A 4-stroke diesel engine has a cycle of four strokes. A stroke is the distance from top dead center to bottom dead center. The piston travels down for one stroke on intake, up for compression, down for power, and back up for exhaust.
In intake, or induction, the inlet valve opens and the piston starts to move down from top dead center. Air enters the cylinder through the inlet port. When the piston reaches bottom dead center, the cylinder is full of air. The inlet valve closes.
The piston starts up from bottom dead center. The exhaust valve is closed so the cylinder is sealed. The piston’s upward motion compresses the air. When the piston reaches top dead center, the air is compressed to about one-sixteenth of its original volume. This is higher compression than in a similar petrol engine. Compressing the air also heats it.
Both valves stay closed as the piston rises. Just before it reaches top dead center, an injector sprays fuel into the chamber. It mixes with the very hot compressed air and ignites. Combustion occurs, the temperature rises much higher and the gases expand and force the piston down in a power stroke. The piston reaches bottom dead center, the exhaust valve opens.
With the exhaust valve open and inlet valve closed, the piston moves up, forcing exhaust gases out of the exhaust port. The piston reaches top dead center, the exhaust valve closes, the inlet valve opens and the cycle starts again.