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. |