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Rotorcraft Flying Menu >Helicopter
Systems>Engines
> Reciprocating Engine
The reciprocating engine consists of a series
of pistons connected to a rotating crankshaft. As the pistons
move up and down, the crankshaft rotates. The reciprocating
engine gets its name from the back-and-forth movement of its
internal parts. The four-stroke engine is the most common type,
and refers to the four different cycles the engine undergoes
to produce power. [Figure 5-1]
Figure 5-1. The arrows in this illustration
indicate the direc-tion of motion of the crankshaft and piston
during the four-stroke cycle.
When the piston moves away from the cylinder
head on the intake stroke, the intake valve opens and a mixture
of fuel and air is drawn into the combustion chamber. As the
cylinder moves back towards the cylinder head, the intake valve
closes, and the fuel/air mixture is com-pressed. When compression
is nearly complete, the spark plugs fire and the compressed
mixture is ignited to begin the power stroke. The rapidly expanding
gases from the controlled burning of the fuel/air mixture drive
the piston away from the cylinder head, thus pro-viding power
to rotate the crankshaft. The piston then moves back toward
the cylinder head on the exhaust stroke where the burned gasses
are expelled through the opened exhaust valve.
Even when the engine is operated at a fairly
low speed, the four-stroke cycle takes place several hundred
times each minute. In a four-cylinder engine, each cylinder
operates on a different stroke. Continuous rotation of a crankshaft
is maintained by the precise timing of the power strokes in
each cylinder.
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