The four-cycle or Otto stroke type of gasoline engine should rightly be called the four-stroke-cycle engine, as it requires four strokes and two revolutions of the crank shaft to complete one cycle of operation.

This type of motor is used almost universally by the manufacturers of pleasure cars due to its reliability, and to the ability it has to furnish continuous power at all speeds with the minimum amount of vibration.

Fig. 17 shows a diagram of one cylinder in the four strokes of the cycle, and the distance traveled by the crank shaft during each stroke. No. 1 begins with a charge of compressed vapor gas in the cylinder and is called the firing or power stroke. The ignition system (explained in a later chapter) furnishes a spark at from five to fifteen degrees early or before the piston reaches top dead center. Although the stroke theoretically starts before the piston reaches its highest point of ascent, the actual pressure or force of the explosion is not exerted until the piston has crossed dead center. This is due to the fact that the piston travels very rapidly, and that it requires a small fraction of a second for spark to ignite the compressed charge of gas. It may, therefore, be easily seen that, if the spark did not occur until the piston is on or has crossed dead center, the piston would have traveled part of the distance of the stroke, and as it is moving away from the highest point of compression the pressure is reduced by allowing more volume space which causes a weak explosion and a short power stroke. The intake and exhaust valves are closed through the duration of the power stroke.

No. 2. The exhaust stroke begins from fifteen to thirty degrees early, or before the piston reaches lower dead center on the firing stroke. The exhaust valve opens at the start of this stroke allowing the pressure of the burnt or inert gas to escape before the piston begins to ascend on the upward part of the stroke, and closes seven to ten degrees late to allow the combustion chamber to clear out before the next stroke begins.

No. 3. The intake or suction stroke begins with the piston descending from its highest level to its lowest level. The intake valve opens ten or twenty degrees late, and as the piston is traveling on its descent, considerable vacuum pressure has formed which draws suddenly when the valve opens and starts the gas from the carburetor in full volume. The entire length of this stroke creates a vacuum which draws a full charge of vaporized gas into the cylinder through the open intake valve. The intake valve closes from ten to twenty degrees late in order that the full drawing force of the vacuum may be utilized while the piston is crossing lower center.

No. 4. The compression stroke begins at the end of the intake stroke with both valves closed. The piston ascends from its lowest extreme to its highest level, compressing the charge of gas which was drawn into the cylinder on the intake or suction stroke; and at the completion of this stroke the cylinder is again in position to start No. 1, the firing stroke, and begin a new cycle of operation. The cam shaft is driven from the crank shaft through a set of gears or a silent chain, and operates at one-half the speed of the crank shaft as a valve is lifted once through the cycle of operation, or two revolutions of the crankshaft.

Fig. 18 shows the operation of a four-cylindered motor as it would appear if the cylinder block were removed. The timing or firing order of the motor shown in this diagram is 1-2-4-3. No. 1 cylinder is always nearest the radiator and on the left in this diagram. No. 1 cylinder is firing. The intake and exhaust valve remain closed while this stroke is taking place. This causes the entire force of the explosion to be exerted on the head of the receding piston. The cylinders, as may be seen in the diagram, are timed to fire in succession, one stroke behind each other. While No. 1 cylinder is on the firing stroke, No. 2 cylinder is compressing with both valves closed and will fire and deliver another power impulse as soon as No. 1 cylinder completes and reaches the lowest extreme of its firing stroke. No. 3 cylinder, being fourth in the firing order, has just completed the firing stroke and is starting the exhaust stroke which forces the burnt and inert gases out of the cylinder through the open exhaust valve. No. 4 cylinder which is third in the firing order has just completed the exhaust stroke and is about to start the intake or suction stroke with the exhaust valve open. This diagram should be studied and memorized as it is often necessary to remove the wires which may easily be replaced if the firing order is known, or found by watching the action of the exhaust valves and made to conform with the distributor of the ignition system. (Note the running direction of the distributor brush and connect the wires up in that direction.) For the firing order given above connect No. 4 wire to No. 3 distributor post, and No. 3 wire to No. 4 post, as this cylinder fires last.

Fig. 19 shows a diagram of the power stroke impulse delivered to the cycle in a one, two, four, and eight cylindered motor. A complete cycle consists of 360 degrees, and as there are four strokes to the cycle an even division would give a stroke of ninety degrees, which is not the case, however, owing to the fact that the valves do not open and close at the theoretical beginning and ending point of each stroke which is upper dead center and lower dead center. The firing or power impulse stroke begins at approximately five to seven degrees before the piston reaches upper dead center on the compression stroke and ends from fifteen to thirty degrees before the piston or cycle of rotation of the crankshaft reaches lower dead center. This results in a power impulse of less than ninety degrees, which varies accordingly with valve timing in the different makes of motors. Consequently we have a power stroke of a little less than ninety degrees in a one-cylinder motor; two power strokes of a little less than 180 degrees in a two cylinder motor, while the power impulse of the four-cylinder motor very nearly completes the cycle. In the six, eight, and twelve cylinder motor the power strokes overlap, thereby delivering continuous power of very nearly equal strength.

Twin, Four, and Six Cylindered Motors.—The operation of the twin cylindered motor varies very little from the single four or six. It is simply a case where two, four, or two six cylindered motors are set to a single crank case at an angle which will allow the piston or connecting rods from the opposite cylinders to operate on a single crank shaft. When the cylinders are set directly opposite each other the connecting rods are yoked and take their bearing on a single crank pin of the crank shaft. This, however, is not always the case, for in some motors the connecting rods take their bearing side by side on the crank pin. The cylinders in this case are set to the crank case in a staggered position to allow the connecting rods from each cylinder to operate in line with the crank shaft.

The cylinder blocks are usually set to the crank case at an angle of ninety degrees and are timed to furnish the power impulse or stroke opposite each other in the cycle of operation. The advantage of this formation is that two power strokes are delivered in one cycle of operation, which increases the power momentum and reduces the jar or shock of the explosion causing a sweet running vibrationless motor.

The valves are usually operated by a single cam shaft located on the upper inside wall of the crank case. Valve timing is accomplished by following the marks on the flywheel or lining up the prick punch marks on the gears, as shown in Chapter II on valves.

When a magneto is used to furnish the current for ignition on an eight cylinder motor it has to be operated at the same speed as the crank shaft, as a cylinder is fired at each revolution of the crank shaft and an interruption of the current is required at the breaker points to produce the secondary or high tension current at the spark plug gaps.

Twelve cylindered motors are usually equipped with two distributors or a dual system, or two magnetos driven separately through a set of timing gears.

Knight or Sleeve Valve Motor.—The Knight or sleeve valve motor operates on the same plan as the ordinary type of motor except that the valves form a sleeve and slide over the piston. The sleeves are operated by an eccentric shaft and are provided with ports which are timed to conform with the ports of the intake and exhaust manifolds at the proper time.

MOTOR HORSEPOWER
S. A. E. Scale
FOUR-CYCLE HORSEPOWER RATING

Formula—S. A. E. D² times N 2.5 equals horsepower.

For sleeve valve timing see Chapter II on Valves.

DISPLACEMENT

There are probably few men operating cars to-day who fully understand what is meant by the term displacement, often used in referring to automobile races. It is one of the main factors or points in determining the class in which a car is qualified to enter under the laws that govern races. In looking over a race program, you will note that there are usually two or more classes, one of which is open, and another with a limited piston displacement, which gives the smaller cars a competing chance in their class.

Consequently piston displacement is merely the volume displaced by all the piston in moving the full length of the stroke. The volume of a single cylinder is equal to the area of the bore multiplied by the length of the stroke, and the total displacement of a four cylinder motor will be four times this and that of a six cylinder motor, six times this.

Piston displacement:

D² times S times N times 3.14 4

Piston Displacement

3.5² times 5 times 4 times 3.14 4

3.5 times 3.5 times 5 times 4 times 3.14 4

equals 173.58 cubic inches.

LUBRICATION SYSTEMS, OILS, AND GREASES

Special attention should be given to regular lubrication, as this, more than any one thing, not only determines the life but also the economic up-keep of the car.

Whenever you hear an owner say that his car is a gas eater, or that it uses twice or three times as much oil as his neighbor’s, which is the same model and make, you know at once that he, or some one who has used the car before him, either did not give sufficient attention to lubrication, or used a poor grade of oil. It is almost impossible to impress the importance of the foregoing facts upon the minds of the average motorist, and we have, as a direct result, a loss of millions of dollars annually through depreciation.

The manufacturers of automobiles and gasoline engines have earnestly striven to overcome this negligence by providing their products with automatically fed oiling systems which alleviate some of the former troubles. These systems, however, also require some attention to function properly.

Grease.—A medium grade of light hard oil grease is best adapted for use in grease cups, universal joints, and for packing wheel bearings and steering gear housings. The transmission and differential operate more successfully when a lighter grade of grease is used, such as a graphite compound, or a heavy oil known as 600 W.

Oils.—Great care should always be exercised in purchasing lubricants. None but the best grades should be considered under any circumstances. The cheaper grades of oil will always prove to be the most expensive in the end. The ordinary farm machinery oils should never be used in any case as an engine lubricant, for in most cases they contain acids, alkalies, and foreign matter which will deteriorate and destroy the bearings of the motor.

An oil to give the best satisfaction must be a purely mineral or vegetable composition which will flow freely at a temperature of 33° Fahrenheit and also stand a temperature of 400° Fahrenheit without burning. Always choose an oil which is light in color as the darker oil usually contains much carbon.

Lubrication (Lat. Lubricus, meaning slippery).—-Lubrication is provided on all types of automobile engines, and at various other places where moving parts come in contact or operate upon each other.

Three different types of lubricating systems are found in common use.

Fig. 24 shows the splash system. The oil is placed into the crank case and maintained at a level between two points, marked high and low, on a float or glass gauge at the lower left-hand side of the crank case. The oil is usually poured directly into the crank case through a breather pipe provided to prevent excessive vacuum pressure.

The lower end of the connecting rod carries a spoon or paddle which dips into the oil at each revolution and splashes it to the cylinder walls and various bearing surfaces within the motor.

Care of the Splash System.—This type of oiling system does not require any adjustments, or special care, except that the oil level be constantly kept between the high and low level marked on the gauge.

Cleaning the Splash System.—Lubricating oils lose their effectiveness and become thin and watery after a certain period of use due to a fluid deposit called residue which remains in the combustion chambers after the charge of gas has been fired. This fluid generally works its way into the crank case, thinning the oil.

The crank case should, therefore, be drained, cleaned, and refilled with fresh oil every fifth week or thousand miles that the car is driven. This will prevent much wear and give a quiet and satisfactory running motor. Draining and washing out the crank case is accomplished by removing a drain plug at the bottom of the crank case. The old oil is drained off and thrown away. Kerosene is then poured into the crank case through the breather pipe until it flows out of the drain clear in color. The plug is then replaced and the crank case replenished with fresh oil until the three-quarter from low level is reached on the gauge. The oil level should be carried as near this point as possible to obtain the most satisfactory result.

Fig. 25 shows the plunger or piston pump pressure system usually used in conjunction with the splash system. The oil is carried in a reservoir at the bottom of the crank case and is drawn through a fine meshed screen by the oil pump, which is of the plunger type operated off the cam shaft. It forces the oil through copper tubes in the three main bearings. The front and center bearings have an outlet which furnishes the oil to the gears in front and to the troughs in which the connecting rods dip. The troughs have holes drilled to keep the level of the oil, the surplus being returned to the reservoir.

There is a pipe line running from the pump to the gear case with a screw adjustment to regulate the oil pressure by turning either in or out. There is a pipe line running to a gauge on the dash which gives the pressure at all times. The cam shaft and cylinder walls get the oil by the splash from the connecting rods. The bottom rings of the pistons wash the oil back into the crank case. The overflow from the front bearings falls into a small compartment immediately under the crank shaft gear where it is picked up by this gear and carried to the other gears and the bearings of the water pump shaft. A small oil throw washer on the pump shaft prevents any surplus oil from being carried out on the shaft or the hub of the fan drive pulley. Any overflow from the gear compartment is carried immediately to the splash pan where it provides for the splash lubrication of the connecting rod bearings and the cylinder walls. The dippers on the connecting rod bearings should go ¹⁄₄ in. beneath the surface of the oil. The upward stroke of the oil pump plunger draws the oil through the lower ball check into the pump body and the downward stroke discharges it through the upper ball check into the body of the plunger which is hollow and has outlets on either side. This allows the oil to flow from the plunger into the by-pass in the oil pump body and then into the lines running to the main crank shaft bearings. The next upward stroke forces the oil through the lines to the main bearings.

The oil pressure regulator is located on the body of the pump and connects to the by-pass. It consists of a hollow sleeve screwed into the body of the pump which has a small ball check held by a short coiled spring the tension of which determines the oil pressure. The tension and the pressure may be increased by turning the nut to the right. The nut should not be given more than one turn at a time in either direction as it is very sensitive. A loose main bearing will allow more oil to pass through it. Consequently the pressure registered on the oil gauge will be reduced. This will come about gradually. It is not advisable to attempt to adjust the oil pressure without first noting the condition of the main crank shaft bearings.

The most common cause of failure to operate is the collection of dust and dirt on the sleeve at the lower end of the pump or from an accumulation of sediment back of the ball check. This needs to be cleaned from time to time.

Force and Gravity Oiling System.—The force and gravity oiling system operates in much the same manner as the plunger pump system, except that the oil is pumped into an elevated tank from which it flows through lines by gravity to the various bearings. Oil pumps, however, differ in construction and some manufacturers use eccentric, centrifugal, and gear pumps. Oil pumps are very simple in construction and action and can be readily understood by recalling their functional action.

Oil pumps rarely give any trouble, and if they fail to function properly, dirt should be immediately suspected, and the ball valves and pipes inspected and cleaned.