Fig. 5.—H. D. O. Power Diagram
Fig. 6.—4-Cylinder Power Diagram
In a 4-cylinder engine one power stroke follows another without any dead stroke intervals, which, of course, makes the crank shaft revolve more smoothly and with a steadier application of power. The power diagram is shown in [Figure 6]; in studying this it should be remembered that if two pistons move in opposite directions, as in [Figure 4], one power stroke follows another, while if they move in the same direction, as in [Figure 3], there is an interval of one stroke between their power strokes.
The crank shaft of a 4-cylinder engine is so made that the middle pistons move in the same direction, and opposite to the end pistons. This construction has been found to make a smoother running engine than if pistons 1 and 3 moved one way while pistons 2 and 4 moved the other.
If piston 1 is on the power stroke, either piston 2 or piston 3 can follow, for they are moving in the opposite direction. If we say that piston 2 is the next, then piston 4 must be the third to give a power stroke, for it is the only one left that is moving in the opposite direction to piston 2. Piston 3 is thus the fourth to move under power, and it is followed by another power stroke by piston 1; the firing order is then said to be 1, 2, 4, 3.
If it is piston 3 that follows piston 1, piston 4 will again be the third to produce power, and piston 2 will be the fourth. The firing order will then be 1, 3, 4, 2. There is no other order in which a 4-cylinder engine can produce power, and there is no choice between them.
The firing order of an engine is established by the manufacturer, and depends on the order in which the valves are operated.