| 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.