Figure 2-3 shows a comparison of the events that occur during the same length of time for both 2-stroke and 4-stroke cycle engines. The graph shows that, during the same amount of time, a 2-stroke engine will have two power events while a 4-stroke engine will have only one. This might lead you to believe that 2-stroke cycle engines are more efficient than 4-stroke engines; however, that is not the case. In order to work properly, a 2-stroke engine must have some method of forcing air into and through the cylinders. And, since this air pump (blower) is driven by the engine, it robs some of the horsepower that would otherwise be available to drive the load. Also, the combustion process in a 2-stroke engine is not as complete as it is in a 4-stroke cycle engine. Since each type of engine has certain advantages over the other, the Navy uses both 2-stroke and 4-stroke cycle engines for main propulsion and electrical generating service.

The illustrations we have used to represent the cycles of operation are for demonstration purposes only. The exact number of degrees before or after TDC or BDC at which an event starts and ends will vary among engines. You can

Figure 2-3.Comparison of the 2- and 4-stroke cycles.

find information on such details in the appropriate technical manuals dealing with the specific engine in question.