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## Crankcase Transfer/Exhaust—Piston at Lowest 

When the piston is near the bottom of its stroke, the transfer port opening from the crankcase to the combustion chamber is exposed, and the high pressure fuel/air mixture in the crankcase transfers around the piston into the main cylinder. This fresh fuel/oil/air mixture pushes out the exhaust (called scavenging) as the piston is at its lowest point and the exhaust port is open. Some of the fresh fuel/oil/air mixture can escape through the exhaust port, resulting in the higher fuel use of the two-stroke engine. [Figure 4-6D] 

## Cylinder Start of Compression Stroke—Piston Initially Moving Up 

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 to minimize the fuel/air/oil mixture escaping through the exhaust port. [Figure 4-6E] 

## Cylinder Compression Stroke—Piston Moving Up 

The piston then rises and compresses the fuel mixture in the combustion chamber. [Figure 4-6E to  4-6F] During this piston compression process, the crankcase vacuum intake 

process is happening simultaneously, as described earlier. This is why four processes can happen in two strokes. [Figures 4-6B and  4-6C] 

## Cylinder Power Stroke—Initial Piston Moving Down 

At the top of the stroke, the spark plug ignites the fuel/oil/air mixture and drives the piston down as the power stroke of the engine. [Figures 4-6F and  4-6G] 

## Cylinder Power Stroke—Final Piston Moving Down 

As the piston passes the exhaust port, the exhaust exits the combustion chamber. As the piston continues down, the transfer port opens and the swirling motion of the fuel/ oil/air mixture pushes the exhaust out of the exhaust port. [Figures 4-6H] 

## Piston Reverses Direction From Down Stroke to Up Stroke 

As the piston reverses direction from the down stroke to the up stroke, the process is complete. [Figures 4-6H and  4-6A] 

