Now, as the piston moves up on the exhaust stroke, it forces the burned gases out of the cylinder through the exhaust port. When the piston reaches the top of its travel, the exhaust valve closes, and the intake valve opens. The cycle repeats again with the intake stroke. The four strokes are continuously repeated during the operation of the engine. In the Diesel, the fuel is not mixed with the air entering the cylinder during the intake stroke.
Air alone is compressed during the compression stroke.
The Diesel fuel oil is injected or sprayed into the cylinder at the end of the compression stroke. This temperature is high enough to spontaneously ignite the fuel as it is injected into the cylinder. The high pressure of the explosion forces the piston down as in the gasoline engine. These four strokes require two revolutions of the crankshaft. The process continuously repeats itself during the operation of the engine.