Four Stroke Engine

The diagram opposite shows the "strokes" involved in a 4cycle spark-ignited internal combustion engine. The intake stroke (a)
starts with the piston at the top of the cylinder (TDC) and the intake valve open to admit air and fuel.
At bottom-dead-center (BDC)
the inlet valve is closed and the
compression stroke (b) starts.

Work is done on the air/fuel mixture as the piston moves to TDC. In the vicinity of TDC the spark ignites the mixture liberating the heat of reaction and increasing the temperature and pressure in the gas. The power stroke extracts work from these gases as they expand and force the piston to BDC. In the exhaust stroke, the exhaust valve is open and the combustion products are expelled from the cylinder. The sequence shown on the next page represents this behavior for a Mitsubishi direct injection engine