Read Combustion of fuel

The process of burning the fuel in the presence of oxygen inside the combustion chamber is known as combustion of fuel. As the fuel burns it gives the power to piston to move. Here the chemical energy derived from the combustion of fuel is converted into the mechanical energy of piston. The combustion process depends upon the type of ignition i.e. spark ignition and compression ignition.

Combustion in petrol or gasoline engine (spark ignition): In petrol engine the fuel and air mixture from the carburetor is inducted into the combustion chamber during the suction stroke. This mixture is compressed during the compression stroke. At the end of compression stroke spark is produced at the spark plug gap. The mixture in between the gap is heated to initiate combustion. Thus the flame is initiated at the spark plug and this flame propagates across the combustion chamber for complete combustion of charge.

Combustion in Diesel engine (Compression ignition): In diesel engine only air is inducted into the combustion chamber during the suction stroke. This air is compressed during the compression stroke so that the temperature of air becomes greater than the self ignition temperature of the fuel. The fuel (diesel) is injected into the combustion chamber in the form of fine spray at the end of compression stroke. The fuel ignites due to the high temperature of air. So burning of fuel does not require spark plug to initiate the combustion as in case of a petrol engine. After combustion has started the flame propagates across the combustion chamber.