Most modern passenger and military aircraft are powered by
gas turbine engines, which are also called
jet engines. The first and simplest type of
gas turbine is the
How does a turbojet work?
On this slide we show a schematic
drawing of a turbojet engine. The parts
of the engine are described on other slides. Here, we are concerned
with what happens to the air that passes through the engine. Large
amounts of surrounding air are continuously brought into the engine
inlet. In England, they call this part the
intake, which is probably a more accurate description, since the
compressor pulls air into the engine. We have shown here a
tube-shaped inlet, like one you would see on an airliner. But inlets
come in many shapes and sizes depending on the aircraft's mission. 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 jump in pressure. A compressor is like an electric fan and we have
to supply energy to turn the compressor. 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. In a typical jet engine, 100
pounds of air/sec is combined with only 2 pounds of fuel/sec. Most of
the hot exhaust has come from the surrounding air. Leaving the
burner, the hot exhaust is passed through the turbine.
The turbine works like a windmill. Instead of needing energy to turn
the blades to make the air flow, the turbine extracts energy from a
flow of gas by making the blades spin in the flow. In a jet engine we
use the energy extracted by the turbine to turn the compressor by
linking the compressor and the turbine by
the central shaft. The turbine takes some energy out of the
hot exhaust, but the flow exiting the turbine is at a higher pressure
and temperature than the free stream flow.
The flow then passes through the nozzle
which is shaped to accelerate the flow.
is greater than the free stream velocity,
thrust is created as described by the thrust
equation. For a jet engine, the exit mass flow is nearly equal to
the free stream mass flow, since very little
fuel is added to the stream. The amount of mass flow is usually set by flow
in the nozzle throat.
The nozzle of the turbojet is usually designed to take the exhaust pressure
back to free stream pressure.
The thrust equation for a turbojet is then given by the general
with the pressure-area term set to zero. If the free stream conditions
are denoted by a "0" subscript and the exit conditions by an "e" subscript,
the thrust F is equal to the mass flow rate m dot
times the velocity V at the exit minus the free stream mass flow rate
times the velocity.
F = [m dot * V]e - [m dot * V]0
contains two terms. Aerodynamicists often refer to the first term
(m dot * V)e as the gross thrust since
this term is largely associated with conditions in the nozzle. The
second term (m dot * V)0
is called the ram drag and is usually associated with conditions
in the inlet. For clarity, the engine thrust is then called
the net thrust. Our thrust equation indicates that net thrust equals
gross thrust minus ram drag. If we divide both sides of the equation by the
mass flow rate, we obtain
an efficiency parameter called the
that greatly simplifies the
for turbine engines.
You can explore the design and operation of a turbojet
engine by using the interactive
Java applet. Set the Engine
Type to "Turbojet" and you can vary any of the parameters which
affect thrust and fuel flow.
You can also explore how thrust is generated within the nozzle by using the
program which runs on your browser.
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