ENGINE CYCLES
Now, with the basic knowledge you have of the parts and the four strokes of the engine, let us see what happens during the actual running of the engine. To produce sustained power, an engine must repeatedly complete one series of the four strokes: intake, compression, power, and exhaust. One completion of this series of strokes is known as a cycle.
Most engines of today operate on four-stroke cycles, although we use the term four-cycle engines to refer to them. The term actually refers to the four strokes of the piston, two up and two down, not the number of cycles completed. For the engine to operate, the piston continually repeats the four-stroke cycle.
TWO-CYCLE ENGINE
In the two-cycle engine, the entire series of strokes (intake, compression, power, and exhaust) takes place in two piston strokes.
-Events in a two-cycle, internal combustion engine.
is engine is a power stroke. Each time the piston moves down, it is on the power stroke. Intake, compression, power, and exhaust still take place; but they are completed in just two strokes. Figure 12-5 shows that the intake and exhaust ports are cut into the cylinder wall instead of at the top of the combustion chamber as in the four-cycle engine. As the piston moves down on its power stroke, it first uncovers the exhaust port to let burned gases escape and then uncovers the intake port to allow a new fuel-air mixture to enter the combustion chamber. Then on the upward stroke, the piston covers both ports and, at the same time, compresses the new mixture in preparation for ignition and another power stroke.
In the engine shown in figure 12-5, the piston is shaped so that the incoming fuel-air mixture is directed upward, thereby sweeping out ahead of it the burned exhaust gases. Also, there is an inlet into the crankcase through which the fuel-air mixture passes before it enters the cylinder. This inlet is opened as the piston moves upward, but it is sealed as the piston moves downward on the power stroke. The downward moving piston slightly compresses the mixture in the crankcase. That gives the mixture enough pressure to pass rapidly through the intake port as the piston clears this port. This action improves the sweeping-out, or scavenging, effect of the mixture as it enters and clears the burned gases from the cylinder through the exhaust port
