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Variable Cycle Engines
By: Praveen Pratap Singh
M.Tech- Aircraft Propulsion
3.VCE Configuration and working.
4. Working cycle of VCE and work calculation
5. Effect of VABI’s on performance parameters
6. Research, Development and Future
7. Revolutionary Turbine Accelerator : Another Air
Aircraft performance lies in the complexity of engine design. The need for supersonic
passenger aircraft has led to a compromise between turbofan and turbojet engine.
Turbofan is optimized for subsonic thrust with better fuel efficiency and less noise but performs
poor at high speeds. On the other hand turbojet could serve purpose for achieving supersonic
speeds but at the cost of higher efficiency and jet noise.
Hence a concept of hybrid engine known as Variable Cycle Engine(VCE) has been formulated
to combine best traits of both turbofan and turbojet engine so as to achieve supersonic speed
at less specific fuel consumption and noise.
Variable Cycle Engine is a next generation supersonic transport engine that is designed to
operate efficiently under mixed flight conditions such as subsonic, transonic and supersonic
which attains high specific thrust at supersonic cruise.
In this presentation the objective and features of VCE with examples of already existing
engines, variation in parameters with variable geometry are discussed and results are
interpreted. Thus, the main aim being to project some light on variable cycle engines, their
characteristics and the present study being made and research carried out on the same.
Gas Turbine Engines
High speed air breathing engines used in aircrafts.
The forward thrust is produced by accelerating hot gases in backward direction.
In a typical gas turbine engine, this is accomplished as follows.
Entry of high speed air
in diffuser (a diverging
Compression of high
speed air in
compressor (a rotating
Burning of fuel air
Expansion of hot gases
in turbine (a rotating
Further expansion of gases
up to ambient pressure of air
in nozzle (a converging
section) and release of gases
at high speed
Gas Turbine Engines
5 Gas turbine engines are classified as follows :
Turbojet (Conventional )
Ramjet (No rotating part)
Turboprop (an additional propeller at front)
Turbofan ( Fan instead of propeller)
Turboshaft (An additional rotor produces shaft power (Torque)which is further
converted into thrust)
Schematic (Ref. 11) of a conventional GT Engine is shown below
Turbojet and Turbofan Engines
Thrust through core engine only
Low bypass engine (BR=1:1)
Operates at supersonic speeds
High specific thrust due to high jet velocity
High operating noise due to high jet velocity
Used in military/fighter aircrafts
Less take off thrust
Low propulsive efficiency
Typically, 80% thrust through air bypassed
by fan, 20% through core engine
High bypass engine (BR= 2-5:1)
Operates at subsonic speeds
High specific thrust at low speeds.
Low operating noise
Used in civil/passenger aircrafts
High takeoff thrust
High propulsive efficiency
Idea of Supersonic Passenger Aircraft
Concorde (in figure, Ref. 12)was developed
jointly by UK and France in 1976 as one of the
commercial supersonic aircraft.
It incorporated a turbojet engines and achieved
speed of Mach no 2.4
Operation of Concorde was stopped in 2001 due
to many reasons.
One of technical reasons being enormous noise
produced by high speed jet during take off .
Specific fuel Consumption was very high 22000
Concorde needed to fly at longer distances to be
For high speed and optimisation of flight,
concord used double delta shaped wings and
variable engine intake ramp system controlled
by digital computers.
Variable Cycle Engines
The need for supersonic passenger aircrafts
has led to the concept of variable cycle
Most of the aircrafts today use Turbofan
Turbofan engines are optimised for subsonic
thrust with fuel efficiency and produce less
noise but they perform poorly at high speed .
Using Turbojet engines for passenger
aircrafts could serve the purpose of achieving
But this speed is achieved at terrible fuel
efficiency and high jet noise.
Hence, a concept of a hybrid engine known
as variable cycle engine was derived which
aims to combine best traits of turbojet and
turbofan engine so as to achieve supersonic
speed at less specific fuel consumption and
Variable Cycle Engines
Quest for Variable Cycle engines began in late 1980s/early 1990s.
GE’s YF120 developed in late1980s was one such engine which was used in Lockheed
YF22 (powered with two YF120) had achieved flight of up to Mach 1.58
These engines are being developed so as to have minimum 25% of fuel saving.
Schematic of YF120 is shown in figure. (Ref. 4)
Objective of VCEs
High specific thrust at low
specific fuel consumption at high
Low noise (of Range that of
Ability to alter mass flow rate
Maintain Overall pressure ratio
High propulsive efficiency
High mechanical efficiency of
Mass flow rate and Thrust
where, ρ- density of fluid (Kg/s)
A- Area ( )
C- Velocity of fluid (m/s)
m- mass flow rate of air and fuel mixture (Kg/s)
Cj- velocity of jet
Ca- Velocity of air (speed of aircraft)
Aj- Jet exit area
Pj- Pressure at jet exit
Pa- Ambient air pressure
From mass flow rate equation, change in
area or velocity will change mass flow rate
also, and hence a variable area geometry is
In thrust equation, if gases are expanded
completely up to ambient air pressure,
second term of equation will be zero.
Again it can be seen from equation that
thrust is directly proportional to jet velocity ,
increasing or decreasing jet velocity will
increase or decrease thrust.
If second term exists, changing jet exit area
will change the value of thrust.
Also, thrust is proportional to mass flow
rate, and hence changing mass flow rate
will change thrust.
Both thrust and mass flow rate are
dependent on jet velocity, an important
parameter to be considered in VCEs
Jet Noise and Propulsive Efficiency
Noise level of an aircraft is measured by
acoustic power (Ref.2) in subsonic turbulent
Kinetic power in jet is proportional to third
power of jet velocity hence a “Acoustic
Efficiency Parameter “may be defined as
where ,Mj is ratio of Cj and ambient speed
of sound .
Therefore to reduce broadband noise of an
aircraft exhaust, jet speed needs to be
Also, reduction in jet speed yields better
It is defined as thrust power to the sum
of thrust power and change in K.E of jet.
It is evident from the equation that lesser
the value of jet velocity, greater will be
the propulsive efficiency
SFC and Overall Efficiency
where m(dot)f is mass flow rate of fuel in Kg/hr
and F is thrust in N.
where Q is calorific value of fuel
Hence it can be seen that
Low SFC = High Efficiency
High SFC = Low Efficiency
For low SFC, Thrust produced should be
high enough while fuel flow rate should
Thrust is dependent on mass flow rate
and jet velocity, hence increasing jet
velocity or mass flow rate or jet velocity
will increase thrust.
This can be achieved by incorporating
variable geometry units in the engine.
Hence evolved the concept the variable
A variable cycle air breathing GT engine includes
A Fan and a high pressure compressor
A High pressure turbine that drives a pressure compressor
A low pressure turbine that drives a fan
A combustor and a afterburner
Variable Geometry components such as
1. Variable Area Bypass Injector (VABI)
2. Variable Inlet Guide vane
3. Variable Area Nozzle
4. Variable Nozzle Area Turbine
There are two VABI’s used in the engine.
The first VABI is a selector valve downstream of fan.
The second VABI is valve which is located between core driven fan stage and inlet to HP
Working Principle of VCE
The schematic of VCE working principle and connection between engine parts are shown in
Working Principle of VCE
There are two kinds of working mode about the variable cycle engine, turbojet and turbofan.
When engine working in subsonic cruise low power state, the mode conversion valve open, make
more air into the deputy external duct, at the same time the former mixer is wide opened. Open the
after mixer, increasing the bypass ratio; reduce fuel consumption, the engine working at the model
of turbofan this time.
When engine working in the state of supersonic cruise, acceleration and climbing, the former mixer
is smaller, selector valve closed, most of the gas forced into the core engine, produce high thrust,
the engine working at the model of turbojet this time
One of the first attempts to achieve this operation was the Variable Pumping Compressor shown in
Variable area bypass injector
It is used to vary the relative proportions of flow in a mixer. Effectively, it acts like a variable area
The key function of the rear VABI is to enable control of the fan operating line independently of
the core gas generator system
It does this by matching static pressures of the streams entering the mixer by varying the Mach
number in the bypass stream to attain the static pressure balance for mixing the flows.
VABI settings may range from 0.5 to 1.5. In any condition other than the nominal position, a
pressure drop must occur across the VABI in order to match static pressures resulting in
increased mixing losses associated with the velocity mismatch of the two streams.
VABI position diagram is shown (Ref. 3)
Variable area nozzle
At constant speed a nozzle downstream of the compressor is closed.
This increases the pressure ratio in the compressor because the compressor must now increase
the density of the gas to push it through the smaller flow area.
In such a way the working line of a compressor can be controlled.
For an engine with an afterburner a variable area nozzle becomes a necessity.
Due to the increase in temperature in the afterburner, the pressure increases to the point where
compressor stall would occur if the nozzle area where not adjusted.
Variable Inlet Guide Vane
The aim of using VIGVs in a VCE is to enable
transition from one cycle to another
Variable inlet guide vanes operate by varying
the inlet axial velocity.
As can be seen in the vector diagrams in
Figure (Ref.3), closing the vanes produces a
smaller value of axial velocity and thus a
smaller mass flow.
Similarly, opening the vanes away from the
nominal position produces a larger axial
velocity and thus higher mass flow.
By varying the axial velocity through changing
the IGV angle, one can effectively vary the
mass flow through the component (as, 𝑚̇ = 𝜌𝑐𝐴
, where c is axial velocity)
In addition to enabling cycle transition, VIGVs
offer better performance at off design, surge
margin control, and increasing the overall
compressor range of operation.
Variable Nozzle Area Turbine
Variable area turbines (also VABI 3) primarily attempt to “change the speed-speed relationship of
the high and low rotors in order to enable optimal engine efficiency over the entire flight
Thus allowing OPR to be controlled as turbine inlet temperature varies to match the fan power
They also allow independent control of high and low rotor speeds and provide increased cycle
This variable geometry occurs at the inlet of the turbine and can be attained two ways.
1.The first method involves re-staggering the stator blades and acts in a similar fashion to
2. The second method is mechanically simpler and involves introducing an obstruction into the
flow or by introducing secondary airflow
Use of variable geometry in an HP turbine is seen as a large technology risk but use in an LP
turbine can lead to modest alteration of bypass ratio, TSFC, and specific thrust at subsonic
The specific improvement depends largely on the cycle engine design
Total Pressure Recovery Through Inlet
The intake is provided with a variable geometry mechanism such that the throat is wide open at
low subsonic speeds and closes progressively towards the supersonic regime.
The mismatch between intake and compressor is avoided during the supersonic regime by
deviating through the bypass duct and the variable geometry bypass nozzle (BN), the excess of
mass low captured.
Hence, it is assumed that the intake delivers the exact mass flow requested by the air compressor
The total pressure recovery through the intake TPR is the ratio of outlet to inlet stagnation
pressures. For air behaving as perfect gas and provided that the flow is adiabatic, the total
pressure recovery is related to the intake kinetic efficiency ηk by: (Ref. 8)
TPR =(1 + 0.5(γ −1)(1−ηk) Ma∞ ^2)^−γ/(γ−1)
where Ma ∞ is the free stream Mach number and γ the ratio of speciﬁc heats.
The kinetic efficiency is the ratio of kinetic energy of the outlet flow (if expanded to ambient
pressure) to the free stream kinetic energy
ηk = v2out/v2 in
Working cycle of VCE
Working cycles for adaptive engines of VCE types are typical cycles of a double flow engine,
where the working area is split between bypass channels, depending on the aircraft flight
conditions and operation range of the engine
On the contrary, the working cycle of the ‘bypass’ engine is a typical cycle of a single-flow
engine with variable area of the corresponding thermal cycle. The curve shape depends on the
amount of air that is bled from downstream of the compressor and fed downstream the turbine.
Real cycle of a turbojet adaptive engine(left) and Real cycle of a turbojet engine of the
‘bypass’ type (right) are shown in fig. (Ref 6)
The work during a full cycle of adaptive engines can be determined for individual channels with use
of the following formulas:
For an adaptive engine that incorporates a mixer of streams, the relationship between the effective
work of the external cycle on one side and the ηM mixer efficiency and φM ratio of velocities of air
stream to be mixed can be expressed in the following form:
Whilst the corresponding relationship for the engine with an afterburner in the external channel is
The work of the full cycle executed by the engine of the ‘bypass’ type can be expressed in the
form that exemplifies the difference between the work during a cycle executed by a turbojet
engine without bleeding facilities and the working cycle of the ‘bypass’ engine, i.e.
VCE Rear VABI and Nozzle Area Effect on Flow and
Bypass Ratio Figure (Ref.4) shows how the two front VABI’s affect
the engine bypass ratio W16/W21.
While VABI 1 (downstream of the fan) is open, both
the nozzle area (varied in the range from nominal to
+10%) and VABI 3 have a certain influence on
If VABI 1 is closed then the bypass ratio is nearly
The numbers describing the VABI 3 position are in
the range from 0.5 to 1.5.
Values lower than 1 indicate that the bypass mixer
area A163 is smaller than its nominal value.
VABI 3 = 0.5 mean that the bypass mixer area is
reduced to 50% of the design point value.
VABI 3 values bigger than 1 mean that the core
mixer area A63 is reduced.
A 50% reduction of A63 is described with VABI 3 =
The sum of A163 and A63 is always equal to the
total mixer area A64.
VCE Rear VABI and Nozzle Area Effect on SFC and
Thrust26 Figure(Ref.4) shows the SFC for the various
geometry positions. These results are
somewhat surprising since the lowest SFC
is achieved with the lowest bypass ratio and
the highest SFC goes with the highest
bypass ratio. This result is affected by the
engine rating limiters:
If VABI 1 is open and VABI 2 closed (this
VABI setting makes the engine a
conventional turbofan), then the engine is
limited by the HP spool speed – T4 is much
lower than in the other cases.
If VABI 1 is closed and VABI 2 open, then
the active limiter is the burner exit
temperature and the HP spool speed is
much lower than in the other cases
If both VABI 1 and VABI 2 are open then the
engine operates with big nozzle and bypass
mixer areas at the spool speed limiter,
otherwise at the temperature limit.
VCE Rear VABI and Nozzle Area Effect on Flow and
One claimed advantage of the VCE
is that it can vary thrust without
changing mass flow which allows for
optimum flow conditions at the
aircraft intake during supersonic
One can adapt the mass flow in such
a way that the intake operates at the
most favourable pressure recovery
and without or reduced spillage drag.
From Figure , one can read at
constant mass flow (∼60 kg/s) a
thrust range of more than 15 to 20%.
Low thrust is connected with big
bypass mixer and nozzle areas, high
thrust with small areas.
Research, Development & Future
Research is being carried out extensively on these types of engines. GE Aviation has been
developing a revolutionary engine that aims to combine the best traits of turbojet and turbofan
engines, delivering supersonic speed capability and fuel efficiency in one package.
The new engines are being developed under the USAF ADVENT project, which is seeking 25
percent fuel saving which will in turn lead to an increase in mission capability.
The variable cycle technology used in the YF120 would be extended to not only turn the engine
into a Turbojet but also into a Ramjet. In that mode all airflow would bypass the core and be
diverted into the afterburner-like "hyper burner" where it would be combusted like a ramjet.
In context of the development of advanced launchers, the Revolutionary Turbine Accelerator
(RTA) was developed by NASA and General Electric (GE) in order to allow space access and
hypersonic commercial transport.
RTA: An Another ABVCE
29 The RTA (Ref.7) is a turbine based combined cycle (TBCC) and variable cycle engine (of GE’s
YF120) with a ramjet running on kerosene.
Ground tests with a prototype, RTA-1, were performed in 2006
RTA: An Another ABVCE
The RTA has following improvements over the YF120:
1. New fan and fan frame,
2. New core driven fan stage,
3. New rear VABI
4. New hyperburner and slave exhaust ,
5. New fuel and thermal management system
the RTA has a core driven fan stage (CDFS) fit with a separation between the tip and hub of the
Using such a fan in combination with variable inlet guiding vanes (VIGV) makes it possible to
change the flow path behind the fan separately.
The new fan and frame are necessary considering the RTA reaches a higher bypass ratio (BPR).
Furthermore, the RTA has a larger rear VABI system which reduces mixing losses by allowing a
smoother transition of the flow.
RTA: An Another ABVCE
The RTA was designed with the following
trajectory (Ref.7)or space access in mind .The
trajectory is divided into the following zones:
1. SLS=M=1.6 :From take-off until Mach 1.6, the
RTA performs like a single-bypass augmented
turbofan engine, i.e. the first VABI is fully closed
and most of the air delivered to the hyperburner
originates from the core stream. At take-off the
engine delivers a thrust of more than 150 kN.
2. 2 < M = 3 : At Mach 2, the engine begins
transition to ramjet mode, with the fuel flow to the
core decreasing while the fuel flow to the
hyperburner increases. Within this range, the
front VABI also reaches its maximum opening
3. 3 < M = 3.5 : Between Mach 3 and Mach 3.5 both
the rotor speed and the turbine temperature
decrease. At Mach 3.5, the core engine is
completely wind milling to ensure a quick engine
starting, to drive the accessory engine parts and
to reduce the mechanical loads on the rotating
components exposed to the highest inlet
4. M > 3.5 : Beyond Mach 3.5, the engine is in full
ramjet mode and at Mach 4 the launch vehicle
1. Variable cycle engine is a very complex power plant, involving fluid mechanics, structural
mechanics, heat transfer, combustion control and many other subjects.
2. Complexity in design with variable geometry is a big challenge to deal with.
3. Cost of manufacturing and maintenance are both very high.
4. VCE’s are in development phase and research is being carried out to make it cost effective.
5. Several optimization techniques are being incorporated for modelling engine design so as to
achieve desired objectives.
6. Mathematical modelling plays an important role in this research.
7. While there is no doubt that with advent of Variable Cycle Engines supersonic speed in
commercial aircrafts would be achieved however it should have benefits over conventional
subsonic commercial aircrafts.
8. Success in VCE technology will even make space access to human easier.
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Thesis. Georgia Institute of Technology , Dec. 2014.
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