Elements of Mechanical engineering (18ME15/25) a VTU syllabus topic.
Module 3: Internal Combustion engines introductions. working of four stroke diesel engine.

4-stroke Diesel Engine

The working principle of a 4-stroke diesel engine is based on theoretical diesel cycle. Hence it is also called diesel cycle engine.

Suction stroke: 

At the beginning of the stroke piston is in TDC and during the stroke, piston moves from TDC to BDC. The inlet valve opens and the exhaust valve will be closed. The downward movement of the piston creates a suction in the cylinder and as a result, fresh air is drawn into the cylinder through the inlet valve. when the piston reaches the BDC, the suction stroke completes and this is represented by the line AB on P-V diagram.

Compression stroke: 

At the beginning of the strike piston is in the BDC and during the stroke piston moves from BDC to TDC. Both inlet and the exhaust valves are closed. As the piston moves upwards, air in the cylinder is compressed to a high pressure and temperature. The compression process is adiabatic in nature and is shown by the curve BC in P-V diagram. At the end of the stroke , the fuel(diesel) is sprayed into the cylinder by fuel injector. As the fuel comes in contact with the hot compressed air, it get ignited and undergoes a combustion at constant pressure. This process is shown by the line CD on PV diagram. At the point D fuel supply is cutoff. The compression ratio ranges from 16:1 to 20:1.

Power stroke/Expansion stroke/working stroke:

At the beginning of this stoke, piston is in TDC and during the stroke, piston moves from TDC to BDC. Both inlet and exhaust valve remain closed. As combustion of fuel takes place, the burnt gases expand and exert a large force on the piston. Due to this, piston is pushed from TDC to BDC. The power impulse is transmitted down through the piston to the crank shaft through the connecting rod. This causes the crankshaft to rotate at high speeds. Thus work is obtained in this stroke.

The expansion of gases is adiabatic in nature and this is shown by the curve DE on P-V diagram. When the piston reaches the BDC, the exhaust valve opens. A part of burnt gases escapes through the exhaust valve out of the cylinder due to self expansion. The drop in pressure at constant volume is shown by the line EB on P-V diagram.

Exhaust Stroke:

At the beginning of the stroke piston is in BDC and during this stroke, piston moves from BDC to TDC. The inlet valve is closed and the exhaust valve is opened. As the piston moves upward, it forces the remaining burnt gases out of the cylinder through the exhaust valve. this is shown by the line BA on P-V diagram. When the piston reaches the TDC the exhaust valve closes. This completes the cycle.

In the next cycle the piston which is at the TDC moves to BDC there by allowing fresh air to enter into the cylinder and the process continues.