While there is no vibration from the movement of the pistons in this engine, the uneven production of power will produce vibration of another kind. These two types may be built with the cylinders standing up or lying down; that is, they may be either _vertical engines_ or _horizontal engines_. The _double opposed_ engine, which is built only in horizontal form, is free from either kind of vibration, but has the disadvantage of occupying more room than the others. The cylinders, instead of being side by side, and on the same side of the crank shaft, are placed end to end, with the crank shaft between them, as shown in Figure 5. The pistons make their inward and outward strokes together, but in so doing they move in opposite directions. Thus every power stroke is followed by a dead stroke, as in the engine shown in Figure 3, while the movement of one piston balances that of the other, as is the case with the engine shown in Figure 4. [Illustration: FIG. 5.—H. D. O. POWER DIAGRAM] [Illustration] [Illustration: 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. CHAPTER III ENGINE PARTS