GAS TURBINE

Materials andENGINE
Hardware
Power Augmentation Systems

SUB - MODULE 15

Cat. Cat.
A B

15.15 - Power Augmentation Systems

Power Augmentation
Operation and applications;

Systems
Water injection, water methanol;
Afterburner systems.
1 3

Issue-I, Rev.-0 15.1

Optional power augmentation systems are used to flat rated. This is not strictly a power augmentation
temporarily increase an engine's available power 'system' but it serves the same purpose.
during hot weather, high altitude, short runway or
emergency operations. Water and water methanol FLAT RATING
injection systems are used to restore the engine's Flat rating is designed to enable an engine to
rated thrust or shaft horsepower to its ISA sea level maintain the rated thrust or rated shaft horsepower
value where this has been reduced by high outside air value over a wider range of outside air temperatures.
temperatures or high runway elevations. Afterburner It will also provide an emergency reserve of power if
systems on the other hand are used to produce a required say in the event of another engine failing at
significant increase in thrust above the engine's rated a critical moment. The concept is simple! An engine
value. The Concorde afterburner system was used for may be rated at say 40,000 lb thrust ISA by the
take-off, to achieve transonic acceleration and to manufacturer but the aircraft operator may not need
provide an improved rate of climb if necessary. to utilize that figure and be happy to use 35,000lb or
Military aircraft would employ afterburning to less. In this case the operator will re-rate the engine
reduce take-off distances with heavy weapon store to a lower rated thrust value than it would ordinarily
loads, improve rates of climb, achieve transonic have on a standard ISA day. The operator can then
acceleration and improve combat performance. The attain that lower rated value over a wider ambient
only other option in both cases would have been to temperature range. There is less chance that the
install a bigger and more powerful engine for these engine will become EGT limited at the de-rated
short duration tasks. That would be a prohibitive maximum power setting and an additional benefit
increase in weight and overall fuel consumption just that the mechanical and thermal stresses on the
for a couple of minutes' work. The normal rated engine will be reduced leading to an increase in the
power or thrust produced by an engine is referred to engine's operational life.
as its dry rated value whereas the value produced
when using power augmentation is referred to as its Flat rating is like having a reserve of power at our
wet rated value. Although not mentioned in the disposal. As the outside air temperature rises the
syllabus for this section, many modern engines are rated thrust value can still be achieved just by

Figure 15-1 : Flat Rating

15.2 Issue-I, Rev.-0

moving the power lever further forward. Eventually a
situation will be reached where rated thrust will be
achieved coincident with the power lever reaching
the fully forward position. Any further increase in
outside air temperature or fall in ambient pressure
beyond that point will now be accompanied by a fall
in thrust. If the ambient temperature where this
occurs is 30°C then the engine is described as being
Flat rated to 30°C.Some turbo-propeller engine
types employ an automatic performance reserve that
comes into operation in the event of another engine
failing on take-off or landing. It is a short term
increase in engine power that is still within the rated

Power Augmentation
power value set by the manufacturers. In effect it is a
form of flat rating. The term flat rating is derived Figure 15-3 : Effect of Water Injection at Airfield Elevation

Systems
from the flat portion of the performance graph where
the de-rated thrust is maintained at a constant value When the engine is employing power augmentation it
over a given ambient temperature range. is said to be producing wet power. A typical water
methanol system is capable of increasing the
WATER INJECTION/WATER METHANOL available engine power by around 15% in order to
If the outside air temperature is high, or the airport regain rated power. If we look first at just water
elevation is very high, the engine will not be able to injection, we can study the effect it has on the
achieve its maximum rated power value for take-off engine. Dependent on engine type there will be one
unless it has such a system. of two points chosen to inject the water, either
compressor inlet injection or combustion chamber
injection. We need to look at both options. Pure
demineralized or distilled water is the most common
water injection fluid. Pure water is also widely used
because it produces a greater cooling effect than a
mixture of water and methyl or ethyl alcohol.
Aircraft, such as helicopters and turboprops, which
make frequent take offs and landings are forced to
use a water alcohol mixture to protect against freeze
up. Demineralized water or distilled water must have
less than 10 Parts per million (PPM) of solids. De-
mineralized water has to be used because tap water
contains suspended elements and impurities and its
use would foul the compressor and turbine vanes and
Figure 15-2 : Effect of Water Injection at High OAT blades with hard calcium and corrosive deposits. The
water or water methanol injection system is
When a turbo-propeller engine is operating without intended to support one take-off only. If water only is
thrust augmentation it is said to be producing dry being used it would freeze in the storage tank and
power. lines at altitude so there is no intention to carry it
around in flight.

Issue-I, Rev.-0 15.3

Compressor inlet injection is not always suited to it will act as an anti-freezing agent when added to
axial flow compressors due to uneven distribution the water. The proportions used are 60% distilled
but it is used in turbo-propeller axial and centrifugal water to 40% methanol. The addition of methanol to
flow engine types. When water is sprayed into the the water is done primarily to provide this anti-
compressor inlet the air will cool rapidly as it gives up freeze protection. There is, however, a secondary
its heat energy to evaporate the water. The reduction benefit to the use of methanol, it is a fuel. You will
in the temperature of the compressor airflow already have read that an increase in fuel supply may
increases its density and thus the air mass flow rate be required to restore the turbine inlet temperature.
rises. The increase in mass airflow increases the The methanol does this by burning in the combustion
thrust of the engine. The cooling effect of the de- chamber and restoring the turbine temperature
mineralized water will also reduce the turbine inlet without having to resort to a complicated engine fuel
temperature and the fuel flow can then be increased control interconnection.
if needed to restore the turbine operating
temperature giving a further increase in thrust. The Even though water does not contain the heating
pressure and temperature drop across the turbine value of alcohol, it has been determined that
stages increase making additional work energy because of its heat absorption capacity, more thrust
available to increase the engine torque at the can be obtained by injecting a given volume of water
propeller. into the engine than an equal mixture of water and
alcohol. Another way to think about this is that
If the water is sprayed directly into the combustion although alcohol can be used as fuel after it is used as
chamber it increases the mass flow of the gas a coolant, the thrust augmentation factor per unit
entering the turbines. The mass of the water volume in a water/alcohol mixture is less that of the
introduced into the combustion chamber is pure water.
responsible for this. The mass airflow through the
compressor is unaffected in this case. The result is Wet thrust rating is restricted to take off, is time
that the pressure and temperature drop across the limited, and has an altitude limitation. Water
turbine stages reduces giving an increase in the injection systems are not normally used on high by
pressure in the jet pipe. This increase in pressure can pass turbofan engines.
then be converted into thrust at the propelling
nozzle. The turbine inlet temperature will be
reduced and more fuel can be supplied if needed to
restore this and increase the engine speed to produce
additional thrust. The combustion chamber injection
method is more suited to turbo-jet engines as it leans
towards thrust production rather than torque.

WATER METHANOL
The addition of methanol to the water serves two
purposes. Firstly, there is a danger that de-
mineralized water on its own could freeze and
produce ice in the compressor inlet. The inlet
pressure probe is a particularly sensitive area and the
inlet spray nozzles are positioned so they do not wet
the probe. Methanol has a very low freezing point so Figure 15-4 : Turbo-prop 'Wet' Take off Performance

15.4 Issue-I, Rev.-0

 Power Augmentation
Systems
Figure 15-5 : Water Methanol Injection System

COMPRESSOR INLET INJECTION CONTROL metering valve will respond to the torque signal and
This system is used on turbo-propeller engines. The reduce the flow.
water or water methanol is stored in a tank in the
aircraft. When the system flight deck switch is COMBUSTION CHAMBER INJECTION
selected, the system isolation valve opens and an This system is used on turbo-jet engines. When the
electrically driven pump produces a head of pressure system flight deck switch is selected the water
at the flow control unit metering valve. methanol is passed from the aircraft storage tank to
an engine bleed air drive turbine pump. The air
A pressure switch positioned in the supply line to the supply to the pump is initiated when the engine
control unit signals a lamp on the flight deck to power level reaches the take-off position. The pump
illuminate showing that water methanol pressure is then produces a head of water methanol pressure at
available. The flow control unit metering valve is a water pressure-sensing valve that is also sensitive
linked to the engine power lever control so that it can to HP compressor delivery pressure. The sensing
only initiate flow to the inlet spray nozzles as the valve will initiate a flow of water methanol based on
power lever reaches the take-off position. The the pressure difference between the water pressure
metering valve will only initiate flow if the ambient and the HP compressor delivery pressure. The water
conditions are such that they would normally reduce methanol flows under pressure from the water
the engines rated power output. The flow control sensing valve to ports in each fuel spray nozzle arm
unit senses the engine torque and atmospheric that spray the coolant onto the inlet side of the swirl
pressure and will meter the water methanol flow to vanes in the combustion chambers. When the system
recover the engine torque to its rated and will meter initiates, the engine speed governor will re-set to
the water methanol flow to recover the engine permit a higher engine speed whilst the system is in
torque to its rated sea level value and maintain it up operation. A final important note for you to
to the limit of the system temperature range. If the remember. You must not select a water or a water
engine torque tries to exceed the rated value, the methanol system ON if the outside air temperature is

Issue-I, Rev.-0 15.5

below 50C. At these temperatures there will be a risk 818°C or 1500K then the temperature ratio would be
of icing in the compressor! 1.5. The square root of 1.5 is approximately 1.2. This
means that the exit gas velocity will increase by 20%
AFTERBURNER SYSTEMS which roughly equates to a 20% increase in the dry
Some low by pass turbofan engines with mixed flow static thrust to give 120% wet thrust.
are used in speed range of 0.8 Mach (military
aircraft). These engines use afterburner to increase
thrust.
It is designed to produce an increase in thrust well
above the engine's rated value. The Concorde
afterburner increased the thrust over the rated value
by around 20%. The thrust increases used in military
aircraft engines will be in the region of 50% or more.
The thrust produced with afterburning is referred to
as wet thrust. There is no way in which increases in
thrust of this nature can be extracted from the
engine through increasing its fuel flow. There is,
however, a large percentage of the engine air that is
not used in combustion. In the case of low by-pass
engines there is an even greater excess of air. The Figure 15-6 : Thrust Increase with Afterburner
principle of afterburning is to introduce fuel into this
air and burn it in the jet pipe to create a bigger A flame stabilizer in the form of an annular V
energy rise at the propelling nozzle to create the sectioned ring called a vapour gutter is positioned in
required increase in exit gas velocity to increase the the jet pipe to create a re-circulation of the gas flow
thrust. In this way the mass air flow through the downstream of the afterburner fuel spray manifold.
engine remains unchanged, the engine speed is This is important because the speed at which
unaffected and the engine operating temperatures kerosene fuels burn is only a few feet per second so
are unaffected. Simply, the engine is to all intents the gutter ensures that the flame is anchored for long
and purposes unaware that after burning is in use and enough to ensure proper combustion. The fuel spray
is unaffected by it. To ensure efficient afterburning it manifold is positioned upstream of the gutter and
is important that a stable combustion area is created sprays the fuel into the gas stream so that it contacts
in the gas flow to the rear of the turbine section. with hydrocarbon fuels at high temperature they will
When the engine is of a low by pass configuration the cause a spontaneous ignition of the fuel. The same
hot and cold stream must be mixed inside the exhaust principle is used in the catalytic converters in cars
unit before the afterburning region. The jet pipe is of where the exhaust gases pass through tubes lined
a larger diameter than normal, is double skinned and with these metals. Another method used to ignite the
insulated to protect the airframe from heat damage. reheat fuel is the hot shot ignition system.
It is constructed of nickel steel alloy to withstand the
increased temperatures. The increase in exit gas This system incorporates a separate fuel feed to the
velocity possible through afterburning is combustion chamber that injects a shot of neat fuel
proportional to the square root of the ratio of the creating a long streak of flame that will pass through
absolute temperature rise in the jet pipe. If the the turbine section and ignite the reheat fuel in the
temperature in the jet pipe before afterburner jet pipe. The increase in the volume caused by the
initiation was say 727°C or 1000K and this rose to expansion of the gas flow raises the jet pipe pressure,

15.6 Issue-I, Rev.-0

P6 in figure. If this were not controlled it would result This may be at the maximum normal power position
in a rapid stall of the compressors and a surge. The or at a pre-determined position below it. An engine
afterburner installation incorporates a variable area driven fuel pump supplies a pressure flow to the
propelling nozzle that is operated by pneumatic or control unit that in turns meters the flow in line with
hydraulic rams. These operate to open the nozzle and the degree of afterburning selected by the power
so increase the exit area as the afterburner fuel flow lever position in the afterburner range of the throttle
is initiated and then increased. This controls the P6 quadrant. The flame pattern in the jet pipe is held
pressure in relation to the HP compressor delivery central by design to avoid overheating the jet pipe
pressure, P3 in the illustration, walls. As the jet pipe P6 pressure starts to rise, the
P3/P6 pressure ratio control unit moves the nozzle
An acoustic shield otherwise known as a screech liner control unit to signal the rams to open the propelling
is fixed to the internal wall of the jet pipe in the nozzle with the assistance of the gas flow. The nozzle
combustion region. This normally consists of a will continue to open until the P3/P6 pressure ratio is

Power Augmentation
corrugated section and a section of overlapping restored to normal.
metal tiles. The corrugated section insulates the jet This means that the pressure drop across the turbine

Systems
pipe wall from the heat of the combustion zone and stages will remain normal and the engine will be
the tiled section absorbs the high amplitude sound unaffected by the operation of the afterburner. If the
waves originating from unstable combustion. The throttle is moved further into the afterburner range,
complete shield is often called a screech liner and it the fuel control unit will increase the fuel flow and
protects the jet pipe from thermal and vibration the pressure ratio control unit will move the
stresses. The afterburner system is selected by propelling nozzle further open to maintain the P3/P6
operating a switch on the flight deck. The pressure ratio. It is essential that there is a failsafe
afterburner fuel control unit is linked to the engine link between the fuel supply and the nozzle
power lever and will initiate a fuel flow to the spray operating system. If the propelling nozzle fails to
manifold at a given throttle position. open then the afterburner will not initiate. The fuel

Figure 15-7 : Afterburner System

Issue-I, Rev.-0 15.7

flow cannot be initiated or increased unless the also incorporated an electrical arc igniter to
nozzle mechanism operates correctly. In the unlikely guarantee ignition under all flight conditions. The
event that the nozzle were to close with the increase in jet pipe temperature was a lot less than in
afterburner in operation, the fuel supply would cease military afterburner systems and the cold thrust
and the afterburner system would cancel out. It pressure loss without afterburning was only half a
takes more force for the rams to close the nozzle percent of the cruise thrust value. As a matter of
against the gas stream pressure so it would be a interest the propelling nozzle exit temperature with
drastic failure of the nozzle control unit in this case. full afterburning was considerably lower than the
turbine entry temperature.
One drawback to the afterburner system is that the
mechanical assembly in the jet pipe will cause Older after burners were two position type. They
turbulence in the exhaust gas flow whether the formed a convergent nozzle in none after burning
afterburner is operating or not. When the mode and a convergent divergent (C-D) nozzle when
afterburner system is not in use, the turbulence open in after burning mode.
causes a pressure loss with a resultant drop in thrust
output. An engine not fitted with afterburning In newer aircraft, the after burner nozzle is C-D
equipment would produce more thrust in normal shaped in both modes, changing both thrust throat
operation than one fitted with it. When afterburning flow area and the final nozzle size to their largest
is used the specific fuel consumption (SFC) will area and flow angles in full afterburner. Electronic
approximately double. The actual increase will be sensors are utilized to match the flow area to the
dependant on the degree of afterburning selected. mass flow in afterburners of this type.
The combustion efficiency of the afterburner is lower
than that of the combustion chamber due to the After burning is used primarily for takeoff with heavy
lower pressure environment in the jet pipe and this aircraft loading and for rapid climb out speeds.
will also raise the SFC. As altitude increases up to the
troposphere the SFC will reduce slightly due to the Afterburner fitted aircraft can have as much as 100%
reduced engine inlet air temperature. Finally, additional thrust in after-burning mode, with fuel
remember that the operation of the afterburning flows increasing by 3 to 5 times. On the other hand,
system does not change the air mass flow through the some modern aircraft have very powerful engines
engine or increase the engine speed. The increase in and require only limited thrust augmentation in the
thrust is gained from the increased gas velocity at the 15 to 20% range. In this case, the tendency today is to
propelling nozzle. refer to the C-D tail pipe as a thrust augmenter rather
than an after burner.
The only civil aircraft to be equipped with an
afterburner system was the Concorde. This was When an aircraft is operating in after-burning mode
necessary to give the aircraft a reduced runway on the ground, gross and net thrust are the same
takeoff length, an accelerated rate of climb and the value. If at that time, the afterburner boosts gross
increased engine power required to pass through the thrust by 25%, in flight the same afterburner
transonic to the supersonic flight range. The system contribution to net thrust would be a much greater
gave between 5% and 20% thrust increase. Fuel was percentage as much as 100%. This occurs because
supplied from an engine driven pump and injected ram drag, which affects the engines thrust, does not
forwards through a single fuel spray ring to be affect afterburner thrust. In other words, ram drag is
deflected downstream by the exhaust gases. The fuel the same whether the engine is in afterburning mode
was ignited spontaneously but the Concorde system or not. Consider the following statements about an

15.8 Issue-I, Rev.-0

aircarft operating on the ground versus its
performance in flight:
Gross (static) thrust without A/B = 16000 lbs
Gross (static) thrust with A/B = 20000 lbs
(That represents an increase of 4000 or 25%)
Net (inflight) thrust without A/B = 4000 lbs
Net (inflight) thrust with A/B = 8000 lbs
(That represents an increase of 4000 or 100%)

Note – Ram drag is defined as aircraft speed times

mass air flow, and net thrust is defined as gross thrust
minus RAM drag.

Power Augmentation
THRUST DECAY SYSTEM
This is not a thrust augmentation system but it is

Systems
worth a brief mention. The system is used on an
engine with a variable area-propelling nozzle. When
the aircraft is taxying the idle thrust from the engines
may be sufficient to accelerate the taxi speed to the
point that the brakes have to be constantly applied.
The thrust decay system operates by opening the
propelling nozzle to form a parallel exit thus
destroying the ability of the nozzle to accelerate the
exit gas flow and create thrust. The system is linked
to the throttle so that if you move it forward to
increase power the thrust decay system will
immediately return the nozzle to its correct
configuration.

Issue-I, Rev.-0 15.9