Figure 4-1. Reed valve is open with low pressure and closes when the pressure increases. Figure 4-2. Intake rotary valve for a two cycle engine. Crankcase Vacuum Intake Stroke—Piston Moving up: Figure 4-3 a to b Crankcase Compression Stroke—Piston Moving down: Figure 4-3 b to c Crankcase Transfer/Exhaust—Piston at lowest:  Figure 4-3 d Cylinder start of Compression Stroke—Piston  initially Moving up: Figure 4-3 e 4-2 The upward stroke of the piston [Figure 4-3a] cre- ates a vacuum in the crankcase and pulls the fuel/ air/oil mixture into the crankcase through the intake  valve system from the carburetor. [Figure 4-3b] This  can be a pressure-actuated reed valve, a rotary valve,  or a third ported inlet system where the lower piston  skirt provides an opening for the fuel/air/oil mixture  to flow in when the piston is reaching its highest  point Top Dead Center (TDC). At this point, the  greatest portion of the fuel/air/oil mixture has filled  the crankcase. During the downward stroke, the pressure valve is  forced closed by the increased crankcase pressure,  the mechanical rotary valve closes, or the piston  closes off the fuel/air oil mixture intake port. The  fuel mixture is then compressed in the crankcase  during the downward stroke of the piston. and the exhaust port is open. Some of the fresh fuel/ air/oil mixture can escape out the exhaust port result- ing in the higher fuel use of the two stroke engine. As the piston starts to move up, covering the transfer  port, the tuned exhaust bounces a pressure wave at  the precise time across the exhaust port (more on  this in the exhaust system discussion) to minimize  When the piston is near the bottom of its stroke, the  transfer port opening from the crankcase to the com- bustion chamber is exposed, and the high pressure  fuel/air mixture in the crankcase transfers around the  piston into the main cylinder. This fresh fuel/air/oil mixture pushes out the exhaust  (called scavenging) as the piston is at its lowest point 