The in-line engine discussed in the preceding section requires only one drive mechanism (gear train) to transmit power to the valve actuating gear and engine accessories. Our discussion will now cover an engine that uses two separate gear drives (gear trains), one at each end of the engine. The front gear train, as shown in figure 5-4, consists of a crankshaft gear and two idler gears. The idler gears serve to drive the water pump (not shown) and balance the engines. (See balance weights.)

The rear gear train (fig. 5-5) consists of a crankshaft gear, three idler gears, and two cam-shaft gears. The rear idler gears, like the front, also serve to balance the engine. (See balance weights.) The two other gears that are mounted on the rear of the engine as shown in figure 5-5

Figure 5-4.Front gear train of a 16-V 149 series Detroit diesel engine.

are the accessory drive gear and the blower drive gear. The blower drive gear supplies the power that operates many of the same accessories as those on the GM 71 in figure 5-3.

The correct relationship between the crankshaft and the two camshafts must be main-tained so that the fuel injection, the opening and closing of exhaust valves, and the engine balance can be properly controlled. Since the camshaft must be in time with the crankshaft, timing marks are stamped on the face of the gears to facilitate correct gear train timing. The timing marks stamped on various gears are shown in figures 5-1 and 5-6. When an engine is assembled, whether it is a 2-stroke or 4-stroke cycle engine, it is important that the appropriate timing marks be lined up on the gears as each gear is installed.