Four Stroke Engine - thermodynamics:

Figure given below shows the working of a four stroke engine. During suction stroke only air (in case of diesel engine) or air with petrol (in case of petrol engine) is drawn into the cylinder by the moving piston.

The charge enters the engine cylinder through the inlet valve that is open. During this stroke, the exhaust valve is closed. At the time of the compression stroke, charge is compressed in clearance space. On the finishing point of compression, if only air is taken in during the suction stroke, fuel is injected into engine cylinders at the end of compression. The mixture is then ignited and the heat is generated, while piston is stationary, sets up high pressure. During the power stroke, piston is forced downward by high pressure. This is the important stroke of the cycle. At the time of the exhaust stroke the products of combustion are swept out through open exhaust valve while piston returns. It is scavenging stroke. The burnt gases are entirely removed from the engine cylinder and cylinder is ready to receive fresh charge for the new cycle.

Therefore, in four-stroke engine there is one power stroke and 3 idle strokes. The power stroke supplies the required momentum to keep the engine running.