This type of engine is mostly used in heavy vehicles. The engine as such is heavier than a Petrol Engine/Spark Ignition Engine. Diesel Engine like in Spark Ignition Engine is a four stroke, but it does not use spark to ignite the fuel, instead it uses high temperature generated as a result of heavy compression.
The four strokes in a diesel Engine are: Suction Stroke, Compression Stroke, Power Stroke, Exhaust Stroke.
1) Suction Stroke: The piston moves down from the Top Dead Centre. The inlet valve is open in this stage. Negative pressure created inside the cylinder sucks air into the cylinder through the inlet valve.
2) Compression Stroke: The piston moves from the Bottom Dead Center to the Top Dead Center, compressing the air inside the cylinder. The compression ratio is 10 to 20 and the pressure becomes as high as 40kg/sq.cm, temperature as high as 100 degree Celsius. As result of compression, the pressure and temperature inside the cylinder rises rapidly.
3) Power Stroke: It is at this point that the fuel is injected into the engine cylinder. And Because of the high temperature prevailing inside the cylinder, the injected fuel burns instantaneously and produces gases. The hot gases produced expands adiabatically pushing the piston down.
4) Exhaust Stroke: At the beginning of the exhaust stroke, the piston is at the BDC. The exhaust valve opens, and the piston moves upwards towards the TDC, pushing the exhaust gases out of the cylinder through the exhaust valve.
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