Motor vehicles and their engines

The following pages represent the result of an attempt to collect in a comparatively small book such elementary, theoretical, and practical information as will assist in the operation, upkeep, and adjustment of the motor vehicles. This book was written with a three-fold purpose; as a guide for the personal instruction of the car owner, as a hand book for chauffeurs, garages, and repairmen, and as a text book for Automobile Schools. The simplest language has been used and technicalities have been reduced to a minimum. The fundamentals of gas motor operation, as well as the care and operation of the principal accessories of the motor vehicles concerned, are
discussed in detail and at greater length than is the usual practice.

To obtain the maximum economy, efficiency, and life of the apparatus the last four chapters of the book should be studied. These chapters are the result of the authors' observations and experience with the great number of trucks, tractors, automobiles, and motor-cycles operating under their supervision.

This book is the outgrowth of the authors' former volume "Motor Transportation for Heavy Artillery," which was prepared for use as a textbook in the Coast Artillery School's course in the subject. The valuable experience gained in connection with their work as instructors in this school has been embodied in this second edition so that the book contains all the information necessary to properly operate and care for motor vehicles.

The term "Gas Engine" is commonly used to designate all types of internal combustion engines regardless of whether they operate on gas or liquid fuel. Liquid fuel is almost universally used in engines adapted for motor transportation. Gasoline is the most commonly used liquid fuel. Kerosene, alcohol, benzol, and fuel oil are used in internal combustion engines, but in general their use is confined to engines of the stationary type.

In the internal combustion engine, the fuel is introduced into the cylinder in a combustible mixture and is there ignited. This type of engine is divided into two classes, that in which the combustion takes place gradually and that in which the combustion takes place almost instantaneously.

The Diesel engine comes under the class in which the combustion is gradual. The liquid fuel is gradually injected into the cylinder which contains only air under high pressure. This air is compressed to such a degree that a temperature far above the ignition point of the fuel is obtained. This causes the fuel to ignite as it is injected into the combustion space. Complete but gradual combustion is obtained in this manner. Engines of this type are not applicable for motor propelled vehicles, largely because of their lack of flexibility.

In the gasoline engine the fuel is burned almost instantaneously. The air is mixed with the fuel outside of the combustion space (Fig. 1) and the resulting combustible mixture is drawn into the cylinder where it is ignited under compression by some outside source of heat., the electric spark being the one universally adopted.

Combustion or burning is always accompanied by the production of heat. The temperature produced depends upon the rapidity and completeness of the combustion. The faster the burning the higher the maximum temperature produced. A slow burning fuel produces a more uniform temperature, but not as high as is produced by a fuel burning almost instantaneously.

When gases and most metals are heated they expand, some expanding more than others. Gases expand more than metals for a given amount of heat. A definite increase of temperature will cause a gas to expand a certain amount and as the heat is increased the expansion increases in proportion. When the mixture in the cylinder is burned, the resulting heat causes the gases to expand, the amount of this expansion depending upon the temperature.

When a gas contained in a closed vessel is heated its expansion exerts a pressure equally in all directions. This condition exists in the cylinder of an internal combustion engine after combustion has taken place. The resulting pressure is exerted on the cylinder walls and piston. The piston, being movable, under the force of the expanding gases, moves outward to the full limit of its stroke.

The energy resulting from this expansion must now be transformed into useful work. In order to accomplish this, a construction such as shown in Fig. 3 is used.

The force exerted on the piston "K" is transmitted through the connecting rod "E" to the crank shaft U H" which is made to revolve, turning through one-half of a revolution as the piston moves outward. Attached to the crank shaft is a fly wheel, which stores up energy and its momentum carries the piston through the balance of its motion until it receives another power impulse. In this way the reciprocating motion of the piston is transformed into a rotary motion at the crank shaft. The operation of the gasoline engine, as already shown, depends upon the production of heat in the cylinder caused by burning the fuel. A given amount of fuel will produce a certain amount of heat when completely burned. However, the total heat value of the fuel cannot be utilized because there are certain losses which must always occur even in the best designed engine. Badly worn engines, imperfect carburetion, and faulty ignition will add to the necessary losses and decrease the percentage of energy actually available for useful work.

The highest thermal efficiency attained in the best types of large stationary internal combustion engines (Diesel) is about 35% while few automobile engines ever exceed 20%. The diagram (Fig. 4) shows the dispersion of energy from fuel as it passes through the engine of a high class touring car traveling at a speed of 40 miles per hour on direct drive.

Referring to Fig. 4, it will be seen that a certain amount of the heat is absorbed by the cooling system. Also a considerable amount of heat is lost in the exhaust gases. Nearly 70% of the total fuel value is lost in this way and this loss cannot be materially reduced below this amount. The loss due to engine friction will vary considerably with the design and condition of the engine. The point indicated as " Motor Full Power" represents the amount of energy remaining for useful work. When this energy is applied to driving an automobile, it is consumed as shown in the diagram. This leaves but a small amount of reserve energy, which will be decreased as the speed of the machine is increased. Every design of engine and car of course has a different energy diagram corresponding to the degree of efficiency attained at different speed and loads.

Engines of all kinds are rated in horse-power the measure of the rate at which they can do work. One horse-power represents 33,000 foot-pounds of work per minute. There are two ways of measuring engine power. The power developed by the expansion of the gases in the cylinder can be determined, in which case the INDICATED HORSE-POWER is obtained. By means of a Prony Brake or Dynamometer, the power which the engine actually delivers can be measured and this is called the BRAKE HORSE-POWER. The brake horse-power of an automobile engine will usually be from 70% to 85% of its indicated horse-power, the losses being energy consumed in friction and other causes in the engine mechanism. However, in obtaining the horse-power of an engine, formulae are used based on the indicated horse-power assuming certain standard conditions. The horse-power obtained in this manner is often inconsistent with the actual horse-power developed on test.

There are a number of quick rules for estimating the power of engines according to their cylinder dimensions and the piston speed.

In order that the operation of the gas engine be continuous, a certain series of events called the cycle must take place which are repeated over and over in the same regular order. In order to clearly understand the events that compose the cycle of an engine, its operation will be compared to the operation of the old style muzzle-loading cannon, which is the simplest form of internal combustion engine.

Referring to Fig. 5, the first step necessary to fire the cannon is inserting the charge; the corresponding step in the gas engine is the ADMISSION of the charge. The second step is ramming the projectile and powder; the corresponding step in the gas engine is the COMPRESSION of the charge. The third step is lighting the fuse; the corresponding step in the gas engine is the IGNITION of the charge. The fourth step is burning the powder and the fifth step expansion of the gases of combustion due to the heat produced which forces the projectile out of the cannon. The corresponding steps in the gas engine are the COMBUSTION of the charge and EXPANSION of the gases. The sixth step in the operation of the cannon is the escape of the burned gases after the projectile has left the muzzle; the corresponding step in the gas engine is the subsequent EXHAUST of the products of combustion. The cannon is now ready to be fired again and the engine to continue its operation.

The steps comprising the cycle of operation of the gas engine may be summarized as follows:

1. Admission of the charge.
2. Compression of the charge.
3. Ignition of the charge.
4. Combustion of the charge.
5. Expansion of the gases.
6. Exhaust of the gases.

In the operation of a gas engine the number of strokes required to complete the cycle varies with the type of engine. In the type almost universally used for motor vehicles the cycle is extended through four strokes of the piston or two revolutions of the crank shaft and is therefore called a four-cycle engine. In a few instances the cycle is completed in two strokes of the piston or one revolution of the crank shaft and is therefore called a two-cycle engine.

In the four-cycle engine, the four strokes are named suction, compression, power, and exhaust in accordance with the operations of the cycle which occur during each particular stroke.

SUCTION STROKE. During this stroke (Fig. 5-A) the piston is moved outward by the crank shaft which is either revolved by the momentum of the fly wheel or some external starting force. This movement of the piston increases the size of the combustion space, thereby reducing the pressure in it and the higher pressure of the atmosphere outside, forces fresh mixture into the combustion space through the open inlet valve.

COMPRESSION STROKE. The compression, ignition, and most of the combustion of the charge takes place during the next inward stroke of the piston. The time elapsed between the mixing of the liquid gasoline and air and its admission into the cylinder is too brief to secure a perfect combustible mixture. What passes into the cylinder consists of air, liquid gasoline, and a more or less perfect mixture of the two. The combustion of this mixture would be slow and incomplete resulting in a loss of power and a waste of fuel. In order to obtain a homogeneous mixture, advantage is taken of the heat produced by compression. This renders the gasoline more volatile, while the compression forces it into intimate combination with the air. Even then a perfect mixture may not result for the air and gasoline vapor instead of being thoroughly combined may be in layers. The combustion will then be slow and uneven. When the mixture of air and gasoline vapor are properly proportioned, this difficulty is seldom encountered. The mixture is ignited while under compression and combustion is practically completed at top dead center.

POWER STROKE. The expansion of the gases due to the heat of combustion exerts a pressure in the cylinder and on the piston. Under this impulse it moves outward.

EXHAUST STROKE. When the exhaust valve is opened, the greater part of the burned gases escape due to their own expansion. The inward movement of the piston pushes the remaining gases out of the open exhaust valve. The space between the cylinder head and the piston, when it is at its inmost point, is called the clearance and will be filled with the remaining exhaust gases. These will dilute the fresh incoming charge.

Thus it is seen that in this type of engine four strokes of the piston are required to complete the cycle.

The two-cycle type of gasoline engine differs from the four-cycle type just described in that the six events composing the cycle are performed during two strokes of the piston or one revolution of the crank shaft. Power is developed during every outward stroke of the piston instead of alternate outward strokes. In order that this result may be attained, the construction of the engine is changed. As shown in Fig. 6, 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. These are covered and uncovered by the piston as it slides in the cylinder. The gas inlet port to the crank case is uncovered when the piston is at the inmost point of its stroke admitting the mixture to the crank case. This is air tight and must have a separate compartment for each cylinder. The exhaust port and the intake port are uncovered when the piston is at the outmost point of its stroke. The exhaust port opens first, which permits the burned gases to escape after combustion has taken place. The intake port opens shortly after the opening of the exhaust port and permits a fresh charge to pass from the crank case to the combustion space.

During an inward stroke, the pressure in the crank case is reduced as the piston moves inward and a fresh mixture is forced into it by the higher atmospheric pressure as soon as the gas inlet is uncovered. This port is covered when the piston makes an outward stroke and the mixture, not being able to escape, is compressed. The tendency of the gas to expand causes it to flow to the combustion space when the inlet port is uncovered, and in entering, it strikes a deflecting plate on the piston. This deflects it to the top of the combustion space instead of allowing it to rush 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. As soon as the exhaust port is uncovered, which is slightly before the uncovering of the inlet port, the gases, which are still expanding, begin to escape. They are further expelled by the fresh charge that enters and drives them before it. Thus the six events of the cycle are performed during an inward and an outward stroke of the piston. On the lower side of the piston a charge of fresh mixture is drawn into the crank case and forced into the combustion space where it is compressed, ignited, and burned.

The type of engine just explained is known as the three-port construction of two-cycle engine. There is also a two-port construction of two-cycle engine. This type of engine differs from the three port in that the inlet port to the crank case is replaced by a check valve as shown in Fig. 7. When the pressure in the crank case is reduced due to the piston moving inward, the higher atmospheric pressure opens the valve and forces a fresh mixture into the crank case. The valve is closed by the action of the spring at all other times and is further assisted in closing by compression in the crank case. The operation of this engine is identical in all other respects with the three-port construction.

In all two-cycle engines a screen is placed in the bypass. The object of this is to prevent any possibility of the incoming charge being ignited by the exhaust gases thus causing a back fire into the crank case.

At slow speeds, two-cycle engines have advantages over the four- cycle in having a power impulse every revolution of the crank shaft and in not having valves and valve mechanism with their weight and possibility of giving trouble. This simplicity makes the two-cycle engine popular for motor boats, where slow and constant speeds are desired. For higher and changing speeds these advantages are out- weighed by disadvantages that show little sign of being overcome.

With the engine running at high speed, the ports are open for only a brief period during each stroke, and 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 is incomplete and contaminated by the portion of the burned gases that have not been able to escape. This results in the "choking up" of the engine, and in the production of lower power than the dimensions and weight of the engine warrant.

Automobile engineers agree that the four-cycle engine is better for automobile work. It has been developed to a greater degree than the two-cycle and it is easier to keep adjusted and in good running condition. Though the two-cycle engine is undoubtedly the simplest form, it is liable to be erratic in operation and it is sometimes difficult to locate the trouble definitely. The following advantages are claimed for the two-cycle engine over the four-cycle:

(1) absence of poppet valves with their springs, push rods, cam shafts, etc.; (2) fewer parts; (3) better turning effect with the same number of cylinders. Offsetting these, the four-cycle engine has the following advantages over the two-cycle: (1) greater fuel economy,

(2) greater flexibility. These advantages far over-balance the advantages of the two-cycle over the four-cycle engine and for that reason the two-cycle engine is very rarely used for automobile propulsion.


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Motor vehicles and their engines