F igure 4-3. Piston ported inlet for a two cycle engine. the fuel/air/oil mixture from escaping out the ex- haust port. The piston then rises, and compresses the fuel mix- ture in the combustion chamber. 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. At the top of the stroke, the spark plug ignites the  fuel mixture and drives the piston down as the power  stroke of the engine. As the piston passes the exhaust port, the exhaust  starts to exit the combustion chamber. As the pis- ton continues down, the transfer port opens and the  swirling motion of the air/fuel/oil mixture pushes the  exhaust out the exhaust port. As the piston reverses direction from the down  stroke to the up stroke the process is complete. Cylinder Power Stroke—Piston Moving Down: Figur e 4-3 f to g Cylinder Power Stroke—Piston Moving Down: Figur e 4-3 g to h Piston Reverses Direction From Down Stroke to  Up Stroke: Figure 4-3 h to a Figure 4-4. The cycles in a four-stroke engine.  4-3 Cylinder Compression Stroke—Piston Moving  Up: Figure 4-3 e to f