Most modern passenger and military aircraft are powered by
gas turbine engines, which are also called
jet engines. The first and most simple type
of gas turbine is the
turbojet.
On this slide we show how the flow pressure varies through a
typical turbojet engine. The pressure is color-coded with blue
indicating the lowest pressure and white the highest pressure. Air is
brought into the turbojet through the inlet
at the left of the computer drawing. At the rear of the inlet, the
air enters the compressor. The compressor
acts like many rows of airfoils, with each
row producing a small increase in pressure. At the exit of the
compressor, the air is at a much higher pressure than free stream. In
the burner a small amount of fuel is
combined with the air and ignited at near constant pressure. Leaving
the burner, the hot exhaust is passed through the turbine.
Energy is extracted from the flow by the turbine to turn the
compressor, which is linked to the turbine
by a central shaft. Some pressure is lost in the hot exhaust during
this process, but the pressure entering the nozzle is still greater
than free stream. The nozzle then converts
the high pressure and temperature into high velocity. Because the
exit velocity is greater than the free stream velocity, thrust is
created as described by the thrust
equation.
The engine pressure ratio (EPR) is defined to be the total
pressure ratio across the engine. Using our station
numbering system, EPR is the ratio of nozzle total pressure pt8
to compressor face total pressure pt2. EPR can be easily measured
on an operating engine and displayed to the pilot on a cockpit dial.
That is why the ratio is not defined in terms of free stream
conditions. Total pressure losses in the inlet are not contained in
the EPR. But if we know the EPR, the inlet losses, and the
corresponding engine temperature ratio, ETR,
we can easily determine the thrust of an
engine using the nozzle performance
information and the thrust equation. The
EPR is simply the product of the pressure ratio across all of the
engine components.
EPR = pt8 / pt2 = (pt3 / pt2) * (pt4 / pt3) * (pt5 / pt4) * (pt8 / pt5)
EPR = compressor pressure ratio * burner pressure ratio * turbine pressure ratio *
nozzle pressure ratio
For a given engine design, we can determine the
pressure ratio of each component as given on each of the component
thermodynamic slides.
Here is an animated version of the graphic:
You can investigate the variation of pressure through an engine
by using the
EngineSim
interactive Java applet.
You can vary the performance of any of the engine parts and investigate
the effects on thrust and fuel flow. EngineSim can also plot the variation
of pressure through the engine.
Activities:
Guided Tours
-
EngineSim - Engine Simulator:
-
Thrust Equation:
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