Lesson 1/Learning Event 1
The piston first uncovers the exhaust port to end the power stroke and to allow gas pressures in the cylinder to
escape. Then the intake port is uncovered. The compressed fuel-air mixture in the crankcase now flows into
the cylinder through the open intake port. Because the mixture is under pressure, it pushes the remaining
exhaust gases out through the exhaust port.
Notice that the piston head is shaped so it deflects the in flowing mixture upward, away from the exhaust port.
This is to keep the fresh mixture from mixing with the exhaust gases and being lost out through the open
The cylinder is now full of a fuel-air mixture as we imagined at the beginning of the cycle. Take particular
notice that the complete cycle of operation occurred in only one turn of the crankshaft and two piston strokes.
The cycle is repeated again and again as the engine continues to run under its own power.
In an engine of this type, engine oil is generally mixed with the gasoline fuel. Then, as the gasoline, oil, and air
mixture passes through the crankcase in vapor form, it lubricates engine parts.