VALVE CONSTRUCTION
If the sleeve rods are removed for some reason, the bearings should be fitted very loosely to the eccentric shaft when they are put back. A looseness of about .008 of an inch is permissible.
CHAPTER III
THE OPERATION OF A 4-CYCLE, 4-CYLINDERED ENGINE
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.
| Firing Stroke | Exhaust Stroke | Intake Stroke | Compression Stroke |
| 1 | 2 | 3 | 4 |
Fig. 17. 4-Stroke Cycle. 1—Cylinder in Action
[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.
| 1 | 2 | 3 | 4 |
| Firing Val. Closed | Compressing Val. Closed | Exhausting Ex. Val. Open | Intake In. Val. Open |
Fig. 18. Diagram of Action, 4-Cylinder 4-Cycle Engine
[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.
| 1-CYL. | 2-CYL. |
| 4-CYL. | 8-CYL. |
Fig. 19 Power Stroke Diagram
[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
| Bore | 1 cyl. | 2 cyl. | 4 cyl. | 6 cyl. | |
|---|---|---|---|---|---|
| 2 | 3⁄4 | 3.00 | 6.00 | 12.00 | 18.00 |
| 2 | 7⁄8 | 3.00 | 6.50 | 13.00 | 20.00 |
| 3 | .00 | 3.50 | 7.00 | 14.50 | 21.50 |
| 3 | 1⁄4 | 4.00 | 8.50 | 17.00 | 25.50 |
| 3 | 1⁄2 | 5.00 | 10.00 | 20.00 | 29.50 |
| 3 | 3⁄4 | 5.50 | 11.00 | 22.50 | 34.00 |
| 4 | .00 | 6.50 | 13.00 | 25.50 | 38.50 |
| 4 | 1⁄4 | 7.00 | 14.50 | 29.00 | 43.50 |
| 4 | 1⁄2 | 8.00 | 16.00 | 32.50 | 48.50 |
| 4 | 3⁄4 | 9.00 | 18.00 | 36.00 | 54.00 |
| 5 | .00 | 10.00 | 20.00 | 40.00 | 60.00 |
| 5 | 1⁄4 | 11.00 | 22.00 | 44.00 | 66.00 |
| 5 | 1⁄2 | 12.00 | 24.00 | 48.00 | 73.00 |
| 5 | 3⁄4 | 13.00 | 26.50 | 53.00 | 79.50 |
| 6 | .00 | 14.50 | 29.00 | 57.50 | 86.50 |
This scale gives the nearest equivalent to the whole or half horsepower, as is required by State where licenses are paid at so much per horsepower.
Formula—S. A. E. D2 times N 2.5 equals horsepower.
For sleeve valve timing see [Chapter II] on Valves.