FOUR STROKE DIESEL ENGINE:
The cycle of events that takes place in a four -stroke diesel engine has been shown. Unlike the petrol which drawn in a mixture of air and petrol, only air from the atmosphere is inducted into the diesel engine cylinder, through the inlet valve, during the suction stroke. This air is then compressed to about 1/22 of its original volume, which raised its temperature. The average automotive diesel engine has a compression ratio of 22:1 and a cranking compression pressure of 2.8Mpa. For every piston is at the top dead centre, the fuel is injected into the cylinder. The raised temperature of the compressed air is sufficient to ignite the fuel. There is an increase in pressure which drives the piston down giving working stroke. The exhaust valve opens at the end of the working stroke and the exhaust gases are driven out as the piston ascends. The combustion in a diesel occurs in four stages. These stages are going on simultaneously in different parts of the combustion chamber during injection of fuel.
(1) Delay period, in which the fuel injected into the cylinder changes from liquid to vapour phase.
(2) Uncontrolled burning of the fuel already injected into the combustion chamber.
(3) Controlled burning as the injector continues to feed fuel into the combustion chamber.
(4) After burning, which is the period for which the fuel remaining in the combustion chamber after the injection stops continues to burn? The amount of fuel injected into the combustion chamber determines the power developed in a diesel engine. Increasing the amount of fuel injected increase the engine speed or torque. The maximum amount of fuel depends upon the amount of air inducted in the intake stroke. However, the fuel injected is limited by a governor, which is usually set to provide fuel so as to result in maximum enrichment of about 10:1 by weight.