How Does A Turbojet Engine Work?
Author: Bill Johnson

A turbojet engine is a type of internal combustion engine often
used to propel aircraft. Air is drawn into the rotating
compressor via the intake and is compressed to a higher
pressure before entering the combustion chamber. Fuel is mixed
with the compressed air and ignited by a small flame in the
eddy of a flame holder.

This combustion technique significantly raises the temperature
of the gas. Hot combustion chemicals leaving the combustor
expand through the turbine, where power is drawn to employ the
compressor. Although this expansion process reduces both the
gas temperature and pressure at exit from the turbine, both
variabels are for the most part still well above ambient
conditions. The gas stream exiting the turbine expands to
ambient pressure through the propelling nozzle, causing a high
velocity jet in the exhaust plume. So if the jet velocity
surpasses the aircraft flight speed it amazingly creates a
forward thrust.

Under normal circumstances, the pumping action of the
compressor prevents any backflow, this kindly helps the flow of
the jet engine. You can compare the whole method with a four
stroke cycle, but with induction, compression, ignition,
expansion and exhaust taking place simultaneously. The
efficiency of a jet engine is strongly dependent upon the
pressure ratio (Combustor Entry Pressure/Intake Delivery
Pressure) and the Turbine Inlet Temperature of the cycle.

It is also perhaps instructive to compare turbojet engines with
propeller engines. Turbojet engines take a rather small mass of
air and accelerate it by a large amount, whereas a propeller
takes a large mass of air and accelerates it by a small amount.


At great speed the exhaust of a jet engine makes it efficient
at high speeds (particularly supersonic speeds) and high
altitudes.

On planes that go slower and fly a shorter range, a gas
turbine-powered propeller engine, commonly known as a
turboprop, is far more common and also more efficient.
Especially small aircraft primarily use conventional piston
engines to drive a propeller but small turboprops are getting
smaller as technology improves each day.

The turbojet mentioned above is a single spool design, where a
single shaft connects the compressor to the turbine Higher
Overall Pressure Ratio designs often have two concentric
shafts, this is to in essence improve the stability of the
compressor during engine throttle movements.

Basically it's the outer shaft that connects the turbine engine
to the high pressure compressor. The combustor together with the
high pressure compressor spool forms the core or gas generator
of the engine. We must not forget that the inner shaft connects
the low pressure compressor to the LP Turbine to create the LP
Spool. Either spools are free to operate at their optimum shaft
speed.


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