ENGINE DRIVE MECHANISMS

Frequently, the source of power that operates one engine part is also the source of power for other parts and accessories of the engine. For example, the source of power that operates engine valves may also be the source of power that operates such items as the governor; fuel, lubricating, and water pumps; and overspeed trips. Since mechanisms that transmit power to operate specific parts and accessories may be related to more than one engine system, we will discuss drive mechanisms before getting into the engine systems.

After reading the information in this chapter, you should be able to recognize the basic design, function, and arrangement of various parts associated with drive mechanisms of 2-stroke and 4-stroke cycle diesel engines.

As used in this chapter, DRIVE MECHAN-ISM identifies the group of parts that takes power from the crankshaft and transmits that power to various engine components and accessories. In engines, the drive mechanism does not change the type of motion, but it may change the direction of motion. For example, the impellers of a blower are driven or operated by a rotary motion from the crankshaft transmitted to the impellers by the drive mechanism, an arrangement of gears and shafts. While the type of motion (rotary) remains the same, the direction of motion of one impeller is opposite that of the other impeller as a result of the gear arrangement within the drive mechan-ism. A drive mechanism may be a gear, chain, or belt type. The gear type is the most common. Some engines use chain assemblies or a combination of gears and chains as the driving mechanism. Belts are not common on marine engines, but are used as drive mechanisms on gasoline engines.

Some engines have a single-drive mechanism that transmits power to operate engine parts and accessories. In some engines, there may be two or more separate mechanisms. When separate assemblies are used, the one that transmits power to operate the accessories is called the ACCESSORY DRIVE. Some engines have more than one accessory drive. A separate drive mechanism that serves to transmit power to operate engine valves is generally called the CAM-SHAFT DRIVE or TIMING MECHANISM.

The camshaft drive, as the name implies, transmits power to the camshaft of the engine. The shaft, in turn, transmits the power through a combination of parts which causes the engine valves to operate. Since the valves of an engine must open and close at the proper moment (with respect to the position of the piston) and remain in the open and closed positions for definite periods of time, a fixed relationship must be main-tained between the rotational speeds of the crankshaft and the camshaft. Camshaft drives are designed to maintain the proper relationship between the speeds of the two shafts. In main-taining this relationship, the drive causes the cam-shaft to rotate at crankshaft speed in a 2-stroke cycle engine and at one-half crankshaft speed in a 4-stroke cycle engine.

There is considerable variation in the design and arrangement of the parts of drive mechanisms found in different engines. The size of an engine, the cycle of operation, the cylinder arrangement, and other factors govern the design and arrange-ment of the components as well as the design and arrangement of the mechanisms. Some of the variations in drive mechanisms are considered in the descriptions and illustrations that follow. The arrangements of the drive mechanisms described in this chapter are representive of those commonly found in marine engines used by the Navy.