Four Stroke Petrol Engine Working Principle and Diagram

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The petrol engine or gasoline engine is a type of internal combustion engine perfoms internal combustion processes to produce power.

Gasoline engines had been discover by German engineer Nikolaus Otto in 1876. As the name implies, petrol engines use gasoline as a source of energy.

In principle, there are two types of gasoline engines;
Two strokes
Four strokes

The two stroke engine is an engine that has two steps in each cycle, while the 4 stroke engine has four steps in each cycle.

Then, how does the 4 stroke engine work? let's discuss in detail

Four stroke petrol engine working principle


Before discussing the principle, first we need to understand some of the main components of a 4 stroke petrol engine.



  1. Engine block, functions as the place where the piston moves
  2. Cylinder head, the top cover of cylinder and also as a combustion place inside the engine
  3. Piston, tubular part that function to vary the volume of the cylinder
  4. Connecting rod & crankshaft, both of these components are used to change the up and down motion of the piston into a rotation motion
  5. Spark plug, a component that functions to burn AFM (air fuel mixture) inside the engine
  6. Inlet & exhaust valve, the inlet valve functions to insert material into the engine while the exhaust valve is used to remove waste gasses from the engine
  7. Carburetor, serves to mix fresh air and gasoline with the correct ratio (recent vehicle no longer uses the carburetor but has used an injector)


Then how does the four stroke petrol engine work?

As we mentioned above, 4 stroke petrol engine works by using four steps in one cycle.

Where each step lasts for half a crank revolution. So as to say, a 4 stroke engine cycle lasts for two crank revolutions.

The strokes can be mentioned;

1. Inlet stroke

Inlet stroke is the step of entering material into a cylinder. The inlet stroke starts when the piston position is above or we usually call it the TOP Dead Center (TDC).


From TDC the piston moves down to reach BDC (bottom dead center). This movement will increase the volume of the cylinder so that the vacuum inside the cylinder increases.

At this time, the inlet valve is open so that the vacuum inside the cylinder will suck the AFM material that is ready in the intake manifold.

AFM is a material consisting of fresh air and gasoline with an ideal composition (1: 14 or 1 gasoline molecule = 14 fresh air molecules).

In past vehicles, to mix gasoline and air was carried out by carburetor part. But in the recent vehicle, this mixing occurred directly in the intake by fuel injection.

2. Compression stroke

The compression step is the AFM compression process to increase pressure and temperature. This is so that the AFM material becomes more flammable and produces more powerful power.


This process starts after the inlet stroke when the piston reaches BDC. The piston will move up from BDC to TDC, this will cause a reduction in cylinder volume.

On the other hand, the inlet valve was tightly closed as well as the exhaust valve. So that the reduction in the volume of the cylinder will compress the AFM inside the cylinder until the pressure and temperature are high.

3. Combustion stroke

Combustion stroke is the process of burning compressed AFM material. This combustion uses a spark fire that is obtained from the ignition system.


This step starts when the piston reaches TDC, at this position the cylinder volume is at the smallest volume so that the AFM pressure and temperature are at the highest level.

A little spark is enough to burn the AFM that is already in high compressed condition.

When the conditions above, spark plugs will sprinkle fire. The spark will burn AFM, the result is the same when the high pressure gasses burn, it will bring up an explosion.

However, the explosion will not explode the engine. It is precisely from the explosion that there is expansion power that is utilized as the main power of the engine. The expansion power is used to push the piston to move to the BDC.

4. Exhaust stroke

Exhaust stroke is the waste gasses discharging process into the atmosphere after the combustion stroke.


This step takes place after the piston reaches the BDC at the end of the combustion stroke. When the piston reaches BDC, the piston will definitely move back to TDC.

In this condition, the exhaust valve opens so that the piston will push the residual combustion gas to exit through the exhaust valve.

But, the exhaust gas is not directly released into the atmosphere. There is an exhaust treatment circuit that aims to treat vehicle exhaust to be more environmentally friendly.

We will discuss the article about the exhaust treatment later, so for this article, we just need to arrive here.