How an internal combustion engine works

The vast majority of vehicles (passenger cars and commercial vehicles) which are sold today are equipped with internal combustion engines. In this article we are going to describe how a four stroke internal combustion engine works.

An internal combustion engine is classified as a heat engine. It’s called internal because the combustion of the air-fuel mixture occurs inside the engine, in a combustion chamber, and some of the burned gases are part of the new combustion cycle.

Basically, an internal combustion engine transforms the thermal energy of the burning air-fuel mixture into mechanical energy. It is called 4 strokes because it takes 4 strokes for the piston to execute a complete combustion cycle. The complete name for an engine which powers a passenger car is: 4 stroke piston internal combustion engine, abbreviated ICE (Internal Combustion Engine).

Now let’s examine which are the main component of an ICE.

Internal combustion engine parts

Image: Internal combustion engine (DOHC) parts


  1. exhaust camshaft
  2. exhaust valve bucket
  3. spark plug
  4. intake valve bucket
  5. intake camshaft
  6. exhaust valve
  7. intake valve
  8. cylinder head
  9. piston
  10. piston pin
  11. connecting rod
  12. engine block
  13. crankshaft

TDC – Top Dead Center

BDC – Bottom Dead Center

The cylinder head (8) usually contains the camshaft(s), valves, valve buckets, valve return springs, spark/glow plugs and injectors (for direct injection engines). Through the cylinder head flows the cooling liquid of the engine.

Inside the engine block (12) we can find the piston, connecting rod and crankshaft. As for the cylinder head, through the engine block flows coolant to help control the temperature of the engine.

The piston moves inside the cylinder from BDC to TDC. The combustion chamber is the volume created between piston, cylinder head and engine block when the piston is close to TDC.

In Figure 1 we can examine the complete set of mechanical components of an ICE. Some of the components are fixed (e.g. cylinder head, cylinder block) and some of them are moving. In the figure below we’ll have a look at the main moving part of an ICE, which transform the gas pressure within the cylinder in mechanical power.

Internal combustion engine moving parts

Image: Internal combustion engine moving parts


  1. camshaft sprocket
  2. piston
  3. crankshaft
  4. connecting rod
  5. valve
  6. valve bucket
  7. camshaft

The rotation of the camshaft is synchronised with the rotation of the crankshaft through a toothed belt or chain. The position of the intake and exhaust valves must be precisely synchronised with the position of the piston, to allow the combustion cycles to take place accordingly.

A complete engine cycle for a 4 stroke ICE has the following phases (strokes):

  1. intake
  2. compression
  3. power (expansion)
  4. exhaust

A stroke is the movement of the piston between the two dead centres (bottom and top).

Now, that we know which are the components of an ICE, we can examine what is happening in each stroke of the engine cycle. In the table below you’ll see the position of the piston at the beginning of each stroke and the details regarding the events that take place in the cylinder.

Stroke 1 – INTAKE

Internal combustion engine intake stroke

Internal combustion engine intake stroke

At the beginning of the intake stroke the piston is close to TDC. The intake valve is opened, the piston starts to move towards the BDC. Air (or air-fuel mixture) is drawn into the cylinder. This stroke is called INTAKE because fresh air/mixture is taken into the engine. The intake stroke ends when the piston is at the BDC.

During the intake stroke, the engine consumes energy (the crankshaft is rotating due to the inertia of the components).


Internal combustion engine compression stroke

Internal combustion engine compression stroke

The compression stroke starts with the piston at BDC, after the intake stroke is finished. During the compression stroke both valves, intake and exhaust, are closed, and the pistons moves towards TDC. With both valves closed, the air/mixture is compressed, reaching maximum pressure when the piston is close to TDC.

Before the piston reaches the TDC (but very close to it), during the compression stroke:

  • for gasoline engine: the spark is generated
  • for diesel engines: fuel is injected

During the compression stroke, the engine consumes energy (the crankshaft is rotating due to the inertia of the components), more than the intake stroke.

Stroke 3 – POWER

Internal combustion engine power stroke

Internal combustion engine power stroke

The power stroke starts with the piston at TDC. Both valves, intake and exhaust, are still closed. The air-fuel mixture combustion started at the end of the compression stroke, which causes a significant increase of pressure inside the cylinder. The pressure inside the cylinder pushes the piston down, towards the BDC.

Only during the power stroke the engine produces energy.

Stroke 4 – EXHAUST

Internal combustion engine exhaust stroke

Internal combustion engine exhaust stroke

The exhaust stroke starts with the piston at the BDC, after the power stroke finished. During this stroke, the exhaust valve is open. The movement of the piston from the BDC towards the TDC pushes most of the exhaust gases out of the cylinder, into the exhaust pipes.

During the exhaust stroke, the engine consumes energy (the crankshaft is rotating due to the inertia of the components).

As you can see, in order to have a complete combustion (engine) cycle the piston has to perform 4 strokes. This means that one engine cycles takes two complete rotations of the crankshaft (720°).

The only stroke which produces torque (energy) is the power stroke, all the others are consuming energy.

The linear motion of the piston is transformed into rotational motion of the crankshaft through the connecting rod.

For a better understanding, we’re summarising the piston initial position, valve position and energy balance for each stroke.

Stroke order Stroke name Piston initial position Intake valve state Exhaust valve state Energy balance
1 Intake TDC Open Closed Consumes
2 Compression BDC Closed Closed Consumes
3 Power TDC Closed Closed Produces
4 Exhaust BDC Closed Open Consumes

In the animation below you can see clearly how the internal combustion engine works. Pay attention to piston position, valve position, the moment when the ignition occurs and the succession of the strokes.

Internal combustion engine animation

Internal combustion engine animation

In the following articles we’ll have a closer look on the parameters, performance and components of an internal combustion engine. If you have questions or comments about this article use the form below for posting.

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