TRACTOR PRINCIPLES

THE ACTION, MECHANISM, HANDLING,
CARE, MAINTENANCE AND REPAIR
OF THE GAS ENGINE TRACTOR

BY

ROGER B. WHITMAN

AUTHOR OF
“MOTOR CAR PRINCIPLES,”
“GAS ENGINE PRINCIPLES,”
“MOTOR-CYCLE PRINCIPLES,”
ETC.

FULLY ILLUSTRATED

D. APPLETON AND COMPANY
NEW YORKLONDON
1920

COPYRIGHT, 1920,
BY
D. APPLETON AND COMPANY

PRINTED IN THE UNITED STATES OF AMERICA

FOREWORD

The tractor of to-day is built in almost as many types and designs as there are tractor makers, and is far from being as standard as the automobile. There are tractors with one driving wheel, with two driving wheels, with three and with four, as well as three arrangements of the crawler principle; there are two-wheelers, three-wheelers and four-wheelers; tractors that are controlled by pedals and levers and tractors that are driven by reins.

Thus if a man who is competent to handle and care for one make is given another make to run, he may be entirely at a loss as to how it works and how it should be handled.

It is the purpose of this book to explain and describe all of the mechanisms that are in common use in tractor construction, to the end that the reader may be able to identify and understand the parts of whatever make he may see or handle.

CONTENTS

ILLUSTRATIONS

TRACTOR PRINCIPLES

CHAPTER I
TRACTOR PRINCIPLES

While tractors and automobiles are the same in general principles, there is a wide difference between them in design, construction, and handling, due to the differences in the work that they do and in the conditions under which they do it.

An automobile is required to move only itself and the load that it carries. While it can run over rough roads, these must be hard enough to support it; on soft ground it will sink in and be unable to get itself out. It can make great speed over smooth, level roads; but only rarely do road and police conditions permit it to run its fastest for more than a few minutes at a time. For the greater part of its life it develops only a portion of the power of which it is capable.

A tractor, on the other hand, is intended not to carry, but to haul. It must run and do its work on rough hillsides, soft bottoms, or any other land where it is required to go. Instead of developing speed it develops pulling power, and must be able to develop its full power continuously.

Appearance and comfort count for a great deal in an automobile, and much attention is devoted to making it noiseless and simple to manage. These things do not apply in a tractor, which is a labor-saving and money-making machine, valuable only for the work that it can do. There is no question of upholstery or nickel-plating; all that is wanted is a machine that will do the required work with the least possible cost of operation.

As is the case with any kind of machine that is purchased as a money-maker, its cost should be as low as is consistent with its ability to do its work. Any extra cost for accessories, or finish, or other detail, is wasted unless it permits the machine to do more work, or, by making the operator more comfortable, allows him to run the machine for a longer stretch of time or with greater efficiency.

It may be taken for granted that any tractor will run and will do its work with satisfaction, provided it is sensibly handled and cared for. Far more troubles and breakdowns come from careless handling and from neglect than from faulty design and material. A tractor that is running and doing its work is earning a return on the money invested in it; when it is laid up for repairs there is not only a loss of interest on the investment, but a loss of the value of the work that it might be doing.

To keep a tractor running is a matter only of understanding and of common sense; common sense to realize that any piece of machinery needs some degree of care and attention, and understanding of where the care and attention should be applied. The more thoroughly a tractor operator understands his machine, the more work he will be able to get out of it, and the more continuously it will run. This is only another way of saying that understanding and knowledge pay a direct return in work done and money earned.

In the early days of the automobile there were as many types of cars as there were manufacturers. As time has gone on, the unsatisfactory ideas have been weeded out, and automobiles have approached what may be called a standard design.

At the present time, tractor designs are varied, and it is hardly possible to speak of any type as standard. The reason for this lies in the fact that many manufacturers start with a design for one special part, and build the tractor around it.

For example, a manufacturer may develop a method of driving the wheels that he feels is especially good for tractor work. In applying it he may find that the engine must be so placed on the frame that when the power pulley is in position the belt will interfere with the front wheels unless they are small; he therefore uses small front wheels, and advocates them for tractors.

Another manufacturer with a patent steering gear may be able to place the power pulley so that there is ample clearance for the belt; he finds that by using high front wheels he can get a better support for the frame, and therefore claims that high front wheels are an advantage.

Other designs may be based on having three wheels, or two; advantages are claimed for each type, and each type undoubtedly has them.

The selection of a tractor is based on one’s own experience or on that of neighbors, or on the ability of the salesman to bring out the advantages of the make that he sells; but when the tractor is bought and delivered, its ability to do the work promised for it depends solely on the care with which it is handled and looked after.

Whatever the design of a tractor may be, there are certain parts that it must have in order to do the work required of it. These parts, or groups of parts, are as follows:

Engine.—This furnishes the power by which the tractor operates.

Clutch.—By means of a clutch the engine may be connected with the mechanism, so that the tractor moves, or it may be disconnected, so that it may run without moving the tractor.

Change Speed Gear.—As will be explained in later chapters, an engine, in order to work most efficiently, should run at a fixed speed; the tractor should be able to run fast or slow, according to conditions. A change speed gear is therefore provided, by which the speed of the tractor may be changed, although there is no change in the speed of the engine.

Drive.—The drive is the mechanism that applies the power of the engine to the wheels, and makes them turn.

Differential.—When a tractor makes a turn, the outside wheels cover a larger circle than the inside wheels, and therefore must run faster in order to get around in the same time. It is usually the case that the power of the engine is applied to both driving wheels; if both were solid on the axle, like the wheels of a railroad car, one would be forced to slip when making a turn, which would waste power. By applying a differential, the engine can drive both wheels, but the wheels may run at different speeds when conditions require it.

The clutch, change speed gear, drive and differential form the transmission.

Steering gear.—By means of the steering gear the direction in which the tractor moves may be changed.

Supports.—A tractor moves on broad-tired wheels, or on crawlers, which are so formed that they grip the ground and do not slip. They give so broad a support that even on soft ground the weight of the tractor will not pack the soil sufficiently to injure it as a seed bed.

Frame.—The frame is the foundation of the tractor, and holds the parts in the proper relation to each other. It is usually made of channel steel, the parts being bolted to it; in some tractors, however, the parts are so attached to each other that they form their own support, and no other frame is needed.

Tractor manufacturers make these parts in different ways; all accomplish the same result, but do it by different methods. The main principles are much the same, and should be known and understood. They are described and explained in the succeeding chapters.

CHAPTER II
ENGINE PRINCIPLES

The working part of a tractor is the engine; it is this that furnishes the power that makes the machine go.

The engine gets its power from the burning of a mixture of fuel vapor and air. When this mixture burns, it becomes heated, and, as is usual with hot things, it tries to expand, or to occupy more room.

The mixture is placed in a cylinder, between the closed end and the piston; it is then heated by being burned, and, in struggling to expand, it forces the piston to slide down the cylinder. This movement of the piston makes the crank shaft revolve, which in turn drives the tractor.

The first step in making the engine run is to put a charge of mixture into the cylinder, and it is clear that if the burning of the charge is to move the piston, the piston must be in such a position that it is able to move. When the mixture is burned, the piston must therefore be at the closed end of the cylinder.

After the charge of mixture has been burned, the cylinder must be cleared of the dead and useless gases that remain, in order to make room for a fresh charge.

The charge of mixture is drawn into the cylinder just as a pump sucks in water. At a time when the piston is at the closed end of the cylinder, a valve is opened connecting the space above the piston with the device that forms the mixture; then by moving the piston outward, mixture is sucked into the space above it. When the piston reaches the end of its stroke the cylinder has been filled with mixture, and the valve then closes.

It would be useless to set fire to the mixture at that time, for the piston is as far down the cylinder as it can be, and pressure could not move it any farther. To get the piston into such a position that the expanding mixture can move it, it is forced back to the closed end of the cylinder. This squeezes or compresses, the cylinderful of mixture into the small space, called the combustion chamber, between the piston and the cylinder head.

If the mixture is now burned, the piston can move the length of the cylinder, and in so doing it develops power.

The cylinder is cleared of the burned and useless gases by opening a valve and pushing them out by moving the piston back to the inner end of the cylinder. When this has been done, the valve is closed, and, by opening the inlet valve and moving the piston outward, a fresh charge is sucked in, and the several steps of the gas engine cycle are repeated.

The name cycle is given to any series of steps or events that must be gone through in order that a thing may happen. Thus the empty shell must be taken out of a gun and a fresh cartridge put in before the gun can be fired again, and that series of steps might be called the gun cycle.

The gas engine cycle requires the piston to make four strokes. An outward stroke sucks in a charge of mixture, and an inward stroke returns the piston to the firing position and compresses the charge. Then comes the outward stroke when the piston moves under power, followed by the inward stroke that clears the cylinder of the burned gases.

For every stroke of the piston the crank shaft makes a half-revolution; the crank shaft therefore makes two revolutions to four strokes of the piston and to each repetition of the gas engine cycle.

Of these four strokes of the piston only one produces power. The other three strokes, called the dead strokes, are required to prepare for another power stroke.

A gas engine cylinder thus produces power for only one quarter of the time that it runs. This is one of the striking differences between the gas engine and the steam engine, for the piston of a steam engine moves under power all of the time that the engine runs.

A one-cylinder gas engine must have something to make the piston go through the dead strokes, for otherwise the piston would stop at the end of the power stroke; the piston is kept in motion by heavy flywheels attached to the crank shaft. These, like any object, try to continue in motion when once they are started; a power stroke starts the crank shaft revolving and its flywheels keep it going.

Thus, the piston drives the crank shaft during the power stroke, and the crank shaft drives the piston during the dead strokes.

To start an engine, the crank shaft is revolved to make the piston suck in a charge of mixture and compress it; then the charge is burned, the power stroke takes place, and the engine runs.

A clear idea of what goes on inside of the cylinder is quite necessary in order to take proper care of an engine and to get the best work out of it. The following description applies to any cylinder, for the action in all cylinders of an engine is the same.

Inlet Stroke.—During the inlet stroke ([No. 1, Fig. 1]), the piston moves outward; the inlet valve is open, and the exhaust valve is closed. This movement of the piston creates suction, and if there are leaks in the cylinder, air will be sucked in and will spoil the proportions of the charge. This will prevent the proper burning of the mixture, and the engine will lose power.

The piston moves at such high speed that the mixture cannot enter fast enough to keep up with it; mixture is still flowing in when the piston reaches the end of its stroke, and even when it begins to move inward on the next stroke. The more mixture there is in the cylinder, the more powerfully the engine will run; the inlet valve is therefore held open for as long a time as the mixture continues to enter.

Fig. 1.—The Gas Engine Cycle

In slow-speed 1-cylinder and 2-cylinder engines the valve closes when the piston reaches the end of its stroke; on high-speed engines the valve does not close until the piston has moved ¼ inch or ½ inch on the compression stroke.

Compression Stroke.—During the compression stroke ([No. 2, Fig. 1]) the piston moves inward, and both valves are closed. This movement places the piston in position to move outward on the power stroke. As the outlets to the cylinder are closed, the charge of mixture cannot escape, and is therefore compressed into the space between the piston and the cylinder head when the piston is at the inner end of its stroke. This space is usually about one quarter the volume of the cylinder; the charge is therefore compressed to about one quarter of its original volume.

This compression of the charge is very important in the operation of the gas engine, and any interference with it will make the engine run poorly.

In the first place, it improves the quality of the charge, and makes it burn very much better. When the charge enters the cylinder, the fuel vapor and air are not thoroughly mixed; much of the fuel is not turned into vapor. By compressing the charge it becomes heated; this vaporizes the fuel, and vapor and air become thoroughly mixed.

Compression also increases the power. Suppose that the cylinder contains a quart of mixture which, when heated, will expand to a gallon. If this quart of mixture is compressed to a half pint, it will not lose its ability to expand to a gallon, and will exert more pressure in expanding from a half pint to a gallon than from a quart to a gallon.

A leaky cylinder will cause a further loss of power because some of the charge will escape during the compression stroke, which will leave less to be burned and to develop power.

Ignition.—Setting fire to the charge of mixture is called the ignition of the charge, and it takes place close to the end of the compression stroke. To get the greatest power, all of the mixture should be on fire and heated most intensely as the piston begins the power stroke.

When the mixture is set on fire, it does not explode like gunpowder, but burns comparatively slowly; the charge is ignited by an electric spark, and the flame spreads from that point until it is all on fire. In order to give the flame time to spread, the spark passes sufficiently before the end of the compression stroke to have the entire charge on fire as the power stroke begins. This is called the advance of the ignition.

The flame takes the same time to spread through the charge when the engine is running fast as when it is running slow. Therefore if the engine is speeded up, the spark must be advanced, for otherwise the piston would be on the power stroke before the flame would have time to spread all through the mixture.

When the engine is slowed down, the spark must have less advance, or must be retarded, for, if it were not, the charge would all be in flame, and exerting its full pressure, before the piston reached the end of its compression stroke.

The subject of ignition, which is of great importance, is covered more fully in [Chapter VI].

Power Stroke.—During the power stroke ([No. 3, Fig. 1]) the piston moves outward, and both valves are closed. As it begins, the mixture is all on fire, and great pressure is exerted against the piston.

As the piston moves outward the combustion space becomes larger, and the gases obtain the room for expansion that they seek. As they expand, the pressure that they exert becomes less. By the time the piston is three quarters the way down the power stroke, the pressure is so reduced that it has little or no effect; the gases are still trying to expand, however, so the exhaust valve is opened at that point, and they begin to escape.

Exhaust Stroke.—During the exhaust stroke ([No. 4, Fig. 1]) the piston moves inward and the exhaust valve is open. This movement of the piston pushes the burned gases out of the cylinder, and it is clear that the more thoroughly the cylinder is emptied of them, the more room there will be for a fresh charge.

In high-speed engines the gases cannot escape as fast as the piston moves; they are still flowing out when the end of the stroke is reached. Therefore the valve is closed, not at the end of the stroke, but when the piston has moved about ⅛ inch outward on the inlet stroke. The inlet valve opens as the exhaust valve closes.

It can be seen that through the inlet and compression strokes a leak will reduce the charge and so interfere with the production of full power. The piston must make a tight fit in the cylinder, the valves must seat tightly, and gaskets and other parts must be in proper condition.

Fig. 2.—1-Cylinder Power Diagram

[Figure 2] shows a power diagram for a 1-cylinder engine, in which the crank shaft moves under power during one stroke out of every four. An engine with two cylinders can be built so that first one cylinder applies power and then the other, in which case the crank shaft moves under power during two strokes out of every four.

Fig. 3.—2-Cylinder Power Diagram

[Figure 3] is a power diagram of an engine of this sort. If piston 1 is moving down under power, piston 2 is also moving down, but on the inlet stroke. The following stroke is exhaust in cylinder 1 and compression in cylinder 2, and cylinder 2 will then deliver a power stroke while cylinder 1 is on inlet. Thus the crank shaft will receive a power stroke, followed by a dead stroke; then another power stroke and another dead stroke, and so on.

There will be the disadvantage, however, that the pistons, moving up and down together, will cause vibration, which in the course of time will be likely to give trouble. To overcome this, a 2-cylinder engine can be built as indicated in [Figure 4].

In this engine the cranks project on opposite sides of the crank shaft instead of on the same side, as in [Figure 3]; the pistons thus move in opposite directions, and produce no vibration. Power will be unevenly applied, however, for both power strokes occur in one revolution, with two dead strokes in the succeeding revolution.

Fig. 4.—2-Cylinder Power Diagram, 180 Shaft

With piston 1 moving down on power, piston 2, moving upward, can only be performing compression or exhaust. If it is on compression, its power stroke will follow the power stroke of piston 1, while if it is on exhaust its power stroke will have occurred immediately before the power stroke of piston 1. In either case one power stroke follows the other, taking place in one revolution of the crank shaft, while in the following revolution both pistons will be performing dead strokes.

While there is no vibration from the movement of the pistons in this engine, the uneven production of power will produce vibration of another kind.

These two types may be built with the cylinders standing up or lying down; that is, they may be either vertical engines or horizontal engines. The double opposed engine, which is built only in horizontal form, is free from either kind of vibration, but has the disadvantage of occupying more room than the others. The cylinders, instead of being side by side, and on the same side of the crank shaft, are placed end to end, with the crank shaft between them, as shown in [Figure 5].

The pistons make their inward and outward strokes together, but in so doing they move in opposite directions. Thus every power stroke is followed by a dead stroke, as in the engine shown in [Figure 3], while the movement of one piston balances that of the other, as is the case with the engine shown in [Figure 4].

Fig. 5.—H. D. O. Power Diagram

Fig. 6.—4-Cylinder Power Diagram

In a 4-cylinder engine one power stroke follows another without any dead stroke intervals, which, of course, makes the crank shaft revolve more smoothly and with a steadier application of power. The power diagram is shown in [Figure 6]; in studying this it should be remembered that if two pistons move in opposite directions, as in [Figure 4], one power stroke follows another, while if they move in the same direction, as in [Figure 3], there is an interval of one stroke between their power strokes.

The crank shaft of a 4-cylinder engine is so made that the middle pistons move in the same direction, and opposite to the end pistons. This construction has been found to make a smoother running engine than if pistons 1 and 3 moved one way while pistons 2 and 4 moved the other.

If piston 1 is on the power stroke, either piston 2 or piston 3 can follow, for they are moving in the opposite direction. If we say that piston 2 is the next, then piston 4 must be the third to give a power stroke, for it is the only one left that is moving in the opposite direction to piston 2. Piston 3 is thus the fourth to move under power, and it is followed by another power stroke by piston 1; the firing order is then said to be 1, 2, 4, 3.

If it is piston 3 that follows piston 1, piston 4 will again be the third to produce power, and piston 2 will be the fourth. The firing order will then be 1, 3, 4, 2. There is no other order in which a 4-cylinder engine can produce power, and there is no choice between them.

The firing order of an engine is established by the manufacturer, and depends on the order in which the valves are operated.

CHAPTER III
ENGINE PARTS

The foundation of an engine is the base, which supports the bearings in which the crank shaft revolves, and to which the cylinders are attached. The cylinders of tractor engines are made of cast-iron, and the cylinder heads, which close the upper ends of the cylinders, are usually in a separate piece, bolted on. The joint between the cylinders and the cylinder head is made tight by placing between them a gasket of asbestos and thin sheet metal.

The crank shaft has as many cranks, or throws, as the engine has cylinders. Crank shafts for 2-cylinder engines are shown in [Figure 7]; the upper one is for an engine of the type shown in [Figure 3], with pistons moving in the same direction. With both cranks projecting from one side the shaft is out of balance, so balance weights are attached to the opposite side.

Fig. 7.—2-Cylinder Crank Shaft

The other shaft shown in [Figure 7] does not need balance weights, for one crank balances the other. A four-cylinder crank shaft, [Figure 8], is also in balance.

Fig. 8.—4-Cylinder Crank Shaft

Fig. 9.—Half of a Plain Bearing

Crank shafts revolve in main bearings, which are set in the engine base. In tractor engines these are usually plain bearings, a half of such a bearing being shown in [Figure 9]. This is a bronze shell lined with a softer metal, making an exact fit on the shaft; with the two halves in place, the shaft should turn freely, but without looseness or side play. The grooves shown are to admit lubricating oil.

Fig. 10.—Connecting Rod Bearings

Fig. 11.—Piston Complete and in Section

The piston is attached to the crank shaft by a connecting rod, which is illustrated in [Figure 10]. Pistons are shown in [Figures 11] and [12]; they are made as light as is consistent with the pressure that they must bear, and are hollow, and open at the lower end.

The piston is attached to the connecting rod by a wrist pin, or piston pin, which is a shaft passing through it from side to side, and also through the bearing in the upper end of the connecting rod. The connecting rod swings on the wrist pin in following the rotation of the crank shaft, and its attachment to the wrist pin must permit this without being loose.

The bearings at the two ends of a connecting rod are usually adjustable, so that wear can be taken up; some of the methods of doing this are illustrated in [Figure 10]. In A, the wrist pin bearing is a plain tube, ground to an exact fit; when it is worn it must be replaced. In B, the bearing is split, and the ends are drawn together by a bolt to the correct fit. The bearing in C is in two parts, held together by a U-shaped bolt, while in D the two parts are held together by a cap bolted to the end of the connecting rod. In E, the end of the connecting rod is a square loop enclosing the two parts of the bearing; the parts are held in the proper position by a wedge adjusted by screws.

The crank shaft bearing of the connecting rod [shown in F] is in two parts which are hinged together. G, H, and K show the forms usually used in tractor engines, which consist of two parts bolted together.

Fig. 12.—Wrist Pin Fastenings

The wrist pin is usually firmly attached to the piston, so that the connecting rod swings on it; methods of securing the wrist pin are shown in [Figure 12], the wrist pin being held in supports cast in the piston. In A, the wrist pin is held by two set screws, and in B, by pins passing through it. The wrist pin shown in D is hollow, as is very common, and a bolt passes through part of the piston and into the wrist pin.

In the construction shown in C the wrist pin is secured to the connecting rod and moves in bearings in the piston. In E, a ring fitting in a groove around the piston prevents the wrist pin from moving endways.

The engine must usually be taken to pieces in order to get at the wrist pin; lock nuts, lock washers or cotter pins are always used to prevent the trouble that would be caused if the wrist pin worked loose.

A leak-proof joint between the piston and the cylinder is made by means of piston rings that fit in grooves around the piston, as shown in [E, Figure 12]. Piston ring grooves are shown in [Figure 11]. Piston rings are not solid, but are split so that they are elastic; they fit snugly in their grooves, and tend to spring open to a greater size than the cylinder. This causes them to maintain a close fit against the cylinder, and the gases are prevented from leaking past.

Fig. 13.—Valve

Each cylinder is provided with two valves: the inlet valve that admits fresh mixture and the exhaust valve through which the burned gases escape. These valves are metal disks with funnel-shaped edges fitting into funnel holes. A valve and its stem are shown in [Figure 13] and also in [Figure 15].

Fig. 14.—Action of a Cam

A valve is opened at the proper time by a cam, and closed by a spring. A cam is a wheel with a bulge on one side, so that its rim is eccentric to its shaft, as illustrated in [Figure 14], which shows a cam in three positions of a revolution. A rod resting on the rim of the cam is moved endways as the bulge passes under it, and the valve is operated by connecting it with the rod.

A valve is opened once during two revolutions of the crank shaft; therefore the cam cannot be placed on the crank shaft, for, if it were, the valve would be opened every revolution. The cam is placed on a separate shaft which is driven by the crank shaft at half its speed. This is usually done with gears, a gear on the crank shaft meshing with a gear on the cam shaft having twice as many teeth; the crank shaft gear must make two revolutions in turning the cam shaft gear once.

The valve in [Figure 13] is held on its seat by a spring. The cam bears against the end of the valve stem, and as it revolves its bulge forces the valve stem and valve to move endways and thus to uncover the valve opening.

As the movement of the piston depends on the crank shaft, the valve can be made to open at the right time by a proper setting of the gears that drive the cam shaft.

The length of time that the cam will hold the valve open depends on the shape of the bulge of the cam. It can be seen that the pointed cam of [Figure 13] will not hold the valve open for as long a time as the flat-end cam of [Figure 14].

Fig. 15.—“Twin City” Tractor Engine

In the design shown in [Figure 13] the cam bears directly against the end of the valve stem, the cam shaft in this case lying along the cylinder head. In the construction shown in [Figure 15] the valves are not placed in the cylinder head, but are in an extension or valve pocket projecting from the combustion chamber; this cam shaft is near the crank shaft. It would not be practicable to make the valve stem long enough to reach down to the cam, so a length of rod, called a push rod, or tappet, is placed between them; the cam moves the push rod and the push rod in turn moves the valve. This is a construction frequently used for automobile engines.

Fig. 16.—“Hart-Parr” Valve Mechanism

In tractor engines the cam shaft is usually placed near the crank shaft, as in [Figure 15], and the valves are in the head, so that a valve moves in the opposite direction to the movement of the push rod. This requires still another part to be used, called the rocker arm. It is shown in [Figure 16]. It is a short bar, pivoted at or near the center, with one end at the push rod and the other at the valve stem. When it is moved by the push rod it in turn moves the valve.

Valves operated by push rods and rocker arms are also shown in [Figures 17], [18] and [19]; Figure 18 is a single-cylinder horizontal engine, while [Figure 19] is a horizontal double opposed engine, in which one cam operates a valve in each cylinder. [Figure 20] shows the valve mechanism of a vertical engine in which all parts, including the rocker arm, are enclosed to protect them from dust, and so they can run in oil.

A small space is always left somewhere between the cam and the valve stem, to give the valve stem room to lengthen, which it will do when it gets hot. If this space were not left, the valve stem, in lengthening as it became hot, would strike the part next to it, and the valve would be lifted from its seat. This would cause the engine to lose power. This space must be kept properly adjusted, and instructions for this will be found in [Chapter XII].

Fig. 17.—“Hart-Parr” Engine

A valve is held against its seat by a spring, which must be compressed when the valve is opened. If this spring is too weak, it will not hold the valve tightly on its seat, while if it is too stiff, the cam shaft and other parts will be needlessly strained in compressing it.

Friction between the cam and the end of the valve stem or push rod would cause rapid wear if these parts were not of hardened steel, and kept well oiled. Still further to reduce wear, there is usually a roller on the end of the push rod, as shown in [Figure 16] and some of the other illustrations. [Figure 15] shows a construction in which the end of the push rod is a flat disk, which rotates as the cam comes into contact with it.

Fig. 18.—“Oil-Pull” Engine

When the mixture burns, the top of the piston, the cylinder head, and the walls of the combustion chamber become heated, and if it is not prevented they will get so hot that they will expand sufficiently to cause the piston to stick, or seize. The upper part of the cylinder is, therefore, provided with a cooling system that keeps these parts from getting overheated. Channels are provided through which water is circulated; the water takes the heat from the metal parts, becoming heated itself, and then passes to a cooler, or radiator, where it gives up the heat to currents of air.

In addition to the channels, or water jackets, around the cylinder, a cooling system includes the radiator, the connections, and usually a pump that keeps the water in motion.

Fig. 19.—Horizontal Double Opposed Engine

In some tractors, notably the Fordson, no pump is used; the water circulates because it is heated. This is called a thermo-syphon system. When the engine runs, the water in the cylinder jackets becomes heated; as hot water is lighter than cold water, it rises and flows out of the jackets to the radiator, its place being taken by cool water from the bottom of the radiator. This circulation continues as long as the water in one part of the system is hotter than the water in some other part of the system.

The lubrication of an engine is described and explained in [Chapter X].

Fig. 20.—“Monarch” Engine

CHAPTER IV
FUELS AND CARBURETION

In order that a thing may burn, it must be provided with oxygen. Oxygen is found in air, so it is usual to say that air is necessary in order that anything may burn.

To prove this, light a candle and place an empty bottle over it, upside down; in a very short time the oxygen in the bottle will be used up, and the flame will flicker and get smoky, and finally die out. If a card is laid on the chimney of a coal-oil lamp so that it covers the opening, that flame also will flicker, get smoky and go out.

In order to deaden the fire in a stove, the dampers are closed to prevent air from entering; the fire is kept alight by the very small quantity of air that leaks in below the fire-box. When the drafts are opened the fire will burn up brightly because a plentiful volume of air can then enter.

In a similar way, air must be used in a gas engine in order that the fuel may burn. It is not possible to mix air with a liquid; the first step in making a gas that will burn is, therefore, to turn the fuel, whether it is gasoline, kerosene, distillate, or other oil, into a vapor; this vapor is then mixed with air.

For good results it is very necessary that the vapor and air be in proper proportions. In the experiment with the candle and the bottle it was seen that as the air was used up, the candle flame became yellow and smoky: this is the effect of insufficient air. If there is not enough air in the mixture, part of the vapor will not be able to burn, and will only smoke.

If, on the other hand, there is too much air, the mixture, if it will burn at all, will burn slowly, and the extra volume of air will reduce the heat.

In a mixture of the proper proportions of air and fuel vapor, the burning, or combustion, will be very rapid, resulting in the sudden production of the greatest possible amount of heat. This, of course, is what is necessary if the engine is to produce its fullest power. With such a mixture, combustion will be complete before the piston has done more than start outward on the power stroke, and the greatest possible, or maximum, pressure will then be produced.

When a mixture burns slowly, the piston will have gone through much of the power stroke before combustion is complete, in which case a considerable part of the pressure that should have been applied at the beginning of the stroke will be wasted.

A mixture that is not correct will burn unevenly; it may burn better during one power stroke than during another, which will make the engine run unsteadily.

If the mixture has too much air in proportion to the amount of vapor, it is known as a thin mixture, or a lean or poor mixture. It burns so slowly that it is quite possible for the mixture that started burning before the beginning of the power stroke to continue burning through the exhaust stroke, and for enough flame to remain in the cylinder to set fire to the fresh charge that enters during the next inlet stroke. This will produce what is known as a backfire; that is, the mixture entering the cylinder will catch fire, and in burning will blow back through the open inlet valve. This is a dangerous condition, for the flame might spread to fuel dripping from the carburetor, or to the fuel tank.

A mixture that has not enough air is called a rich mixture; the air that is present will burn part of the vapor, while the rest will go out of the exhaust unburned, or will work past the piston into the oil in the crank case. This is wasteful of fuel.

The most serious result of a rich mixture, however, is in the production of carbon, and the carbonization of the engine. The flame of a rich mixture is smoky; the smoke of this flame, as is the case with smoke from all other sources, is composed of fine particles of carbon, or soot. These particles of carbon will deposit on all parts of the combustion space: on the top of the piston, on the valves, on the spark plugs, and on the inner wall of the cylinder head. At first it is gummy, but it rapidly hardens and forms a crust that must be scraped off with a steel tool.

Carbon in an engine will reduce the power through causing preignition, or, in other words, by setting fire to the fresh charge before the proper point in the stroke. The heat of the combustion will cause the carbon deposit to become so heated that it will glow, these glowing particles being sufficient to ignite the incoming fresh charge. The remedy for this condition is to remove the carbon, which is usually done by taking off the cylinder head and scraping away the deposit.

It may be added that carbon is also formed by the use of too much lubricating oil, as will be explained in the chapter on lubrication.

Thus it is seen that if the engine is to run properly, and is to be kept in good condition, the proportions of the mixture must be very carefully maintained.

The mixture is formed in a carburetor, or mixer. This is, roughly, in the form of a tube through which air is sucked during the inlet stroke; projecting into it is a fine tube called a spray nozzle through which the fuel enters. In action it is somewhat similar to the atomizer that is used for spraying the nose and throat. By forcing the fuel to flow rapidly through this small tube it comes out in the form of spray, and the tiny drops are picked up by the current of air and are carried into the cylinder.

It is much easier to form a mixture of gasoline than of kerosene or distillate, because gasoline vaporizes more readily at ordinary temperatures. If saucers of gasoline and kerosene are placed in the sun, the gasoline will evaporate rapidly and completely, leaving only a faint oily deposit. The kerosene, on the other hand, will evaporate slowly, and much of it will not evaporate at all.

To make kerosene and distillate evaporate completely, they must be heated, just as water must be heated to make it evaporate.

In the case of a carburetor for gasoline, the current of air needs only to be warmed; the spray of gasoline will evaporate on coming into contact with the warmed air, and much of it will enter the cylinder as vapor. In order to evaporate kerosene and distillate much more heat must be provided, and it is usually considered necessary to heat not only the current of air, but the liquid fuel as well. Methods of doing this will be explained in the next chapter.

Fig. 21.—Principle of Carburetor

When kerosene or distillate is used, there are conditions that make it necessary to add water vapor to the mixture, which prevents the overheating of the cylinder and reduces the deposit of carbon. The difficulty of making a complete vapor of kerosene and distillate results in a tendency on the part of these fuels to carbonize the cylinders; the use of water aids in keeping the cylinders clean.

The general principle of a carburetor is shown in [Figure 21], one drawing illustrating conditions when the inlet valve is closed and the other when it is open. It shows an engine cylinder connected with an inlet pipe or mixing chamber, through which there is a swift flow of air during an inlet stroke.

Projecting into the intake pipe is the spray nozzle, which is connected with a small chamber containing fuel; inside of this chamber is a float, usually made of cork, although it is sometimes a light metal box. The fuel is intended to fill the chamber to a certain height, at which the valve will be closed by the float rising on the fuel. This level is such that the fuel does not quite reach the tip of the spray nozzle.

During the compression, power, and exhaust strokes, the fuel stands at this level, for it cannot run out of the spray nozzle, and the float holds the valve closed. As soon as the inlet valve opens, air rushes through the intake pipe and sucks fuel out of the spray nozzle. This, of course, takes fuel out of the float chamber; the float in sinking opens the valve, and enough fuel enters to restore the level.

The fuel comes out of the nozzle in the form of fine spray; it is in such small drops that it evaporates quickly, and the resulting mixture of fuel vapor and air passes into the cylinder. By using a spray nozzle of the proper size, any desired proportion of fuel and air may be obtained.

If an engine runs at a single speed, a carburetor as simple as this one would be satisfactory, for if the suction is always the same, there will be little or no change in the proportions of the mixture that is formed.

To get the best results, the proportions of fuel vapor and air should be the same for all running speeds of the engine. The proportions of the mixture, however, depend on the violence of the suction, which changes as the engine speed changes, becoming greater as the speed increases. The simple carburetor illustrated in [Figure 21] can be adjusted to give a correct mixture for any particular speed, but will be out of adjustment for any other speed.

The speed of a 1-cylinder engine does not change very greatly; it is built to run at practically a constant speed, and a simple carburetor is satisfactory for it. The speed of engines with a greater number of cylinders may be greatly changed, and the carburetor must be so made that it will give the same proportions of vapor and air at low speed as at high.

In the simple carburetor described, the speeding-up of the engine will result in a greater rush of air through the intake pipe, which in turn will suck out a much greater quantity of fuel. If the carburetor is adjusted to give the proper quantity of fuel for the air that passes at low speed, at high speed it will give far more fuel than will be required by the quantity of air that then passes. Thus at high speed the mixture will be too rich.

If, on the other hand, this carburetor is adjusted to give a proper mixture at high speed, too little fuel will be sucked out when the engine runs slowly, and the mixture will be too lean.

A carburetor must thus have an additional device that will keep the mixture correct, regardless of the speed at which the engine runs. This is sometimes done by changing the size of the spray nozzle so that a greater or less quantity of fuel flows out, but more usually by permitting an extra quantity of air to enter the carburetor as the engine speeds up. This is done with an extra air intake, the principle of which is illustrated in [Figure 22].

As will be seen, this carburetor has two openings for air, one being the main air inlet and the other the extra air inlet. The latter is an opening provided with a valve which is held on its seat by a spring. The suction created by an inlet stroke is exerted in the carburetor, but at low speed is not sufficient to suck the extra air valve from its seat. Air then enters only through the main air inlet, and the spray nozzle is adjusted to give the proper proportion of fuel.

Fig. 22.—Principle of Extra Air Inlet

As the engine speed increases the mixture becomes richer; but there is also an increase in suction, which becomes strong enough to pull the extra air valve from its seat. This provides another opening into the carburetor, through which enough air enters to keep the mixture in proper proportion. The higher the speed of the engine the more the valve will open, and the greater will be the quantity of air admitted.

In order to get the fullest power from an engine, the carburetor is built to give its most perfect mixture at the usual working speed. This will be the speed at which the engine will run under ordinary conditions. As the engine will run at this speed most of the time, the carburetor should then deliver its best mixture on the least possible quantity of fuel.

As an engine is run at low speed so little of the time, it is not necessary that the mixture should then be so perfect or that the fuel should be used so economically.

The design of a carburetor is a complicated matter, because the production of mixture is due to the flow of air, which is a very changeable thing. On a cold, damp day, the air will be heavier and denser than on a day that is hot and dry, and different quantities of fuel will be necessary for the formation of the mixture. The carburetor manufacturer cannot make a commercial carburetor that will take care of such a difference as this; he strikes an average that gives good general results, and expects the user to change the adjustments when weather and temperature make it necessary.

The formation of the mixture is affected by the condition of the engine. When all of the parts of the engine are tight, the suction in the carburetor is more violent than when there is a leakage of air past the piston rings, or through a leaky valve or spark plug.

On a dry, hot day the fuel evaporates much more readily than on a day that is cold and damp; more of the fuel that flows out of the spray nozzle will be vaporized and the formation of the mixture will be easier. On a cold, damp day the fuel will not vaporize in the carburetor to any extent, and much of it will pass to the cylinder in drops that even there will not vaporize in time to form a mixture. In order to assure the vaporization of enough fuel to form a mixture under such conditions, the fuel and the air must be heated to a greater degree.

As the engine becomes heated up, more and more of the fuel will vaporize, and the amount flowing out of the spray nozzle may therefore be cut down.

With fuels like kerosene and distillate, which do not vaporize as readily as gasoline, it is not unusual to have them condense on the walls of the inlet pipe, which produces a condition known as loading. This condensation is similar to the sweating of an ice-water pitcher on a hot day. If an engine is running at a constant speed, loading does not make much difference, because the carburetor is so adjusted that it gives a proper mixture. If the engine is suddenly speeded up, however, the greater rush of air will pick up the condensed fuel, and the mixture will instantly become too rich, continuing so until this extra supply of fuel is used up. The result will be to choke the engine and make it lose power just at the time when extra power is needed.

Loading can be prevented by heating the inlet pipe to such an extent that the fuel will not condense on it.

The speed of a tractor engine is practically always controlled by a throttle, which is a valve set in the passage of the carburetor. It operates exactly the same as a damper in a stovepipe; when it is closed, it shuts the passage and prevents the flow of mixture to the engine. As it is opened, it permits a greater quantity of mixture to flow, and it follows, of course, that as the charges of mixture become larger, the engine runs with more power. A tractor carburetor usually has two throttles, one being operated by hand and the other by the governor.

It is usual for a carburetor to be fitted with a strangler, or choke, which makes it easier to form a mixture at slow starting speed. When an engine is cold, the fuel evaporates slowly; and, furthermore, when an engine is cranked by hand its speed is so low that the suction in the carburetor is not sufficient to draw out enough fuel to form a mixture. The strangler is a valve similar in every way to the throttle, but placed between the main air inlet and the spray nozzle. When it is closed and the engine is cranked, very little air can enter the carburetor; the suction is therefore very great. Far more fuel than usual is then sucked out of the spray nozzle, and of this greater amount enough reaches the cylinder to form a combustible mixture. The engine will start, but as soon as it does so, the strangler must be opened so that the normal amount of air enters. If this is not done, the excessive suction will draw so much fuel out of the spray nozzle that the mixture formed will be too rich to burn.

CHAPTER V
CARBURETORS

The apparatus that forms the mixture is in two parts, one being the carburetor that proportions the fuel to the quantity of air drawn into the cylinder, and the other the mixing chamber, or manifold, that connects the carburetor with the valve chamber. The mixing chamber has no adjustments; it is a passage, often a pipe, that is shaped to fit the conditions, and according to the ideas of the manufacturer. When kerosene and distillate are used, the mixing chamber must be heated, so it is frequently built into the exhaust manifold, which is the pipe that conducts the burned gases away from the engine. In some cases it gets heat from the water jacket of the engine, a water jacket formed around it being connected with the cooling system.

The carburetor, on the other hand, has adjustments that must be understood in order to run the engine economically. The understanding of these adjustments is simplified if it is remembered that the object of the carburetor is to maintain a correct proportion of fuel to the volume of air that passes through it.

All tractor carburetors operate on the same principles, and the principles are applied in much the same way. If these principles are understood, and there is an understanding of what the parts of a carburetor are for and what they do, there should be no difficulty in adjusting and caring for any kind of a carburetor that may be offered.

The main body of the carburetor is the tube through which the air passes. This is a casting, and cannot be adjusted or altered. Into this passage projects the spray nozzle, which is usually provided with an adjustment to control the amount of liquid that may flow out of it. When no adjustment is provided, the spray nozzle is made removable, so that a nozzle with an opening of any desired size may be inserted.

Fig. 23.—“Kingston” Carburetor, Model L

On some carburetors the extra air valve is set by the manufacturers, while on others it is adjustable by controlling the strength of the spring that holds it against its seat.

The carburetor shown in [Figure 23] has a spray nozzle adjustment of a very usual type. A rod is so arranged that its pointed end projects into the opening of the spray nozzle; by screwing it up or down the opening may be made larger or smaller, so that more or less fuel may flow out. The extra air valve is a flap valve that closes the air passage until the suction is great enough to lift it from its seat. Around the spray nozzle is a tube that connects the passage below the extra air valve with the passage above it; when the suction is too low to lift the extra air valve from its seat, any air flowing through the carburetor passes through this tube. The tube is so small that even a little air passing through it is enough to suck fuel out of the spray nozzle, and the spray nozzle is so adjusted that enough fuel comes out to make a proper mixture with that volume of air.

This is the low-speed adjustment, which gives a mixture on which the engine will start and will run at its lowest or idling speed. At this speed the engine produces just enough power to keep itself going.

When the engine speeds up, and suction increases, the extra air valve is lifted off its seat, and a greater volume of air flows through the carburetor. The increased suction also draws more fuel out of the spray nozzle. If the greater amount of fuel were in proportion to the greater volume of air, there would be no change in the mixture, but this is not the case. As suction increases, the proportion of fuel drawn out of the spray nozzle becomes too great for the air, and the mixture becomes too rich. To overcome this, the extra air valve permits a still greater volume of air to pass, so that the proportions of fuel and air do not change.

The chamber below the air passage in [Figure 22] is the fuel cup, into which fuel flows from the tank. Inside the fuel cup is a ring of cork attached to a pivoted lever, on the other end of which is a needle valve that can close the opening through which the fuel enters the cup. As the cup fills, the cork floats on it, and in rising it moves the lever on its pivot. When the fuel reaches such a level that it is near the tip of the spray nozzle, the valve closes the opening and prevents more fuel from entering.

Fig. 24.—“Kingston” Carburetor, Model E

In the carburetor shown in [Figure 24], the principal air passage is past the spray nozzle, and all air goes by this passage when the engine is running at low speed. The extra air inlet consists of a number of holes through which air can pass without going past the spray nozzle. On each of these holes is a ball; when the suction is low the balls completely close the holes. When speed increases, the suction becomes great enough to lift the balls off the holes, and the extra volume of air that is necessary is permitted to enter. By making the balls of different weights, it can be seen that the volume of air admitted for any speed is under good control.

Like the carburetor shown in [Figure 23], this carburetor is of the float feed type; that is, the flow of fuel to it is controlled by a valve that is operated by a float.

Either of these two carburetors may be adjusted for gasoline or for kerosene, but the adjustment that is right for one is wrong for the other. Thus, if an engine is started on gasoline, with the intention of running on kerosene, the carburetor must be readjusted when the change is made. This is unsatisfactory, so a double carburetor is sometimes used, as shown in [Figure 25]. This consists of two carburetors of the kind shown in [Figure 24], having a single mixture outlet, one being adjusted for gasoline and the other for kerosene. Either of them can be connected with the mixture outlet by means of a switch valve.

Fig. 25.—“Kingston” Carburetor, Dual Model

In order to run on kerosene or distillate it is necessary to apply heat for the reason that these oils do not evaporate readily at ordinary temperatures. Gasoline, on the other hand, evaporates readily, and a cold engine can be started on it. Tractors that run on kerosene or distillate are therefore started on gasoline and run on it until they are hot enough to vaporize the heavier oil.

A carburetor that will run on either gasoline or kerosene is shown in [Figure 26]. The main air inlet is at E, which leads the air around the spray nozzle and into the chamber G. The mixture flows to the cylinder by the passage B. The control of the fuel at working speeds is by the high-speed adjustment, which is a needle valve screwing into the spray nozzle. Above this is another needle valve that adjusts the flow of fuel for slow speed.

Extra air enters through the opening A, which is closed at slow speed by a valve held against it by a spring. This valve bears against one end of a pivoted lever, the other end of which is attached to the slow-speed needle valve; when the extra air valve opens it moves the lever and the slow-speed needle valve is lifted to permit the flow of a greater volume of fuel from the spray nozzle.

Fig. 26.—“E-B” Carburetor

This carburetor is started on gasoline. When the engine is hot, a switch valve is operated to permit the burned gases from the engine to flow through the carburetor; they pass through the pipe C, D, and as the chamber G is directly in their path it becomes intensely heated. The carburetor can then be switched to kerosene. A side view of this carburetor is shown in [Figure 27].

These carburetors are all of the float feed type, and are used on engines of which the speed is variable. A carburetor that is fed by a pump is shown in [Figure 28]. This is a simple tube with a fuel cup cast on one side of it. Fuel is pumped to the bowl, and the proper level is maintained by an overflow through which excess fuel passes back to the tank.

This carburetor is intended for an engine of which the speed does not change greatly. Its only adjustment is the spray nozzle, and this is altered to correspond with changes in engine speed.

Fig. 27.—“E-B” Carburetor,
Side View

Fig. 28.—Pump-fed Carburetor

If an engine is clean and in good condition, it will run as well on kerosene as on gasoline, although the heating effect of kerosene is greater. When an engine is carbonized, as is usually the case, a condition known as preignition will occur unless it is prevented. Carbon from unburned fuel or from lubricating oil will deposit on the piston head and the parts of the combustion chamber, and particles will become heated to the glowing point, when they will set fire to the fresh mixture during the compression stroke and before the proper time. The effect is to make the engine lose power, and it also gives rise to a sharp metallic knocking. By reducing the temperature in the cylinder during the compression stroke this condition can be prevented. This can be done by adding water vapor to the mixture, and kerosene carburetors are therefore built with a water attachment. As can be seen in [Figure 28], this is a water cup and spray nozzle like those for the fuel. When the engine knocks, and shows that preignition is occurring, water is turned on, and, being carried into the cylinder, keeps the mixture from being heated to the point of ignition before the proper time.

[Figure 29] shows the attachment of this carburetor to an engine which, in this case, is horizontal. To start the engine, gasoline is injected into the carburetor, as shown; this will give a sufficiently good mixture for the purpose, and enough heat for running on kerosene is thus obtained.

Fig. 29.—“Titan” Carburetor

Fig. 30.—Pump-fed Carburetor
With Two Fuel Nozzles

The carburetor shown in [Figure 30] is similar, but has a bowl and spray nozzle for gasoline, to use in starting. It is also provided with a heating jacket through which hot water or hot gases may circulate.

In many cases the fuel is heated before reaching the carburetor. This is done by coiling the feed pipe around the exhaust pipe or putting it in a jacket through which hot water circulates.

Another device sends the mixture through a chamber heated by the exhaust, as shown in [Figure 31]. [Figure 32] shows an arrangement in which the mixture passes through a jacket around one branch of the exhaust pipe. By means of a switch valve, A, more or less of the exhaust gases may be permitted to flow through this branch, so that the mixture may be heated to any desired degree.

Fig. 31.—“Hart-Parr” Mixture Heater

All of these heating devices are so arranged that the heat is under the control of the driver, which permits him to heat the mixture as much as he judges to be necessary. Enough heat must be used to prevent the fuel from condensing; but too much heat will cut down the efficiency of the engine because it will cause so much expansion of the mixture that a cylinderful of it will not produce the maximum power.

Fig. 32.—“Twin City” Manifold

[Figure 33] shows the pump that is used in a force feed carburetor of the type shown in [Figure 28]. Its plunger is forced through an inward stroke by a cam, and makes an outward stroke as its spring returns it to position. The inlet and outlet openings of the cylinder are closed by ball check valves, the inlet check being open on the outward strokes, and the outlet check being open on the inward strokes. A pump of this sort requires no attention beyond seeing that the check valves work properly, and that there are no leaks.

[Figure 34] shows the connections between the fuel tank and the carburetor. Under the tank, 1, is a chamber containing a fine wire strainer, 4, through which the fuel must pass to reach the carburetor; any dirt that may be present is strained out, and collects in the cup, 2. Water in the fuel also settles here, and the cup is cleaned out by unscrewing the plug, 3. 5 is the shut-off cock; it should always be closed when the tractor is not working.

Fig. 33.—Fuel Pump

A complete fuel system is illustrated in [Figure 35], showing the connections of the tanks, pumps, and carburetor.

As dirt is injurious to an engine, the air that forms the mixture must be clean, so when a tractor works in a dusty field, it should be equipped with an air cleaner, of which there are three kinds. In one of these the air is required to pass through water, which washes it. A cleaner of this type is shown in [Figure 36]. The dusty air enters the central passage, and is forced to pass through the water in order to reach the outlet. Passage through the water and through the baffle plates frees the air of all its dust.

In the cleaner shown in [Figure 37], the air is passed through loose wool, which filters out the dust. Another type of cleaner works on the same principle as a cream separator; the air is given a whirling motion, which throws the dirt out at the sides, and it is collected in a glass jar.

Fig. 34.—“Avery” Fuel Connections

Fig. 35.—“Oil-Pull” Fuel System

These air cleaners must be emptied frequently, for if they are not kept clean it cannot be expected that they will do their work.

A tractor engine is built to develop its maximum power at a certain speed; if it runs at greater speed, it will not operate efficiently, and there will be unnecessary wear of its parts. These engines are therefore usually fitted with governors which hold them at their most efficient speed. A governor operates by centrifugal force.

Anything in motion tries to move in a straight line; if it is forced to move in a circle, it will exert force in trying to move away from its center. It is this that is called centrifugal force. It is centrifugal force that holds water in a pail that is being swung around the head, and that makes the pail fly off if it is released.

Fig. 36.—Air Washer

In applying this principle to a governor, weights are attached to a plate and made to revolve; springs hold them together, but in spite of this, centrifugal force throws them outward. In moving, they act on a rod that operates the throttle; as the speed increases, the move outward more and more, and it is a simple matter of adjustment to cause them to close the throttle when the speed reaches a desired point.

Fig. 37.—Air Strainer

Fig. 38.—“E-B” Governor

A governor and its connections are shown in [Figure 38]. The weights, R, are L-shaped, and pivoted at the angle to a plate driven by the engine. The shaft that drives the plate also supports a collar, P, that is loose on it and can slide endways; the collar rests against the short bar of the L-shaped weights. The other end of the collar touches the lever, E, which is moved when the collar moves. As the lever is connected with the throttle, a movement of the collar will control the position of the throttle.

Fig. 39.—“Case” Governor

When the shaft revolves, the long arms of the L-shaped weights tend to fly outward; this moves them on their pivots, and the short arms thereupon force the collar to slide on the shaft, which moves the lever and operates the throttle. The speed at which the throttle will begin to close is determined by the setting of the spring that holds the weights in.

Fig. 40.—“Hart-Parr” Governor

Governors and governor connections are shown in [Figures 39] and [40].

The governor shown in [Figure 41] is enclosed in a housing that can be locked or sealed. This prevents the unauthorized changing of the adjustment.

Fig. 41.—Vertical Governor

CHAPTER VI
IGNITION

In order that a gas engine may run properly, the mixture must be set on fire, or ignited, at exactly the right time; if ignition occurs too early or too late, there will be a loss of power.

The greatest pressure will be obtained at the instant when all of the mixture is burning, and this should take place just as the piston begins to move outward on the power stroke. A little time is required for the mixture to burn; there is a brief interval between the instant when it is set on fire and the instant when it is all in flame. Thus it is clear that if the mixture is all to be burning as the piston starts the power stroke, it must be set on fire before that time, or, in other words, toward the end of the compression stroke.

The point at which ignition should occur depends on the speed of the engine and should change when the speed changes. The time required for the flame to spread throughout the mixture does not change; let us say that, with the engine running at 1200 revolutions a minute, the mixture can be ignited when the piston is ¼ inch from the end of the compression stroke, and will all be in flame by the time the piston starts on the power stroke. If the engine is slowed down to 600 revolutions a minute and no change is made in the ignition, the mixture will all be in flame before the piston reaches the end of the compression stroke; pressure will then be produced before the piston is in position to perform the power stroke. The pressure will try to make the engine run backwards; it will sometimes be sufficient to make the engine stop. If the momentum of the flywheel is sufficient to force the piston to the end of the stroke against the pressure, this condition will cause a loss of power. This is called preignition, or ignition that occurs too soon. One effect of it is to produce a hard, metallic knocking, due to the oil being squeezed out of the bearings by the great pressure, which permits the bearing and shaft to strike. The remedy is to make ignition occur later in the stroke.

If the engine is speeded up above 1200 revolutions, the piston will have had time to move some distance on the power stroke before the mixture is all in flame; the combustion space will then be too large to permit the mixture to produce its greatest pressure, and again there will be a loss of power. The remedy in this case is to make ignition occur earlier in the compression stroke.

When ignition is made to occur early in the compression stroke, it is said to be advanced; when it is made to occur late in the stroke, it is said to be retarded.

To get the best results, the engine should be run with ignition advanced as far as is possible without causing knocking.

The charge of mixture is always set on fire by an electric spark, and the parts that produce and control this spark are called the ignition system.

An ignition system consists of: First, the apparatus that produces the electric current, which is usually a magneto; second, a timer, which controls the instant at which the spark occurs; third, the spark plugs, which project into the cylinders, and at which the sparks take place; fourth, a switch, by which the sparking current can be turned on or off, and fifth, the wires, or cables, by which the parts are connected.