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Most modern passenger and military aircraft are powered by
gas turbine engines, also called jet
engines. There are several different
types of gas turbine engines,
but all turbine engines have some parts in common.
engines have a compressor to increase the pressure of the
incoming air before it enters the combustor.
has a large influence on total engine
As shown in the above figure, there are two main types of
compressors: axial and centrifugal.
In the picture, the compressor on the left is
called an axial compressor because the flow
through the compressor travels parallel to the axis of rotation. The
compressor on the right is called a centrifugal
compressor because the flow through this compressor is turned
perpendicular to the axis of rotation. Centrifugal compressors, which
were used in the first jet engines, are still used on small turbojets
engines and as pumps on
engines. Modern large
engines usually use axial compressors.
Why the change to axial compressors? An average, single-stage,
centrifugal compressor can increase the pressure by a factor of 4. A
similar average, single-stage axial compressor increases the pressure by only
a factor of 1.2. But it is relatively easy to link together several
stages and produce a multistage axial compressor. In the
multistage compressor, the pressure is multiplied from row to row (8
stages at 1.2 per stage gives a factor of 4.3). It is much more
difficult to produce an efficient multistage centrifugal compressor
because the flow has to be ducted back to the axis at each stage.
Because the flow is turned perpendicular to the axis, an engine with
a centrifugal compressor tends to be wider, having a greater cross-sectional
area than a corresponding axial. This creates additional undesirable
For these reasons, most high performance, high compression
turbine engines use multi staged axial compressors. But, if only a
moderate amount of compression is required, a centrifugal compressor
is much simpler to use.
Parts of a Jet Engine:
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