk and the other wire to the spring, it will be seen that the circuit will be complete when the revolution of the disk brings the two together, and broken when the further revolution of the disk separates them.

Fig. 26.—Types of Timers.

There is a great variety in the methods by which this result is secured, the chief essential being that the contact must be made as positively as possible in order that the resistance to the current in passing from one part of the timer to the other may be as low as possible, and broken equally positively. The number of contacts that the timer makes during one revolution depends on the number of cylinders of the engine, the revolving part touching two, three, four, or more springs or contact points on the stationary part as it turns. A timer for a two-cylinder engine is shown in the second diagram on Fig. 26, the third diagram being of a form of timer in which the revolving part is not connected into the circuit, but serves to bring together a spring and a contact point, the circuit being completed when they touch. A modern form of timer consists of a ring of hard rubber on the inside face of which are set the metal plates, while the moving part revolves within it, and may be a flat spring, or a plunger, or a roller, contact being maintained by a coil spring. The difficulty of attaching a wire to the moving part of the timer is overcome by having its contact in connection with the half-time shaft, the current flowing from the contact on the stationary part to that on the moving part, and from there flowing by the half-time shaft to the ground connection of the battery, for, as all parts are of metal, the current can pass from the shaft to the ground connection through the bearings and the engine.

If the spark were always required to pass in the combustion space at the same point in the stroke of the piston, this could be secured by setting the stationary part of the timer in such relation to the moving part that contact would be made when the half-time shaft reached any desired point in its revolution. As it is necessary to change the point at which ignition occurs, however, the timer must be so arranged that it closes the circuit earlier or later, according to conditions. To effect this, the stationary part of the timer is so made that it may rotate around the shaft for about a half turn, and it is controlled by a lever on the steering column, which holds it in any desired position. If the stationary part is rotated in the direction opposite to that in which the half-time shaft revolves, contact will be made and the circuit closed at an earlier point in its revolution than if it is rotated in the same direction. It must be remembered that the revolution of the half-time shaft, in operating the exhaust valve, revolves exactly in accordance with the crank shaft and the movement of the piston, and that as the moving part of the timer is attached to it, contact can be made at any desired point in the stroke of the piston by the position in which the stationary part is placed.

THE VIBRATOR

The secondary current occurs whenever the core is magnetized and demagnetized, and it is clear that the oftener the magnetization occurs the more constant will be its presence. The timer closes and opens the primary circuit, which results in the production of but two impulses of the secondary current, and as these are not sufficient to produce the necessary spark, additional means are employed to make and break the circuit while the timer makes contact. This is accomplished by means of a vibrator, or trembler, which consists of a flat steel spring secured at one end, so that it may vibrate after the manner of a tuning fork. About midway between its ends the point of an adjusting screw touches it, and when the flat spring, or blade, vibrates, it springs away from this screw and returns to it, both screw and blade being tipped with platinum to offset corrosion. If one wire of the circuit is attached to the blade and the other to the screw, the circuit will be complete when the blade touches the screw, and broken when it springs away from it. In the early type of vibrator the blade was set in vibration by mechanical means. At the end of the blade was a weight, which rested on the edge of a cam, the blade then being out of contact with the adjusting screw (Fig. 27). During the revolution of the cam the weight dropped into a deep notch, thus setting the blade into vibration and the circuit being made and broken. This type has been almost entirely superseded by the magnetic vibrator, which is more rapid and accurate. In this the blade is so placed that its free end is close to the end of the core of the coil (Fig. 28). When the timer makes contact and the current passes through the primary winding, the core becomes magnetized and attracts to it the free end of the steel blade, this movement drawing the blade away from the adjusting screw with which it was in contact. The vibrator is so connected into the primary circuit that the current flows from the battery to the adjusting screw, thence to the blade and through the primary winding; when the attraction of the blade by the core draws it away from the adjusting screw, the circuit is broken, and the magnetism then ceasing to exist, the elasticity of the blade causes it to spring back to the screw, to make contact, and to be again attracted by the magnetism.

Fig. 27.—Mechanical Vibrator. When the weight drops into the notch on the cam, the flat spring touches the adjusting screw and the current passes.

Fig. 28.—Magnetic Vibrator. When the magnetism of the core attracts the vibrator the contact is broken at the adjusting screw.

This action continues as long as the timer holds the circuit closed, and a wave of secondary current is set up in the secondary winding for every make and break that results. By means of the adjusting screw, the vibrations may be made long and slow, or short and quick, with a corresponding difference in the secondary current. As the object to be attained is the production of a spark that will ignite the mixture rapidly, the vibrator must be adjusted with this end in view; but it is also advisable to obtain the spark with the least possible expenditure of current, and with the least wear of the parts. The passing of the battery current through the primary winding establishes the condition that exists in the make-and-break ignition system, and the demagnetization of the core will set up an induced current that will result in a spark at the points where the circuit is broken; that is, at the vibrator and timer contacts. This spark can be greatly overcome in the timer by packing it with vaseline, which, being an insulator, breaks down the spark in flowing between the contacts of the stationary and moving parts as they separate. If the vibrator contacts presented no resistance—an impossible condition—there would be no spark, but as they will always be slightly burned and corroded, the spark must be kept as small as the requirements of the secondary current will justify by the adjustment of the screw. In other words, adjust the vibrator so that it gives the best results in the production of the secondary spark with as little sparking as possible at the vibrator contacts.

THE SPARK PLUG

The secondary circuit includes the secondary winding of the coil and the spark plug, which is the device in the combustion space at which the spark occurs. The spark plug provides two points of metal at a fixed distance apart, which are so connected into the circuit that the current must jump from one to the other in order to complete its circuit. The secondary current, as well as the primary, may make use of the metal of the engine as a ground return, and one of the points of the spark plug is therefore in contact with the metal, being supported on a metal sleeve that is screwed into the cylinder. The other point must of course be insulated from it, for if it were not the current could pass from one to the other without jumping the space between them. The insulation is secured by a tube of porcelain or mica set in the sleeve, the lower end of a metal rod that passes through it being the required distance from the wire or projection on the sleeve (Fig. 29). This device must be strong enough to resist the pressure in the combustion space during the compression and power strokes, and must be unaffected by the intense heat.

Fig. 29.—A, Spark plug; B, spark plug in section.

THE CIRCUIT

Fig. 30 shows the primary and secondary circuits of the jump-spark system, as applied to a one-cylinder engine, the light lines representing the primary circuit and the heavy lines the secondary. Each circuit forms a complete path by which the current passing over it may return to the point at which it started. In practice the wiring is simplified by the use of but one ground return for both circuits, the primary and secondary windings of the coil being connected at one point, and the secondary current returning either through the timer as it makes contact or through the battery.

Fig. 30.—Ignition Circuit. Wiring diagram for one-cylinder engine, with coil, timer, and two batteries; coil with magnetic vibrator.

The flow of the primary circuit is from the battery to the switch, to the vibrator, which is built into the coil, through the primary winding and the timer contacts, whence it passes by the metal of the engine and ground wire back to the battery. The secondary circuit consists of a wire from the secondary winding of the coil to the spark plug, and back to its winding by the metal of the engine and a ground wire, which, as has been explained, may be the separate ground wire shown in the diagram, or the return that is common to both circuits.

The timer makes contact as the half-time shaft revolves, and the vibrator operates as long as the circuit is closed; while this occurs, the secondary current is set up, and the spark passes.

Included in the primary circuit and built into the base of the coil is a condenser which, while of such delicate construction that it is most inadvisable for the automobilist to attempt to examine or repair it, is of such advantage that it should be understood. A current flowing in a circuit possesses momentum, and if the circuit is broken, the momentum will cause the current to flow across the break for a greater or less time, according to the pressure. This will produce a spark, and a spark at the vibrator as it breaks the circuit will not only burn away the contacts, but will prevent the instant demagnetization of the core. The passing of a spark between the vibrator contacts indicates that the current is still flowing through the primary winding, and that the core is still magnetized. The function of the condenser is to prevent sparking at the vibrator by absorbing the momentum of the current, and, what is more important, to cause the instant demagnetization of the core by the instant breaking of the circuit.

While it is usual to provide a multicylinder engine with one coil for each cylinder, it is possible to equip it with but one, in such a case a secondary distributer being used. This consists of two parts: a primary timer, and a somewhat similar device by which the secondary current induced in the coil is switched from cylinder to cylinder as it is required. For a four-cylinder engine the timer has four contacts, as in the usual type; but the primary winding is connected to all of them, so that as the moving part of the timer revolves, the core of the single coil is magnetized four times to two revolutions of the crank shaft. A single wire leads the secondary current that is thus induced to the distributer, by which it is switched to the several cylinders. The moving part of the distributer rotates in unison with the moving part of the timer, making contact as it does so with contact pieces, each of which is connected to one of the spark plugs.

When the timer makes its contact the distributer is in position to pass the induced current to the proper cylinder, the connections being made in accordance with the firing order of the engine. While this system has advantages in that there is but one vibrator to adjust, the fact that the coil is operating for the greater part of the time makes it more liable to injury through heating and the consequent breaking down of its insulation.

CHAPTER VIII
TRANSMISSION

The transmission of an automobile consists of those parts that transmit to the driving wheels the power developed by the engine.

THE CLUTCH

Because a gasoline engine must be in operation before it can deliver power, a clutch is provided by means of which it may run free or be so connected that it drives the car, one of its two chief parts being attached to the engine and the other to the transmission. When the two parts are in contact, the transmission is driven, and when separated, the engine and transmission are independent of each other, and may be stationary or in motion. A clutch must be of such a nature that it does not apply the power of the engine instantly, but gradually, so that the car starts slowly and without jerking. If the power were to be applied suddenly, the effort of starting the stationary car would either overcome the momentum of the engine and stop it, or would jerk the car into such sudden motion that it might be badly wrenched. By making the clutch so that it is permitted to slip when first applied, the part that is driven is gradually brought to the speed of the part that drives, when slipping ceases and the two make firm contact.

The most usual form of clutch is the friction cone, in which the fly wheel of the engine is utilized as the driving part, the rim being broad and thick, with its inner side funnel-shaped, or beveled. A metal cone that fits the bevel is carried on the end of a shaft of the transmission, the shaft at this point being square, to fit a square hole in the hub of the cone. This arrangement permits the cone to slide along the shaft while always revolving with it. When the cone is pressed to a seat in the fly wheel, which is accomplished by means of a heavy coil spring, the friction between its leather-covered surface and the surface of the fly wheel causes the two parts to revolve together, and by its fit on the square shaft the transmission is set in motion, and through that the car. The clutch is thrown out of contact by means of a foot pedal that acts on a ring fitting in a groove around the hub of the cone; when the pedal is released, the spring forces the cone to its seat (Fig. 31). In order to support it, the end of the shaft carrying the cone projects into the hub of the fly wheel, where it rests in a bearing, this arrangement in no manner preventing the two parts from acting independently of each other.

In the reversed type of friction-cone clutch a funnel-shaped ring is bolted to the rim of the fly wheel and forms the seat, the cone fitting inside of it (Fig. 31). Depressing the pedal moves the cone toward the fly wheel instead of away from it, as in the regular type, and while there is no difference in the effect of one as against the other, the reversed type is more compact.

Fig. 31.—Friction Cone Clutches.

The multiple-disk clutch, which is rapidly coming into use, depends on the friction between the flat surfaces of metal when pressed together. An experiment illustrating this is to place a silver dollar between two half dollars, and to press them together between the thumb and finger. It will be found that a light pressure is sufficient to produce friction that will make it difficult to revolve the large coin between the smaller ones.

The parts of a simple form of multiple-disk clutch, as shown in Fig. 32, are a flange on the engine shaft, a smaller flange with a square-hole, square-shaft arrangement on the transmission shaft, and large and small rings placed alternately. The large rings are driven by the large flange, fitting loosely on pins, or studs, projecting from it, and the small rings are similarly attached to the transmission shaft. The openings in the large or driving rings are large enough to contain the studs carrying the small rings, so that when the parts are assembled the outer surfaces of the small or driven rings are in contact with the inner surfaces of the driving rings. A heavy coil spring is arranged to press the small flange toward the flange on the engine shaft, binding the rings that it carries between the driving rings, and the latter are often faced with leather to increase the friction between them. When the small flange is released from the pressure of the spring by depressing the pedal, the driving and driven flanges with their rings are independent of each other, and the engine may run free while the transmission shaft is stationary or revolving. A clutch of this type is incased and runs in oil, which prevents the rings from gripping suddenly; when the pressure of the spring is applied, the oil is gradually squeezed out from between them, and the slipping of the driving and driven rings is reduced as they are forced into contact.

Fig. 32.—Multiple Disk Clutch.

An internal-expanding clutch consists of a broad ring, or drum, against the inner surface of which bear two pieces of metal shaped to fit. The pieces of metal, or shoes, are pivoted together at one end so that they may be moved in or out, after the manner of the handles of a pair of scissors; when open they bear against the inside surface of the drum, and when closed they are free from it. The drum is attached to the engine shaft and the shoes to the transmission shaft, the friction between them being so great that the transmission shaft is carried around as the drum revolves. The shoes are kept in contact with the drum by a coil spring, the depression of a pedal releasing them from its pressure.

CHANGE-SPEED MECHANISM

The change-speed mechanism, to which the clutch transmits the power of the engine, is to the engine what a block and tackle is to a man who lifts a heavy weight, and is necessary because of the varying resistance to the movement of the car in traversing steep, rough hills and smooth avenues. A change-speed mechanism may be defined as an arrangement by which the relative number of revolutions of the crank shaft and driving wheels may be altered to suit conditions. If the driving wheels revolve but once while the crank shaft makes twelve revolutions, the car will move at one sixth the speed that it would have if the wheels revolved once to every two revolutions of the crank shaft, but it will have six times the ability to overcome the resistance presented by a hill or sandy road.

If a gasoline engine were so connected that the relative number of revolutions of the crank shaft and wheels could not be changed, a slowing down of the car through the resistance presented by a rough hill would slow the engine to correspond, and as speed is an important factor in the power that the engine delivers, it would be prevented from doing the work of which it is capable at the time when it was most necessary. By means of the change-speed mechanisms in most general use, the relative number of revolutions of the crank shaft to one of the wheels may be varied to from two to eighteen, the former giving the car high speed over a smooth road, and the latter slow speed, but greater ability to overcome hills and heavy roads.

To attain this result, gears are used. If two gears having the same number of teeth are in mesh, they will make the same number of revolutions, and the force with which the driven gear will revolve will be the same as that of the driving gear, less the friction of the teeth. If the driven gear has twice the number of teeth of the driving gear, it will revolve at half the speed, but with twice the force.

The forms of change-speed mechanism most largely used are based on this principle, which is so applied that the gear driven by the engine may be in mesh with a gear that has many more teeth and revolves much slower in consequence, or a gear that has possibly one and a half times the number of teeth, or a gear that has the same number of teeth, and therefore revolves at the same speed. The sliding-gear mechanism takes its name from the arrangement by which the changes in the combination of gears is effected by sliding them along a shaft, to mesh with other gears on a shaft driven by the engine.

Fig. 33.—Sliding Gear—Progressive Type. A, Sleeve driven by engine; B, gear on sleeve; C, gear on countershaft; D, low-speed gears; E, second-speed gears; F, idler for reverse; G, clutch for high speed; H, connected to rear wheels; J, gears sliding on square shaft.

The driver changes the gears by moving a lever that in the progressive type moves forward by degrees to move the car on the slow speed, the intermediate speeds, and the high. A typical arrangement of the progressive type of sliding change-speed mechanism is shown in Fig. 33. The power of the engine is transmitted to a short, hollow shaft, called a sleeve (A), which carries a gear (B) that is in permanent mesh with a gear (C) on the end of the countershaft. Parallel to the countershaft is another shaft, one end of which is held in a bearing in the hollow sleeve; while the sleeve supports this shaft, the two may revolve independently of each other. The second shaft is square, or of such a construction that while the two gears that it carries may slide along, they revolve with it. The gears on the square shaft are of different sizes, and in sliding on it come successively into mesh with gears carried on the countershaft. Because of the gears between them, the countershaft revolves when the engine revolves the sleeve; but the speed of the square shaft depends on the combination of gears in mesh between it and the countershaft. When the sliding gears are in such a position that they are not in mesh with the countershaft gears, the square shaft is independent of the countershaft, and may revolve or be stationary, the gears then being in the neutral position. When the sliding part is moved so that its largest gear is in mesh with the smallest of the countershaft gears (D), the square shaft will revolve at a slower speed than the countershaft, because its gear is larger than the one driving it. Again sliding the moving part will separate these gears, and bring the next pair (E) into mesh, the square shaft then moving at a higher speed, but still slower than the countershaft because of the difference in the size of the gears. Sliding the moving part still farther along the shaft will disengage the second-speed gears and engage the high speed, in which the square shaft revolves at the speed of the sleeve and crank shaft, this being effected by locking the moving part to the sleeve by means of a clutch (G). This clutch consists of several fingers projecting from the moving part, corresponding to the spaces between similar fingers on the end of the sleeve. The locking together of the square shaft and sleeve gives what is known as the direct drive, which is of comparatively recent development; many designs of sliding gears still use a third pair of gears which, being of the same size, give the square shaft the speed of the crank shaft. By the use of the direct drive, the power of the engine is directly applied to the square shaft, avoiding the loss that occurs through the friction of the teeth of the gears.

The revolution of the square shaft is transmitted to the driving wheels, the speed of the car therefore corresponding to the speed at which the square shaft is driven by the gear combinations between it and the countershaft. To obtain the reverse, which enables the car to be backed without reversing the engine, a third gear is introduced between the low-speed gears of the square shaft and countershaft. When two gears are in mesh, they revolve in opposite directions, but when one of them is in addition meshed with a third gear, the first and third will revolve in the same direction, and opposite to the direction in which the middle gear revolves. When the car is going forward, the square shaft and countershaft revolve in opposite directions, but when the reverse gear is introduced between them, the square shaft is revolved in the same direction as the countershaft, reversing the rotation of the driving wheels.

The ends of the teeth of the gears are chisel-shaped, instead of being flat, as in ordinary gears, so that they will go into mesh easily.

The greatest economy in the operation of a gasoline engine results from its running at as nearly constant a speed as possible, and the gear is therefore changed when the resistance of the road to the movement of the car decreases the speed of the engine, or permits it to increase.

The selective type of sliding change-speed mechanism as shown on Fig. 34 is in use on many of the high-grade cars, and its control differs from that of the progressive type described in that the lever has only a short movement forward and back, and in addition may slide sideways. To the lower end of the lever is attached a shaft that rocks in its bearings as the lever is moved forward or back, and slides lengthways when the lever is moved toward or away from the car.

Fig. 34.—Selective Type. A, Sleeve driven by engine; B, fixed gear on sleeve; C, fixed gear on countershaft; D, low-speed gears; E, second-speed gears; F, third-speed gears; G, clutch for direct drive; H, rod and arm for third-speed and direct drive; J, rod and arm for low and second speeds; K, rod and arm for reverse, L, rocking-shaft finger in groove; M, guide plate and control lever; N, bevel gears on square shaft and jack shaft; O, idler gear for reverse.

The arrangement of the countershaft and square shaft is the same as in the progressive type, but there are two sets of sliding gears instead of one, and these are moved by means of arms that extend from rods sliding in bearings at the side of the gear case. When these rods are slid endways, the gears attached to their arms slide on the square shaft to correspond, and go in or out of mesh with the countershaft gears. Across the ends of these rods are grooves, which when the gears are in the neutral position are in line with the rocking shaft attached to the control lever. From the inward end of the rocking shaft a finger (L) projects downward into the groove; when the grooves are in line, the rocking shaft may be slid endways, the finger passing from one groove to the next without affecting the rods. When the shaft is rocked, however, the finger in engaging one of the grooves slides the rod endways, shifting the gears controlled by its arm. Moving the control lever into such a position that it may enter the middle slots of the guide plate (M) slides the rocking shaft so that its finger projects into the groove of the central sliding rod (J), and if the control lever is then pushed forward so that it enters the front half of the slot, the sliding rod will be moved by the finger in the opposite direction, and the low-speed gears (D) will be brought into mesh. Bringing the lever back to the central position will separate the gears, and moving it to the back half of the slot will slide the same gears in the opposite direction, meshing the second-speed combination (E). Moving the control lever outward so that it is in line with the outside slot brings the finger into the groove of the sliding rod (H) that moves the third and high speeds (F and G), the latter being direct drive, and the reverse is obtained through the movement of the sliding rod (K) that is engaged by the finger when the control lever is in the inside slot. The movement of this rod brings a third gear (O) into mesh with the low-speed gears on the square shaft and countershaft, and the rotation of the countershaft is reversed.

While this type is in general use, the gears are often so arranged that the direct drive combination is reached when the control lever is in the third-speed position, the gears meshed by the fourth-speed position driving the square shaft at a still higher speed. This high speed can only be used for running under the best road conditions.

The advantages of the selective type over the progressive are the shorter movements of the control lever and the ability to pass from one speed to any other without the necessity of first meshing and unmeshing those between, or, in other words, there is a neutral position between every combination of gears, and from neutral any desired combination may be obtained directly without reference to the others.

In starting up a car fitted with either of these types of change-speed mechanism, it is necessary to withdraw the clutch before sliding the gears, and this is also necessary in changing from one combination to another. The square or corresponding shaft of a change-speed mechanism is always connected with the driving wheels, and is at rest when the car is standing. With the engine running, the countershaft will be revolving, and it will obviously be difficult to slide the stationary gear into mesh with a gear that is revolving. When the clutch is withdrawn, the countershaft moves only through momentum, and will be brought to a stop by the contact of the teeth of the sliding gear as that is moved against it, the two then easily going into mesh. The clutch is then thrown in slowly, and will bring the speed of the countershaft to the speed of the crank shaft.