With the help of a P-V diagram explain the working of a four-stroke diesel engine.

5.A] With the help of a P-V diagram explain the working of a four-stroke diesel engine.


Four Stroke Diesel Engine: Diesel engine works on the diesel cycle (constant pressure combustion cycle). In this type of engine fuel injector is used for injection of fuels. They are also called as compression ignition engines (C.I engines). The compression ratio is 14-20 for C.I engines. Due to the use of high compression ratio, the temperature a compression stroke is significantly high to self-ignite the fuel which is injected into the chamber.

P-V diagram:

Working: The schematic representation of sequence of operations for 4-s diesel engine is shown in figure

1. Suction stroke: Suction stroke starts when the piston is at TDC and about to move downwards. At this time, inlet valve is open and exhaust valve is closed. In this situation, piston moves from TDC to BDC in downward direction. As the piston moves downward, suction is created (or) negative pressure is created in the cylinder and the fresh air is sucked into the cylinder through inlet valve. The pressure inside the cylinder will be slightly less than the atmospheric pressure. When the piston reaches BDC, suction stroke ends and inlet valve closes.

2. Compression stroke: During this stroke, piston moves from BDC to TDC and both the valves are kept closed. As the piston moves upward, the air gets compressed to high pressure and temperature. Just at the end of compression stroke, the fuel is being injected in the form of spray into the compressed air by a fuel injector and combustion of fuel takes place. The temperature of the compressed air is sufficient to auto ignite the fuel without the need of spark. Combustion of fuel takes place at constant pressure. The products of combustion (burnt gases) are formed.

3. Working stroke: In expansion stroke both the valves remain closed. Expansion of products of combustion takes place and the piston moves in the downward direction to BDC. The work is done during this stroke.

4. Exhaust stroke: During this stroke, exhaust valve opens and inlet valve remains closed. The piston moves from BDC to TDC. The piston while moving from BDC to TDC drives the burnt gases into atmosphere through the open exhaust valve so as to make room for the fresh charge (only air) to enter the cylinder. The exhaust valve closes after the piston reaches the TDC. This completes the cycle.

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