As the air passes through the engine various adjustments must be made to its velocity and
pressure. For example throughout the compression stage the air must be compressed, but no appreciable
increase in its velocity can be easily tolerated.
Another example which we've already seen occurs at the exhaust nozzle where the pressure of
the gas is dropped to that of ambient and a considerable increase in its velocity results.
These changes in pressure and velocity are accomplished by the different shaped passages
or ducts through which the air must pass before it exits the engine. The design of these ducts
is extremely important because the efficiency with which the changes from kinetic energy
to pressure energy and vice-versa occur are reflected in the overall efficiency of the
engine. In this example of the use of different duct shapes within the engine. It can be seen
that the use of a divergent duct will increase the pressure of the air after it leaves the
final stage of the compressor and before it enters the combustion chamber.
This air sometimes called compressor delivery air is the highest pressure air in the engine.
Using a divergent duct here gives a two-fold advantage. First, an increase in pressure
has been achieved with no expenditure of energy in driving the compressor. Secondly, a decrease
in velocity has been contrived which will serve to make the task of the combustion chamber
in keeping the flame burning less difficult.
This example which we've already experienced shows how a convergent duct is used to accelerate
the gas as it passes through the nozzle guide vanes on its way to the turbine blades. The
torque which is applied to the turbine blade is dependent amongst other things upon the
rate of gas flow into it. It follows then that the faster we can make the gas flow into
the turbine the more torque we can transfer to the turbine.
Logically therefore if we convert some of the considerable pressure energy of the gas
stream into kinetic energy it will be more efficient in imparting a turning effect upon
the turbine and its shaft.
When a compressor and turbine are joined on one shaft the unit is called a spool. This
diagram shows a single spool axial flow compressor turbojet engine. This type of engine was for
a long time considered to be the most useful where an engine with a small frontal area
was required such as in fighter aircraft. Where a high forward speed was the main criterion.
There are however problems with the control of the smooth flow of air through the engine
throughout its rotational speed range, more of this later.
As it flows from the compressor the air is fed directly into the combustion chambers
and fuel is added in the resulting mixture ignited. the resultant increase in temperature
will cause the substantial increase in volume which is required. The energy required to
drive the compressor approximately 50% of the energy in the gas stream is now extracted
from the stream as it passes through the turbine. The remaining energy in the gas stream acts
as thrust as the gas is passed atmosphere by the end of the jet pipe.
This diagram illustrates both a centrifugal compressor turboprop engine and an axial flow
compressor turboprop engine. the output from a turboprop engine is the sum of the shaft
horsepower developed at the turbine and the residual jet thrust. This is called equivalent
shaft horsepower or ESHP. The major difference between the turbojet and the turboprop in
how they handle the generated power is that in the turbojet virtually all the energy that
remains after the compressor has been powered is used as thrust. Whereas in the turbo problem
almost all the energy in the gas stream is converted into mechanical energy to drive
both the compressor and the propeller. Only a small amount of jet thrust is available
from the exhaust system of a turboprop with an efficient turbine it can fairly be described
as residual thrust only.
Apart from this difference the airflow through the engines is virtually the same in either
case. The compressor passes the air to the combustion chambers, where the fuel is added
and the mixture is burnt. And because of the temperature rise a substantial increase in
the volume of the air is obtained at a nominally constant pressure. The gas is now expanded
in the turbines where are dropping the pressure and temperature is converted into the kinetic
energy which is exchanged for the torque used to drive the compressor or compressors and
the propeller through its reduction gear.
The turboshaft engine can be thought of as a turboprop engine where the propeller has
been replaced by a shaft turboshaft engines can be used to drive helicopter rotors, they
can also be used in applications.
Where a compact supply of electrical power is required their output shaft being attached
to an alternator. This is the type of engine normally used to see Auxiliary Power Unit
or APU on most modern transport aircraft.
Most if not all turboshaft engines incorporate a free power turbine. A free power turbine
is a turbine that is not connected to any of the compressors. This frees the turbine
from the constraint of having to rotate at a speed that suits the compressor and this
gives the turbine a much wider operating speed range. The single spool turboshaft engine
illustrated here has a reverse flow combustion chamber system. This allows the engine to
be much shorter stiffer and lighter than it otherwise would be. But does add the requirement
for a centrifugal compressor to be used in the high-pressure stage this allows for the
air to be thrown out radially in order that you can enter the combustion chamber in the
correct direction. Other than this deviation the airflow is similar to that previously
described for the turbojet engine, up to the point where it leaves the first stage turbine.
The first stage turbine having converted sufficient energy from the gas stream to drive the compressor,
the free power turbine converts any remaining energy to mechanical energy which is utilized
to drive whatever is attached to its shaft.
The bypass ratio of an engine is due find out the ratio of the amount of air which is
bypassed around the hot core of the engine to the amount of air which passes through
the hot core. An engine with a bypass ratio in the region of about 1 or 2 to 1 would be
considered to be a low bypass ratio engine. Whereas an engine with a bypass ratio of around
5 to 1 would be considered to be a high bypass ratio engine.
The engine shown here is a twin spool low bypass ratio engine. The airflow as far as
the end of the low-pressure compressor is identical to that of a pure turbojet. But
Then the airflow splits into two. An amount depending on the bypass ratio will flow down
the bypass duct and the remainder continues into what is the start of the hot core of
the engine the high-pressure compressor.
From the high-pressure compressor the air follows the usual path through the combustion
chambers and into the turbine. Before it leaves the hot core and rejoins the bypass air in
the mixer unit of the exhaust system.
The propulsive efficiency of both the low and high bypass ratio engines is much greater
than that of the pure turbojet at the speeds normally associated with jet transport aircraft.
The term propulsive efficiency is explained later in this lesson.
The specific fuel consumption of both the low and high bypass ratio engines is also
much lower than that of the pure turbojet.
The experience which was gained by many and operating the low-bypass ratio type of engine
proved that engines which dealt with a larger comparative air mass flow and lower jet velocities
could deliver propulsive efficiencies comparable to those of turboprops. And indeed a higher
propulsive efficiency than turbo jets operating at normal cruising speeds. The advent of the
fan jet engine had arrived
This model of a triple spool front fan turbojet engine shown here represents probably the
most successful early example of this type of engine the Rolls-Royce RB211.
The air enters the intake and passes immediately into the low-pressure compressor more commonly
called the fan. Here its pressure is raised before it splits to go either through the
bypass duct or into the intermediate pressure compressor. The amount going into either depending
upon the bypass ratio. The thrust of this type of engine is almost completely dependent
on the relatively cold air going through the bypass duct which has a high mass and relatively
low velocity, hence its high propulsive efficiency.
The air which passes through to the hot core which consists initially of the intermediate
and high pressure compressors has a great deal of energy added in the combustion chambers.
But this energy is required to drive all the compressors which includes the fan. In fact
it is the rear most turbine or the low-pressure turbine which is responsible for driving the
front fan, by extracting virtually all of the energy that remains in the gas stream.
If it's efficient in doing its job then there should be only residual thrust remaining where
the hot gases emerge from the turbine
We previously explained that thrust is the product of mass times acceleration. It can
be demonstrated that the same amount of thrust can be provided either by imparting a low
acceleration to a large mass of air or by giving a small mass of air a large acceleration.
In practice the former is preferred since it's been found that the losses due to turbulence
are much lower and the propulsive efficiency is higher. In this graph the levels of propulsive
efficiency for several different types of gas turbine engine will be shown.
The highest propulsive efficiency at lower speeds is offered by the turbo propeller combination.
However, above about 350 miles per hour the propellers efficiency drops off quite rapidly
due to the disturbance of the airflow at the tips of the blades.
In comparison with the turbo problem the propulsive efficiency of the pure turbojet appears quite
poor at the lower air speeds. As the airspeed increases in excess of 800 miles per hour
however the propulsive efficiency of the turbojet starts to improve beyond the capability of
the turboprop engine to match it and from then on there is no comparison. The eventual
outcome being a propulsive efficiency close to 90%.
Cruising speeds in the order of 800 miles per hour are at present out of the reach of
most transport aircraft. And this fact means that in the mid speed range where most of
the world's jet transport aircraft operate. There is a niche for the bypass engine both
low ratio and high ratio.
The low and high bypass ratio engines which includes the ducted fan or turbofan engine
have a propulsive efficiency which fits neatly between that of the turboprop and the pure
turbojet.
By dealing with comparatively larger mass air flows at lower jet velocities the bypass
engine attains a propulsive efficiency which exceeds that of both the turboprop and the
pure turbojet at the speeds normally associated with jet transport aircraft.
The use of larger and larger aircraft has meant that more passengers can be accommodated
in each flight thus air travel has become less and less expensive for the individual.
This concept of using large aircraft works well. As long as the aircraft themselves remain
serviceable. If however, one restricting component such as an engine becomes unserviceable on
a large aircraft then the expense involved in keeping three or four hundred passengers
fed, accommodated and happy becomes exorbitant.
Engine manufacturers in an attempt to minimize the financial burden imposed upon the users
of their equipment in the event of its failure. Have started to use modular construction methods,
which facilitate changing sections of an engine rather than a whole engine. This diagram shows
how the engine is split into several modules. This concludes the introduction to the gas
turbine engine.