We have talked about the events taking place in a cycle of engine operation, but we have said very little about piston strokes except that a complete sequence of events will occur during a cycle regardless of the number of strokes made by the piston. The number of piston strokes occurring during any one cycle of events is limited to either two or four, depending on the design of the engine. Thus, we have a 4-stroke cycle and a 2-stroke cycle. These cycles are known as the mechanical cycles of operation.

From your study of Fireman, NAVEDTRA 10520-H, you should recall that the terms 4-stroke and 2-stroke identify the number of strokes the piston makes during a cycle of events. You should also recall that both types of mechanical cycles, 4-stroke and 2-stroke, are used in both diesel and gasoline reciprocating engines. Most gasoline engines in Navy service operate on the 4-stroke cycle. Most diesels operate on the 2-stroke cycle. You may be required to operate and maintain engines that operate on either of these mechanical cycles. Therefore, you should be familiar with the principal differences in these cycles. The relation-ship between the events and piston strokes occurring in a cycle of operation involves some of these differences. A thorough understanding of the relationship will aid you in carrying out your duties in connection with engine operation and maintenance.

Relationship of Events and Strokes in a Cycle

A piston stroke is the distance a piston moves between limits of travel. The cycle of operation in an engine that operates on the 4-stroke cycle involves four piston strokesINTAKE, COMPRESSION, POWER, and EXHAUST. In the 2-stroke cycle, only two strokes are involved POWER and COMPRESSION.

In figure 2-2 the strokes are named to correspond with the events.

However, since six events are listed for diesel engines, more than one event must take place during some of the strokes, especially in the 2-stroke cycle. Even so, it is common practice to identify some of the events as strokes of the piston. This is because such events as intake, compression, power, and exhaust in a 4-stroke cycle involve at least a major portion of a stroke and, in some cases, more than one stroke. The same is true of power and compression events and strokes in a 2-stroke cycle. In associating the events with strokes, you should not overlook other events taking place during a cycle of operation. You will have to consider all events in the operation of an engine when you begin to deal with maintenance problems involving the timing of fuel injection systems.

4-Stroke Cycle Diesel Engine

To help you understand the relationship between events and strokes, we will discuss the number of events that occur during a specific stroke. We will also discuss the duration of an event with respect to a piston stroke and the cases where one event overlaps another. We can demonstrate the relationship of events to strokes by showing the changing situation in a cylinder during a cycle of operation. Figure 2-2 illustrates these changes for a 4-stroke cycle diesel engine. The relationship of events to strokes is more readily understood if the movements of a piston and its crankshaft are considered first. In figure 2-2, each view showing piston travel is reflected on the graph and illustrates the approximate piston position and valve action. Top center and bottom center identify points where changes in direction of motion take place. In other words, when the piston is at top center, upward motion has stopped and downward motion is ready to begin. With respect to motion, the piston is dead. Likewise, when the piston is at bottom center, downward motion has stopped and upward motion is ready to begin and the piston is said to be at bottom dead center.

The points which designate changes in direction of motion for a piston and crank are frequently called TOP DEAD CENTER (TDC) and BOTTOM DEAD CENTER (BDC). You should keep TDC and BDC in mind since they identify the start and end of a STROKE (180 degrees of crankshaft rotation) and since they are the points from which the start and end of EVENTS are established.

Lets pick up the action of the piston of a 4-stroke cycle engine as it moves up the cylinder towards TDC. Refer to figure 2-1 and locate the top arrow labeled EXHAUST EVENT OF PRECEDING CYCLE. This arrow indicates that the exhaust valve or valves at the top of the cylinder are open as the piston reaches TDC. And, several degrees after TDC, the dashed line indicates that the exhaust valves are closed. Now, locate the arrow labeled INTAKE EVENT (AIR). Notice that this arrow starts at the same point in the cycle as the exhaust event for the preceding cycle. The shaded parts of the two top arrows indicate that during this part of the cycle, both the intake and exhaust valves will be open. This is necessary because the incoming air must be allowed to flow through the cylinder to sweep out or scavenge any exhaust gases remaining from the preceding combustion event. The remainder of the arrow labeled INTAKE EVENT (AIR) shows that the intake valves remain open as the piston moves toward BDC and slightly past BDC. The dashed line just past BDC indicates when the intake valves are closed, and the compression stroke begins. Now, the piston continues moving up to compress the air trapped in the cylinder, because during this event, both the intake and exhaust valves are closed. The action of the pistons in compressing the air rapidly increases the pressure within the cylinder. This action, in turn, results in a rapid increase in temperature. The increase in temperature occurs because the molecules of air are being squeezed into a smaller and smaller space. Just before the piston reaches TDC, fuel is injected into the cylinder as a spray of finely atomized droplets (very tiny drops). As some of the molecules of the hydrocarbon fuel come into contact with hot molecules of oxygen, the combustion process begins. The rapid burning of the fuel as it mixes with the oxygen results in a drastic increase in temperature and pressure as the gases released by the combustion process expand. Since the piston is free to move, the rapidly expanding gases force the piston to begin its downward stroke, which is indicated by the arrow labeled POWER EVENT (EXPANSION OF GASES). The power stroke continues to the point indicated by the dashed line, at which time the exhaust valves open. The arrow labeled EXHAUST EVENT (WASTE GASES) shows that the exhaust valves are open as the piston moves towards BDC . . . past BDC . . . and up towards TDC, at which point the cycle of events repeats.

In summary, during the 4-stroke cycle, the piston moves up and down within the cylinder, and the crankshaft makes two complete revolutions, for a total travel of 720 degrees. In other words, 4-stroke cycle engines have only one power event per cylinder during 720 degrees of crank-shaft rotation.