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cycles (fig. 2-1) show that in each instance there is
induction, compression, combustion and exhaust.
These processes are intermittent in the case of the
piston engine whilst they occur continuously in the
gas turbine. In the piston engine only one stroke is
utilized in the production of power, the others being
involved in the charging, compressing and
exhausting of the working fluid. In contrast, the
turbine engine eliminates the three ’idle’ strokes, thus
enabling more fuel to be burnt in a shorter time;
hence it produces a greater power output for a given
size of engine.
3. Due to the continuous action of the turbine
engine and the fact that the combustion chamber is
not an enclosed space, the pressure of the air does
not rise, like that of the piston engine, during
combustion but its volume does increase. This
process is known as heating at constant pressure.
Under these conditions there are no peak or
fluctuating pressures to be withstood, as is the case
with the piston engine with its peak pressures in
excess of 1,000 lb. per sq. in. It is these peak
pressures which make it necessary for the piston
engine to employ cylinders of heavy construction and
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to use high octane fuels, in contrast to the low octane
fuels and the light fabricated combustion chambers
used on the turbine engine.
4. The working cycle upon which the gas turbine
engine functions is, in its simplest form, represented
by the cycle shown on the pressure volume diagram
in fig. 2-2. Point A represents air at atmospheric
pressure that is compressed along the line AB. From
B to C heat is added to the air by introducing and
burning fuel at constant pressure, thereby considerably
increasing the volume of air. Pressure losses in
the combustion chambers (Part 4) are indicated by
the drop between B and C. From C to D the gases
resulting from combustion expand through the
turbine and jet pipe back to atmosphere. During this
part of the cycle, some of the energy in the
expanding gases is turned into mechanical power by
Working cycle and airflow
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Fig. 2-1 A comparison between the working cycle of a turbo-jet engine and a piston engine.
Fig. 2-2 The working cycle on a pressurevolume
diagram.
the turbine; the remainder, on its discharge to
atmosphere, provides a propulsive jet.
5. Because the turbo-jet engine is a heat engine,
the higher the temperature of combustion the greater
is the expansion of the gases. The combustion
temperature, however, must not exceed a value that
gives a turbine gas entry temperature suitable for the
design and materials of the turbine assembly.
6. The use of air-cooled blades in the turbine
assembly permits a higher gas temperature and a
consequently higher thermal efficiency.
THE RELATIONS BETWEEN PRESSURE,
VOLUME AND TEMPERATURE
7. During the working cycle of the turbine engine,
the airflow or ’working fluid’ receives and gives up
heat, so producing changes in its pressure, volume
and temperature. These changes as they occur are
closely related, for they follow a common principle
that is embodied in a combination of the laws of
Boyle and Charles. Briefly, this means that the
product of the pressure and the volume of the air at
the various stages in the working cycle is proportional
to the absolute temperature of the air at those
Working cycle and airflow
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Fig. 2-3 An airflow through divergent and convergent ducts.
stages. This relationship applies for whatever means
are used to change the state of the air. For example,
whether energy is added by combustion or by
compression, or is extracted by the turbine, the heat
change is directly proportional to the work added or
taken from the gas.
8. There are three main conditions in the engine
working cycle during which these changes occur.
During compression, when work is done to increase
the pressure and decrease the volume of the air,
there is a corresponding rise in the temperature.
During combustion, when fuel is added to the air and
burnt to increase the temperature, there is a corresponding
increase in volume whilst the pressure
remains almost constant. During expansion, when
work is taken from the gas stream by the turbine
assembly, there is a decrease in temperature and
pressure with a corresponding increase in volume.
9. Changes in the temperature and pressure of the
air can be traced through an engine by using the
airflow diagram in fig. 2-5. With the airflow being
continuous, volume changes are shown up as
　
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