In comparing views A and B of figure 6-3, note that the length of the supercharging and scavenging periods in a 2-stroke cycle engine is not the same as those in a 4-stroke cycle engine. Also, there is considerable difference in piston location between the times when these processes take place in the two types of engines. In a 4-stroke cycle, scavenging takes place while the piston is traveling through the latter part of the upstroke and the early part of the downstroke, and supercharging takes place when the piston is in the vicinity of BDC. In a 2-stroke cycle, the processes of scavenging and supercharging both take place while the piston is in the lower part of the cylinder. In a 4-stroke cycle engine, a piston does much of the work of intake and exhaust. In a 2-stroke cycle engine, however, the piston does very little work in these two processes. Because of this, many 2-stroke cycle engines use a blower to force air into the cylinder and to clear out the exhaust gases.

View B of figure 6-3 is based on the 2-stroke cycle of operation. If you compare view B with view A (fig. 6-3), the differences in the scavenging and supercharging processes in 2- and 4-stroke cycle engines are more apparent. Start your study of the cycle with the piston at TDC in view B of figure 6-3. Fuel has been injected, ignition has occurred, and combustion is taking place. The power developed forces the piston through the power event until the piston is 92 1/2 (as compared to 125 for the 4-stroke cycle in view A) past TDC, just a little more than halfway through the downstroke. At this point, the exhaust valves open, gases escape through the manifold, and cylinder pressure drops rapidly. When the piston reaches a point 48 before BDC, the intake ports are uncovered as the piston moves downward and scavenging begins. (Compare this with the opening of the intake valves in a 4-stroke cycle in view A.) The scavenging air, under blower pressure, swirls upward through the cylinder and clears the cylinder of exhaust gases. The situation in the cylinder when scavenging starts is approximately the same as that illustrated in figure 6-2. Note the position of the piston, the open scavenging ports, the open exhaust valves, and the flow of air through the cylinder. The flow of scavenge air through the cylinder also helps to cool the parts, which are heated by combustion.

Study again view B of figure 6-3. Scavenging continues until the piston is 44 1/2 past BDC (a total of 92 1/2 as compared with 160 in the 4-stroke cycle in view A), at which point the exhaust valves close. In a 2-stroke cycle engine, the exhaust valves remain open during only 132, as compared with the 320 in the 4-stroke cycle. The scavenge ports remain open for another 3 1/2 of shaft rotation (45 in the 4-stroke cycle), and the blower continues to force air into the cylinder. Even though the ports are open for only a short interval after the exhaust valves close, enough time is available for the blower to create a supercharging effect before the compression event starts.

The piston closes the intake ports at 48 past BDC. The compression event takes place during the remainder of the upstroke, with injection and ignition occurring at TDC. At this point one cycle is ended and another is ready to start.