Four stroke diesel engine: Definition, Diagram, Principle, Working

What is Four stroke diesel engine?

Four stroke diesel engine is also known as the compressed ignition (CI) engine since in this type of Internal combustion engine, the combustion occurs due to the compressing of air to more than fuel self-ignition temperature.

The compression ratio of this engine is higher as compared to SI engine and it not uses the spark plug. This engine works on the thermodynamic diesel cycle.

In this type of engine, the diesel is used as the fuel. The four strokes in this engine are suction, compression, power as well as exhaust.

Four stroke diesel engine diagram:

Four stroke diesel engine diagram

Four stroke diesel engine working principle:

The four stroke diesel engine works on the Diesel cycle.

diesel engine pv and ts diagram

The four processes in the Diesel engine are as follows:-

  • Process 1-2: Isentropic compression
  • Process 2-3: Constant pressure heat addition
  • Process 3-4: Isentropic expansion
  • Process 4-1: Constant volume heat rejection

Working of Four stroke diesel engine:

Four stroke diesel engine working

At the start, when the piston moves from TDC to BDC, from the intake manifold air enters into the cylinder through the inlet port.

[ When the piston moves from TDC to BDC, the piston creates a vacuum inside the cylinder. Hence due to the pressure difference between intake manifold and cylinder, air enters into the cylinder. ]

During this suction stroke, the inlet valve is open and at the end of the suction stroke inlet valve closes.

After suction stroke is done, the piston starts to move from BDC to TDC.

During this movement, the piston compresses the air to high pressure inside the cylinder.

During Compression, both valves (Inlet & Exhaust) are closed.

After the compression of air, the fuel injector sprays the fuel inside the cylinder, hence fuel gets ignited (catch fire).

Therefore Combustion starts, high-pressure gases expand (spread) inside the cylinder and push the piston downwards ( towards BDC).

During expansion, both valves (Inlet & Exhaust) are closed.

Due to expansion stroke, power is obtained from the engine. Hence expansion stroke is also called a Power Stroke.

After expansion stroke, the piston moves from BDC to TDC.

During this movement of the piston (BDC to TDC), the Burnt exhaust gases from the exhaust valve of a cylinder are released through the engine exhaust pipe in the environment.

During this exhaust stroke, only the exhaust valve is open and at the end of this stroke, exhaust valve closes and the inlet valve opens to suck (pull) fresh air.

Hence In this way cycle continues and power obtains from the engine.

Four stroke diesel engine working video:

This four-stroke diesel engine video will help you to better understand its working.

Four stroke diesel engine advantages:

The four stroke diesel engine has the following advantages:-

  1. Higher torque generation:- Due to the higher compression ratio, the diesel engine can generates higher torque.
  2. High power generation:- The higher torque leads to the high power generation.
  3. No need of carburettion for mixing of air and fuel.
  4. Lower fuel cost in comparison with gasoline or petrol engine.
  5. The spark plug is not necessary for ignition.
  6. Higher thermal efficiency in comparision with petrol engines.
  7. Suitable for heavy load applications.
  8. It generates higher torque at lower speed.
  9. Less fuel consumption.

Four stroke diesel engine disadvantages:

The four stroke diesel engine has the following disadvantages:-

  1. It requirres high pressure fuel injection system.
  2. The engine works at higher compression ratio.
  3. The maintainance cost is higher in comparison with gasoline engines.
  4. Most of the diesel engines are noisy in operation.
  5. The diesel engines are bulky than petrol engines.
  6. During winter season, it becomes difficult to start the engine.
  7. It creates more vibrations.
  8. The heater is necessary during cold season to start engine.

Applications of four stroke diesel engine:

The four stroke diesel engine has applications in many kind of automobiles such as follows:-

  1. Train
  2. Boat
  3. Truck
  4. Wagon


  1. What is Four stroke diesel engine?

    The Four stroke diesel engine is the internal combustion engine that works on thermodynamic diesel cycle in which one power cycle consists of four consecutive strokes which are suction, compression, power and exhaust.

  2. How does a Four stroke diesel engine Work?

    The four stroke diesel engine works on principle of diesel cycle in which the fuel is ignite by increasing the temperature of the air above the self-ignition temperature of fuel.

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Media credits:

Pratik is a Graduated Mechanical engineer. He enjoys sharing the engineering knowledge learned by him with people.

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