The engine-operating cycle of most petrol and
diesel engines. The `stroke' is an upward or
downward movement of a piston in a cylinder.
In a petrol engine the cycle begins with the
induction of a fuel mixture as the piston
goes down on its first stroke. On the second
stroke (up) the piston compresses the mixture
in the top of the cylinder. An electric spark
then ignites the mixture, and the gases
produced force the piston down on its third,
power stroke. On the fourth stroke (up) the
piston expels the burned gases from the
cylinder into the exhaust. The four-stroke
cycle is also called the Otto cycle. The
diesel engine cycle works in a slightly
different way to that of the petrol engine on
the first two strokes. The diesel engine
cycle works in a slightly different way to
that of the petrol engine on the first two
strokes.