Four-Stroke Engine

An engine operating on a cycle which is completed in four strokes, or two revolutions of the crankshaft.

The four strokes are:
Induction Stroke - The stroke where the fuel-air mixture is drawn into the cylinder.

Compression Stroke - The stroke of the internal Combustion Engine where the piston moves into the cylinder, compressing this fuel-air mixture to high density, pressure and temperature. Ignition - the fuel air mixture ignites with a spark as the initiator or fuel is injected into the high temperature gas and it ignites, either way it converts into extremely hot burned gas.

Working Stroke - The piston stroke during which the combustion gases exert a pressure on the moving piston.

Exhaust Stroke - The piston stroke during which the exhaust gases are expelled from the cylinder ready for the next induction stroke.



There are two primary types of four-stroke engine and this classification is based on how the air-fuel mixture is ignited:

  1. Spark Ignition (SI) - a spark that is timed based on the crank rotation ignites the compressed mixture.
  2. Compression Ignition (CI) - the air is compressed and so heats up, an injection of fuel then ignites with the heat from the compressed air.

Bore and Stroke

The bore and stroke of the piston sets the swept volume. The ratio of bore to stroke is a good indicator of the engine characteristic such as how high it will rev and basic fuel economy. More simply it also defines some of the basic dimensions of the engine such as height and length.

Bore versus stroke for a number of production engines.

Historical Notes

  1. 1861 Alphonse Beau de Rochas patented the concept of four strokes, with the vital compression of the mixture before ignition
  2. 1876 German engineer Nikolaus August Otto built the first practical four-stroke gas engine, the most direct ancestor to today′s automobile engines.
  3. 1890 First four-cylinder, four-stroke engine built by Wilhelm Maybach.

See also: Internal Combustion Engine, Petrol Engine, Two-Stroke Engine, Working Stroke.

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Subjects: Automotive Engines