=Exhaust Stroke.=—During the exhaust stroke (No. 4, Fig. 1) the piston moves inward and the exhaust valve is open. This movement of the piston pushes the burned gases out of the cylinder, and it is clear that the more thoroughly the cylinder is emptied of them, the more room there will be for a fresh charge. In high-speed engines the gases cannot escape as fast as the piston moves; they are still flowing out when the end of the stroke is reached. Therefore the valve is closed, not at the end of the stroke, but when the piston has moved about ⅛ inch outward on the inlet stroke. The inlet valve opens as the exhaust valve closes. It can be seen that through the inlet and compression strokes a leak will reduce the charge and so interfere with the production of full power. The piston must make a tight fit in the cylinder, the valves must seat tightly, and gaskets and other parts must be in proper condition. [Illustration: FIG. 2.—1-CYLINDER POWER DIAGRAM] Figure 2 shows a power diagram for a 1-cylinder engine, in which the crank shaft moves under power during one stroke out of every four. An engine with two cylinders can be built so that first one cylinder applies power and then the other, in which case the crank shaft moves under power during two strokes out of every four. [Illustration: FIG. 3.—2-CYLINDER POWER DIAGRAM] Figure 3 is a power diagram of an engine of this sort. If piston 1 is moving down under power, piston 2 is also moving down, but on the inlet stroke. The following stroke is exhaust in cylinder 1 and compression in cylinder 2, and cylinder 2 will then deliver a power stroke while cylinder 1 is on inlet. Thus the crank shaft will receive a power stroke, followed by a dead stroke; then another power stroke and another dead stroke, and so on. There will be the disadvantage, however, that the pistons, moving up and down together, will cause vibration, which in the course of time will be likely to give trouble. To overcome this, a 2-cylinder engine can be built as indicated in Figure 4. In this engine the cranks project on opposite sides of the crank shaft instead of on the same side, as in Figure 3; the pistons thus move in opposite directions, and produce no vibration. Power will be unevenly applied, however, for both power strokes occur in one revolution, with two dead strokes in the succeeding revolution. [Illustration] [Illustration: FIG. 4.—2-CYLINDER POWER DIAGRAM, 180 SHAFT] With piston 1 moving down on power, piston 2, moving upward, can only be performing compression or exhaust. If it is on compression, its power stroke will follow the power stroke of piston 1, while if it is on exhaust its power stroke will have occurred immediately before the power stroke of piston 1. In either case one power stroke follows the other, taking place in one revolution of the crank shaft, while in the following revolution both pistons will be performing dead strokes.