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Four-stroke Otto engine

Four-stroke Otto engine

This animation demonstrates the type of engine most commonly used in cars.



Otto engine, engine, four-stroke, automobile, radial engine, crankshaft, valve, cylinder, piston, spark plug, combustion, spark, intake, compression, explosion, power stroke, work, cycle, petrol engine, internal combustion engine, petrol, carburettor, mechanical energy, combustion product, environmental pollution, air pollution, car, automobile factory, car manufacturing, thermodynamics, heat engine, heat energy, physics

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  • engine block
  • gearbox
  • air filter
  • intake manifold
  • exhaust manifold
  • timing belt
  • ignition distributor


  • intake port
  • spark plug
  • exhaust port
  • intake valve
  • exhaust valve
  • piston
  • cylinder
  • connecting rod
  • crankshaft

Stroke 1

Stroke 2

Stroke 3

Stroke 4


  • radiator
  • engine
  • gearbox
  • drive shaft
  • petrol tank
  • differential gear
  • driven shaft
  • exhaust pipe


  • crankshaft
  • camshaft
  • piston
  • valves


We know that cars are powered by engines, but how do they do this? The rotating motion of an engine´s crankshaft is transmitted to the wheels by the drive shaft. The gearbox changes the number of rotations of the driven wheels during one turn of the crankshaft. In low gear the output of the engine is high but the speed is low, in high gear the car is faster and uses less fuel but accelerates more slowly.

The most common type of engine used in cars is the four-stroke Otto engine.

It converts the alternating vertical motion of the pistons into the rotation of the crankshaft. The crankshaft drives the drive shaft and the camshaft through the timing belt. The camshaft operates the valves, which ensure fuel intake and the expulsion of exhaust gases through a coordinated, rhythmic opening and closing.

The first stroke is intake. The piston moves downwards, decreasing the pressure in the cylinder. The intake valve opens, and a mixture of air and fuel flows from the carburettor into the cylinder.

The second stroke is compression: both the intake and the exhaust valves are closed then. The momentum of the crankshaft and the counterweight causes the piston to move upwards, compressing the mixture of air and fuel and thereby increasing its temperature.

The third stroke is the power stroke. The spark plug ignites the compressed, heated mixture of fuel and air. The explosion pushes the piston down.

The fourth stroke is the exhaust stroke. The piston moves upwards, the exhaust valve opens and the exhaust gases are expelled.

As you can see, the linear motion of the piston is converted into the rotating motion of the crankshaft in the engine. The energy necessary to move the piston is provided by the combustion of fuel. The fuel used in the four-stroke Otto engine is petrol. An important attribute of petrol is the octane number. The higher it is, the higher its boiling point. Therefore it can be compressed more, resulting in greater efficiency.

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