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Saturday 19 April 2014 • 17:03 GMT • Last update 16:46

Two stroke cycle

The two stroke cycle is a combustion process which takes two movements, or strokes, of the piston.

The four processes required in the cycle - induction, compression, ignition and exhaust - are completed within the two cycles by using the volume of the crankcase as part of the process, with a transfer port between crankcase and combustion chamber.

Air/fuel mixture is sucked into the crankcase as the piston rises to compress an air/fuel mixture which is already present in the combustion chamber. When the mixture in the chamber is ignited, causing the piston to be driven downwards on the power stroke, the mixture in the crankcase is pressurized. As the piston continues downwards it uncovers an exhaust port in the wall of the combustion chamber, which allows the exhaust gases to escape. Soon after, the descending piston uncovers the transfer port and the mixture from the crankcase is pushed into the combustion chamber. The piston then rises, compressing the mixture in the combustion chamber and beginning the cycle again.

The advantages of the two stroke cycle are:

  • power - there is a power stroke on every engine revolution, so two stroke engines can produce more power than four stroke engines
  • simplicity - the mechanical layout of a two stroke engine can be simpler than a four stroke engine, leading to cheaper production and potentially allowing a higher safe engine speed

The main drawback to a two stroke engine is that the intake charge is polluted with remaining exhaust gases, as both are in the combustion chamber at the same time. Therefore it is difficult to obtain good emissions performance from a two stroke engine.

Compare four stroke cycle.

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