Diesel Engine is an internal combustion engine in which air is working substance and diesel is fuel. The working of diesel engine is divided into four steps as explained below and its PV diagram is shown.
- Suction stroke: Air valve I is opened and pure air at atmospheric pressure is drawn into the cylinder by the outward motion of the piston. This process is represented by PA on the PV-diagram.
- Compression stroke: All the valves are closed and air is compressed adiabatically to about 1/16th of its initial volume by the backward motion of the piston. In this process, the pressure of air inside the cylinder becomes 34 atmospheres and temperature equal to about 1000°C. This process is represented by AB on the PV-diagram.
- Working stroke: The oil valve O is opened and diesel oil is sprayed into the hot air inside the cylinder. Due to high temperature inside, the oil burns spontaneously. The rate of supply of oil is so adjusted that combustion takes place at constant pressure. This process is continued upto the cutoff point C. During this process, the burning fuel supplies heat to the air at constant pressure. At a certain stage when the temperature has reached the maximum value about 2000K, the oil supply is cut off. The hot air expands adiabatically and the piston moves in the outward direction in which useful work is done. This process is represented by CD on the PV-diagram.
- Exhaust stroke: The exhaust valve E opens and the burnt out gases are driven out until the pressure in the cylinder falls to the atmospheric pressure. This process is represented by DA on the PV-diagram. Now, from position A, the piston moves inside and the rest of the combustion products are exhausted at constant pressure. It is called exhaust stroke and is represented by AP on the PV-diagram.
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