Two of the more common small boat engines used in the Navy today are the 6-71 General Motors Detroit Diesel engine and the Westerbeke Model 4-107. The reason for their popularity is that they are reliable and parts are easy to obtain.Medium-sized combatant ships and many auxiliary vessels are powered by large (~50,000 Bhp ) single-unit diesel engines or, for more economy and operational flexibility, by combinations of somewhat smaller engines. Diesel engines have relatively high efficiency at partial load, and much higher efficiency at very low partial load than steam turbines. They also have greater efficiency at high speeds than any of the other fossil-fueled plants. Thus they require the least weight of fuel for a given endurance. Other advantages include low initial cost and relatively low RPM, the latter resulting in small reduction gears. Additionally, diesel engines can be brought on-line from cold conditions rapidly. They are reliable and simple to operate and maintain, having a long history of active development for marine use.
Large diesel plants have been adapted for use in the Navy on the LSD-41 (Whidbey Island) class (two medium-speed diesel engines drive each of the two shafts). In general, however, the use of diesels on intermediate sized combatants and larger requires that several smaller units be combined to drive a common shaft. This requirement results in severe space and arrangement problems. Among other disadvantages is the fact that periodic engine overhaul and progressive maintenance are required. These result in frequent down periods, which, because of the number of similar units, may not increase the amount of necessary in-port maintenance time, but do decrease the amount of time the ship has full power available while at sea. Finally, the marine diesel has a high rate of lube oil consumption, which may approach 5% of the fuel consumption; thus large quantities of lube oil must be carried.The moving parts of the diesel engine provide for controlling the elements necessary for combustion and the transformation of combustion to mechanical shaft energy. The major moving components are the crankshaft, piston assembly, connecting rod, camshaft, valves, operating gear, flywheel, vibration dampener and various gears.
For a diesel engine to start, it must turn over fast enough to obtain sufficient heat to ignite the fuel-air mixture. Combustion forces the piston down or outward (power stroke) from rapid expansion of the gases. Some factors that effect the starting of an engine include: ambient temperature, compression in the cylinders from excessive wear (low) or newly overhauled engine (high), load on the engine (attached generator) and proper pre-lubrication of the engine bearings prior to starting.There are two extremes of diesel engine maintenance and overhaul philosophy: run the engine until it breaks and then repair it (usually at the most inopportune time) or continuously tear it down to inspect and replace worn parts. Neither of these philosophies is cost effective nor do they increase engine availability. Monitoring the operational parameters of a diesel engine by plotting and analyzing their values is a proven way to disclose the mechanical condition of a diesel engine. As engine components wear, the operating parameters gradually change with time. Diesel engine trend analysis is the collection and analysis of diesel engine operating parameters for the purpose of predicting the need for corrective maintenance/overhaul. This data can be analyzed to determine the condition of the engine's internal components and critical support systems. Trend analysis and condition based maintenance are logical methods of determining the need for major engine maintenance actions and has been in use in Navy applications since the late 1960's.