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What does 4-stroke mean in an engine? Follow

This is often asked and the answer is deceptively simple.

To be very basic, it means that there are four separate stages (or strokes) to the function of a 4-stroke engine. These can be described as...

  1. The intake stroke  (suck)
  2. The compression stroke (squeeze)
  3. The combustion/explosion stroke (bang/power stroke)
  4. The exhaust stroke. (blow)

The 4-stroke internal combustion engine works in cycles.

The ‘action’ happens inside a cylinder, in which is a piston.

As follows. 


  1. Intake.(Suck) The piston starts at top end of the cylinder. Then an intake valve opens up,(top left of diagram A ) and as the the piston moves down it allows air and petrol in. This action only needs a small amount of petrol.
  2. Compression.(Squeeze) The piston then moves up and compresses the petrol and air which will create a more a powerful explosion
  3. Combustion/explosion (Bang) As the piston reaches the end of its upstroke, the spark plug lets out a spark (top left of diagram C) then ignites the petrol and powerful explosion drives the piston down. Also known as the Power Stroke*.
  4. Exhaust. (Blow) The piston hits the bottom end of this stroke, the exhaust valve is opened (top right of diagram D) piston moves back up and the exhaust exits the engine, forced by both the movement of the piston and the power of the ignition. *

* This action is often called the power stroke as it is what drives a crankshaft and makes wheels, propellors and blades etc go round.

The cycle then continues.

Factfile: The successful production of the first internal combustion engine is largely attributed to Belgian, Jean Joseph Etienne Lenoir in 1860 with Germans August Otto and Eugen Langen being the first to build and sell in bulk.

For information on 4-stroke engine maintenance and the difference between a two-stroke engine and a four stoke, please go to our keeping going section.

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