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FOUR STROKE, DIESEL CYCLE, COMPRESSION IGNITION ENGINE

 


        FOUR STROKE, DIESEL CYCLE, COMPRESSION IGNITION ENGINE

The engine of heavy motor vehicle ( like truck, buses) stationary power plant big industrial units and ships mostly operate on diesel cycle or constant pressure cycle. It was introduced by Dr Rudolph Diesel in 1897. Diesel cycle differs from Otto cycle in one respect. The heat is added at constant pressure instead of at constant volume. Thus, it comprises two adiabatic process, one constant pressure heat addition process and one constant volume heat rejection process. Diesel cycle engines mostly use heavy oils. Diesel oil is the most common. Diesel cycle engine only air is compressed in the cylinder to a high pressure, the temperature of nice compressed air Sufficiently high to ignite fuel. Diesel is injected in the cylinder at the end of compression stroke which itself ignitor new to high temperature of the compressed air. There is no spark plug in diesel engine. In a four stroke diesel cycle, compression ignition engine the four stroke are as follows.



1. Suction stroke. During the suction stroke, the piston moves down from top dead centre position. Air is sucked into the cylinder through the open inlet valve which closed at the end of the stroke. The exhaust valve remains closed during this stroke.

2. Compression stroke. The piston moves upward from bottom dead centre position. Inlet and exhaust valve remains closed. The air is compressed in the cylinder with the upward movement of the piston. As the compression ratio in the engine is high ( 10-20 ) the air is finally compressed to a pressure as high as 40kg /cm square at which it’s temperature is as high as 100 degree Celsius enough to ignite the fuel.

3. Working stroke or power stroke. At the end of compression stroke the fuel (diesel) is injected into the hot compressed air where it starts burning maintaining the pressure constant. At the point 4, the fuel supply is cut off. Theoretically the fuel is injected at the end of compression stroke and injection continued till the point of cut off. Meanwhile the piston moves from point 3 to 4 and this stroke is called constant pressure stroke. In actual practice, the ignition starts before the end of the compression stroke.

Theoretically, the power stroke starts from point 4. The hot gasses now expand adiabatically to the point 5in the cylinder pushing the piston down. The piston finally reaches to bottom dead centre. Both the valves remain closed during this stroke.

4. Exhaust stroke. The piston moves upword the inlet valve remains closed and the exhaust valve opens. The greater part of the burnt gases escape because of their own expansion. The upward movement of the piston pushes the remaining gasses out through the open exhaust open valve. The cycle is thus completed.



Actual and theoretical p-v diagram for a four stroke, diesel engine are shown respectively

Shows valve timing diagram for four stroke diesel cycle engine . Inlet valve open 10° to 25° in advance the top dead centre : and closes 25° to 50° after the bottom dead centre. Exhaust valve opens 30° to 50° in advance of the bottom dead centre; and closes 10° to 15° after the top dead centre.



Fuel injection starts 5 to 10° before the top dead centre in compression stroke and continues upto 15° to 25° after the top dead centre in working stroke, depending upon the speed of the engine.




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