Four Stroke Engine
Figure. shows the working of a 4 stroke engine. During the 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.
Fig. 4.7. Cycle of events in a four stroke petrol engine
The charge enters the engine cylinder through the inlet valve which is open. During this stroke, the exhaust valve is closed. During the compression stroke, the charge is compressed in the clearance space. On completion of compression, if only air is taken in during the suction stroke, the fuel is injected into the engine cylinders at the end of compression. The mixture is ignited and the heat generated, while the piston is nearly stationary, sets up a high pressure. During the power stroke, the piston is forced downward by the high pressure. This is the important stroke of the cycle. During the exhaust stroke the products of combustion are swept out through the open exhaust valve while the piston returns. This is the scavenging stroke. All the burnt gases are completely removed from the engine cylinder and the cylinder is ready to receive the fresh charge for the new cycle.
Thus, in a 4-stroke engine there is one power stroke and three idle strokes. The power stroke supplies the necessary momentum to keep the engine running
