4 Stroke Diesel Engine Working Principle and Diagram

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As we know, there are many vehicle that using diesel engine as the source. Like truck or bus, that vehicle need big torque to  make it run. So that, diesel engine takes place.

It's true, diesel engines have large torque. Besides, diesel engines also have advantages in fuel efficiency, the reason is the air and fuel ratio is very thin. So that fuel consumption becomes more efficient.

For those of you who are curious about how the 4-stroke diesel engine works, we will explain in detail this article

4 stroke diesel engine working principle


The vehicle engine has a main mechanism on the piston part. Inside the engine, the piston moves up and down. The upward movement of the piston will increase the combustion chamber volume, while the downward movement of the piston will reduce the combustion chamber volume.

from changes in the volume of the combustion chamber the diesel engine can work.

Now what about the 4-step cycle?

4 stroke diesel engine means there are four processes in one engine cycle. where each process works on one piston movement. that means if the 4-step engine has 4 processes, the piston will move for 4 times (twice moving up, twice moving down).

There are 5 main part that you need to understand first ;

Cylinder block, this cylindrical component is used as a track of piston movement

  • Piston, this part is tubular and moves up and down inside the cylinder block
  • Cylinder head, is used as a cover of the top of the cylinder and a place to put some engine attachments such as injectors and valves
  • Injector, use to injected diesel fuel inside the combustion chamber with high pressure
  • Connecting rod & crankshaft, these two components are used to change the form of energy from directional motion to circular motion


Then what are the processes? This is it

1. Suction step


The suction step is also called intake stroke, that is the process of entering the air into the cylinder chamber. This air intake occurs when the piston moves down from TDC (top dead center) to BDC (bottom dead center). This movement will increase the volume in the engine cylinder.

In other side inlet valve open, as a result the piston will suck the air from the intake manifold, so that air from the outside enters the intake valve filling the cylinder chamber.

2. Compression Steps

The compression step is the process of compressing the air inside the cylinder chamber. Why does the air need to be compressed? this is related to the process of fuel combustion.

Maybe you know that the diesel engine is not equipped with spark plugs, it is because the diesel engine can burn without sparking from the spark plug. This is known as self combustion.

However, to occur self combustion, the air must be compressed until the temperature rises above the diesel fuel burning point. Thus, the diesel injected into the high temperature can burn by itself.

This compression step takes place after the suction step, when the piston has reached BDC at the end of the suction step, the piston will again rise to the TDC. As a result there is a narrowing of the cylinder volume. In this condition both the intake valve and exhaust valve are closed, so that the narrowing of the cylinder space will compress the air inside.

3. Combustion step


The combustion step is the main process of the engine. In this process the diesel is fed through the injector into the combustion chamber.

As we discussed, the air temperature will rise until above of diesel burning point. And when the piston reaches TDC, the air is already in its highest temperature level (exceeding the diesel burning point). In this condition, the diesel injected via injectors, it make the fuel atomized through the injector. As a result, there was a burning that produced expansion power.

This expansion power will push the piston to move down to the BDC. The expansion, is also use to make the vehicle run. Because the power is so high.

4. Exhaust step


The exhaust step is the process of removing combustion residual gases from the combustion chamber. This process occurs when the piston returns to TDC after being exposed to the expansion of combustion power.

At this step, the exhaust valve opens so that the upward movement of the piston will push the combustion residual gas to get out into the exhaust manifold.

The conclucion is

When the piston reaches TDC at the end of the exhaust step, the cycle is fully done. That mean, there are 4 processes, suction step when piston moving down, compression step when piston moving up, burning step when piston moving down, the last is exhaust step when piston moving up.

From that statement, we can learn that 4 stroke engine produce combustion in every 2 revolutions of crankshaft.

The final step is exhaust stroke, when piston rise TDC at the last of exhaust step, piston will move down and it go to the next cycle.

Thank’s for your attention, hope usefull for all of you.