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The invention of the internal combustion engine in the mid-nineteenth century marked a vital step in the development of modern society. These engines work through the combustion of petrol, diesel or even gas to produce heat energy which produces movement, i.e. mechanical energy. These engines can therefore be used for many different purposes, such as for generating electricity (if they are coupled up to an electric generator) or for powering different kinds of vehicles, such as automobiles, ships and aeroplanes.


The components of a petrol and diesel engine are similar. They consist of a sealed cylinder or combustion chamber where the fuel is burned.

They both use a four-stroke cycle to receive the fuel, compress it, use it and finally to expel the gases. The explosion generates a force that moves the piston, which in turn moves the connecting rod and crankshaft. This causes a rotating movement which is used by the vehicle's transmission system to make the wheels turn.

  1. Intake valve
  2. Spark plug
  3. Exhaust valve
  4. Combustion chamber
  5. Piston
  6. Connecting rod
  7. Crankshaft

Petrol engines

In these engines - also known as Otto-cycle engines - the spark plug produces an electrical spark which causes the petrol to explode. The four-stroke cycle operates in a loop to generate the necessary movement.

These are the four strokes in the cycle of a petrol engine:

  • Intake
  • Compression
  • Combustion
  • Exhaust


The piston descends into the cylinder and sucks in the mixture of air and petrol (previously mixed in the carburettor).


The piston rises and compresses the mixture, which cannot escape because the valves are now closed.


When it is compressed, the mixture heats, facilitating the explosion caused by a spark from the spark plug. As a result, the piston is forced down with considerable force.


Finally, combustion produces gases which are pushed out through the hole in the valve by the final upward movement of the piston.

Diesel engines

Diesel engines use compression to make the mixture explode. The air is compressed in the cylinder until it reaches a temperature at which the diesel spontaneously explodes. In this case too, the cycle consists of four movements.

These are the four strokes in the cycle of a diesel engine:

  • Intake
  • Compression
  • Combustion/Expansion
  • Exhaust


The piston begins moving downward while the air valve draws as much air as possible into the combustion chamber.


With the intake and exhaust valves closed, the piston rises and compresses the air, thus raising the temperature.


The fuel is injected and when it comes into contact with the hot air, combustion occurs. This pressure forces the piston down at great force. This is what causes the power stroke.


Finally, the exhaust valve opens completely, and the burnt-off gases are expelled by the upward movement of the piston.

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This is the attention service of the Basque Energy Agency.