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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 compressed.
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 providing
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.
TURBINE ENGINE
The gas turbine engine mounted on most helicopters is
made up of a compressor, combustion chamber, turbine,
and gearbox assembly. The compressor compresses the
air, which is then fed into the combustion chamber
where atomized fuel is injected into it. The fuel/air
mixture is ignited and allowed to expand. This combustion
gas is then forced through a series of turbine
wheels causing them to turn. These turbine wheels
provide power to both the engine compressor and the
main rotor system through an output shaft. The
Figure 5-1. The arrows in this illustration indicate the direction
of motion of the crankshaft and piston during the fourstroke
cycle.
Intake Compression
Power Exhaust
Intake

Valve
Exhaust

Valve
Spark

Plug
Piston
Connecting

Rod
Crankshaft
1 2
3 4
5-2
combustion gas is finally expelled through an exhaust
outlet. [Figure 5-2]
COMPRESSOR
The compressor may consist of an axial compressor, a
centrifugal compressor, or both. An axial compressor
consists of two main elements, the rotor and the stator.
The rotor consists of a number of blades fixed on a
rotating spindle and resembles a fan. As the rotor
turns, air is drawn rearwards. Stator vanes are arranged
in fixed rows between the rotor blades and act as a
diffuser at each stage to decrease air velocity and
increase air pressure. There may be a number of rows
of rotor blades and stator vanes. Each row constitutes
a pressure stage, and the number of stages depends on
the amount of air and pressure rise required for the
particular engine.
A centrifugal compressor consists of an impeller, diffuser,
and a manifold. The impeller, which is a forged
disc with integral blades, rotates at a high speed to
draw air in and expel it at an accelerated rate. The air
then passes through the diffuser which slows the air
down. When the velocity of the air is slowed, static
pressure increases, resulting in compressed, high-pressure
air. The high pressure air then passes through the
compressor manifold where it is distributed to the
combustion chamber.
COMBUSTION CHAMBER
Unlike a piston engine, the combustion in a turbine
engine is continuous. An igniter plug serves only to
ignite the fuel/air mixture when starting the engine.
Once the fuel/air mixture is ignited, it will continue to
burn as long as the fuel/air mixture continues to be
present. If there is an interruption of fuel, air, or both,
combustion ceases. This is known as a “flame-out,” and
the engine has to be restarted or re-lit. Some helicopters
are equipped with auto-relight, which automatically
activates the igniters to start combustion if the engine
flames out.
TURBINE
The turbine section consists of a series of turbine
wheels that are used to drive the compressor section
and the rotor system. The first stage, which is usually
referred to as the gas producer or N1 may consist of
one or more turbine wheels. This stage drives the
components necessary to complete the turbine cycle
making the engine self-sustaining. Common components
driven by the N1 stage are the compressor, oil
pump, and fuel pump. The second stage, which may
also consist of one or more wheels, is dedicated to
driving the main rotor system and accessories from
the engine gearbox. This is referred to as the power
turbine (N2 or Nr).
Compressor Discharge Air Tube
Exhaust Air Outlet
Igniter Plug
Fuel Nozzle
　
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