Piston moves up Piston is at top Piston moves down Piston is at bottom Lower crankcase chamber Upper combustion chamber Vacuum created in  crankcase A Fuel/Oil/Air mixture  enters from carburetor B Fuel/Oil/Air mixture  pressurized C Pressurized Fuel/Oil/ Air mixture transferred D Fuel/Oil/Air mixture pressure E Fuel/Oil/Air ignited F Piston forced down G Fuel/Air enters and exhaust exits H Piston Ported Two-Stroke Cycles 1 2 Connecting Rod Piston Exhaust port Crank Crankcase Tuned exhaust pressure wave Fuel/Oil/Air intake port Transfer port Figure 4-6. Piston ported inlet cycles for a two-stroke engine. 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] Crankcase Transfer/Exhaust—Piston at Lowest Cylinder Power Stroke—Initial Piston Moving  Down Cylinder Start of Compression Stroke—Piston  Initially Moving Up Cylinder Power Stroke—Final Piston Moving Down Cylinder Compression Stroke—Piston Moving Up Piston Reverses Direction From Down Stroke to Up  Stroke 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] 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] 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] 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] As the piston reverses direction from the down stroke to the up  stroke, the process is complete.  [Figures 4-6H  and  4-6A] 4-5