content_image/550690.jpg content_image/550689.jpg
<img src='content_image/550690.jpg'>

## Crankcase Vacuum Intake Stroke—Piston Moving up: Figure 4-3 a to b 

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. 

## Crankcase Compression Stroke—Piston Moving down: Figure 4-3 b to c 

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. 

## Crankcase Transfer/Exhaust—Piston at lowest: Figure 4-3 d 

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 

<img src='content_image/550689.jpg'>

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. 

## Cylinder start of Compression Stroke—Piston initially Moving up: Figure 4-3 e 

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 

