The working principle of four stroke diesel engine.

Suction stroke

Suction stroke is the first stroke. Piston moves down. The inlet valve is open and allows air to enter the cylinder and fills the space. Piston reaches the bottom centre and again starts moving up.

Compression stroke

The piston starts moving up. The inlet valve is closed. Air is compressed. The pressure and temperature of the air increase due to compression. Just before the piston reaching top dead center, diesel fuel is sprayed inside the cylinder. The fuel is sprayed by means of a fuel injector. The fuel is sprayed at a very high pressure overcoming the pressure of the compressed air. The fuel gets mixed up with air. The air contains oxygen. Since the fuel is a hydrocarbon the temperature of compressed air is sufficient to initiate the combustion. The fuel is burnt.

Four stroke diesel engine

Power stroke

The piston moves from top dead centre to bottom dead center. The heat released by combustion increases the pressure and temperature of the air further. The maximum pressure and temperature of the burnt gases act on the top of the piston and forces the piston to move fast, accelerates the piston downwards. This is the power stroke. Just before the piston reaches the bottom dead centre exhaust valve opens,

Exhaust stroke

The piston reaches the bottom dead centre and starts moving up. The burnt gases are being pushed out by the piston. The gases escape through exhaust valve. Before the piston reaches top dead centre the inlet valve is opened. The cycle is repeated.


The Two stroke Diesel engine  works as that of the Petrol engine but it injects diesel at end of compression stroke. The working principle is given below:

Two stroke cycle diesel engine

As the piston moves down on the power stroke, it first uncovers the exhaust port, and the cylinder pressure drops to atmospheric pressure as the products of combustion come out from the cylinder. Further downward movement of the piston uncovers the Transfer port and slightly compressed air enters the engine cylinder from the crank case. Due to deflector on the top of the piston, the air will move up to the top of the cylinder and expels out the remaining exhaust gases through the Exhaust port..

During the upward movement of the piston, first the Transfer port and then the Exhaust port closes. As soon as the exhaust port closes the compression of the air starts. As the piston moves up, the pressure in the crankcase decreases so that the fresh air is drawn into the crank case through the open inlet port. Just before the end of the compression stroke the fuel is forced under pressure in the form of fine spray into the engine cylinder through the nozzle into this hot air. 

At this moment, the temperature of the compressed air is high enough to ignite the fuel. It suddenly increases the pressure and temperature of the products of combustion. The rate of fuel injection is such as to maintain the gas pressure constant during the combustion period. Due to increased pressure the piston is pushed down with a great force. Then the hot products of combustion expand. During expansion some of the heat energy produced is transformed into mechanical work. 

When the piston is near the bottom of the stroke it uncovers exhaust port, which permits the gases to flow out of the cylinder. This completes the cycle and the engine cylinder is ready to suck the air once again.

See More: Two Stroke Petrol Engine

See More: Petrol Engine-Working Principle

See More: I.C. Engine

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