The smooth operation of an engine and its steady production of power depends, to a great extent, on the arrangement of the cranks on the shaft and on the firing order of the cylinders. For uniform rotation of the crankshaft in most multicylinder engines, the power impulses must be equally spaced with respect to the angle of crankshaft rotation. Whenever possible, they must also be placed so that successive explosions do not occur in adjacent cylinders. (This arrangement is not always possible, especially in 2-, 3-, and 4-cylinder engines.)

Crankshafts may be classified according to the number of throwsl throw, 2 throws, and so forth. The 6-throw shaft illustrated in figure 4-19 is for a 6-cylinder, in-line, 2-stroke cycle engine. Shafts of similar design can be used in V-type engines.

The number of cranks and their arrangement on the shaft depend on a number of factors, such as the arrangement of the cylinders (in-line or V-type), the number of cylinders, and the operating cycle of the engine. How these factors influence throw arrangement and firing order can be seen in a comparison of examples a through e of figure 4-23. The arrangement of throws with respect to one another and with respect to the circumference of the main journals is generally expressed in degrees. In an in-line engine, the number of degrees between throws indicates the number of degrees the crankshaft must rotate to bring the pistons to TDC in firing order. This is not true in engines where each throw serves more than one cylinder. Figure 4-23 lists the examples of throws with respect to cylinder arrangement, the number of cylinders served by each throw, and the firing order of the cylinders. (The sketches are not drawn to scale and do not indicate relative size, but are for illustrative purposes only.)

In studying the examples in figure 4-23, remember that the crankshaft must make only one revolution (360) in a 2-stroke cycle; whereas two revolutions are required in a 4-stroke cycle. Note the throw arrangement in example a of a 4-stroke cycle engine. Since the 4-cylinder engine in example a operates on the 4-stroke cycle, throws 1, 3, 4, and 2 (see firing order), must be 180 apart in order for the firing to be spaced evenly in 720 of crankshaft rotation. Note too, that in all the other examples, the throws are equally spaced, regardless of cylinder arrangement, cycle of operation, or number of cylinders.

In examples b and c, the shaft design and the number of degrees between throws are the same. Yet the shaft in example c fires twice as many cylinders. This is possible because one throw, through a fork and blade rod, serves two cylinders which are positioned in 60 banks. Thus, even though both engines operate on the 4-stroke cycle, the 12-cylinder engine requires only 60 shaft rotation between power impulses.

There are six throws shown in examples b and d, yet they are 120 apart in one and 60 apart in the other. Why? The cylinder arrangement, the total number of cylinders, and the number of cylinders served by each throw are the same. In examples b and d, the operating cycle is the controlling factor in throw arrangement. In examples d and e, other variations in shaft throw arrangement and firing order are shown. Note that the differences are governed to a great extent by the cylinder arrangement, the number of cylinders served by the shaft and by each throw, and the operating cycle of the engine.