# Diesel Engine - Four Stroke Explanation, Cycle and Efficiency | Physics Grade 11 Notes

### Thermodynamics

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# Diesel Engine - Four Stroke Explanation, Cycle and Efficiency

Diesel Engine
The cylinder of diesel is provided with three valves namely inlet, oil supply and exhaust valve, I, O, and E respectively. It also works on four stroke principle;

1. 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.
2. Compression stroke: All the valves are close, as the piston start moving inwards, air is compressed adiabatically to about on-seventeenth of its initial volume by the inward motion of the piston. In this process the temperature is reaised to about 1000C.
3. Working stroke: The oil valve O is opened and diesel oil is forced in under pressure. At high temperature, the oil burns spontaneously. The supply of oil is so regulated that during combustion, as the piston moves forward, the pressure remains constant. In this process, the burning fuel supplies heat to air at constant pressure. At a certain stage when the temperature has reached the maximum value, about 2000K, the supply of oil is cut off. This is the stroke in which useful work is done.
4. Exhaust stroke: As the piston moves inwards, exhaust valve E is opened and the useless gas mixture is forced out of the cylinder as shown in the figure.
Efficiency of diesel engine
Suppose Q1 be the amount of heat absorbed by the working substance at a constant pressure during combustion DA. Let Q2 be the amount of heat rejected at constant volume during the release of heat BC.
Then the efficiency is given by,
η = 1 – Q/ Q1
= 1- (1/ϱ)γ-1

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