A gas turbine is a machine delivering mechanical
power or thrust. It does this using a gaseous
working fluid. The mechanical power generated can
be used by, for example, an industrial device. The
outgoing gaseous fluid can be used to generate
thrust. In the gas turbine, there is a continuous
flow of the working fluid.
This working fluid is initially compressed in the
compressor. It is then heated in the combustion
chamber. Finally, it goes through the turbine.
The turbine converts the energy of the gas into
mechanical work. Part of this work is used to
drive the compressor. The remaining part is
known as the net work of the gas turbine.
History of gas turbines
We can distinguish two important types of
gas turbines. There are industrial gas turbines
and there are jet engine gas turbines. Industrial
gas turbines were developed rather slowly. This
was because, to use a gas turbine, a high initial
compression is necessary. This rather troubled
early engineers. Due to this, the first working gas
turbine was only made in 1905 by the Frenchman
The first gas turbine for power generation became
operational in 1939 in Switzerland. It was
developed by the company Brown Boveri.
Gas turbines had a rather low thermal efficiency.
But they were still useful. This was because they
could start up rather quickly. They were therefore
used to provide power at peak loads in the
electricity network. In the 1980’s, natural gas made
its breakthrough as fuel. Since then, gas turbines
have increased in popularity. After world war 2,
the gas turbine developed rapidly.
New high-temperature materials, new cooling
techniques and research in aerodynamics
strongly improved the efficiency of the jet
engine. It therefore soon became the primary
choice for many applications.
Currently, there are several companies
producing gas turbines. The biggest producer of
both industrial gas turbines and jet engines is
General Electric (GE) from the USA. Rolls
Royce and Pratt & Whitney are also important
manufacturers of jet engines.
The cycle that is present is known as the Joule-Brayton
cycle. This cycle consists of four important points.
We start at position 1where the gas has passed through
the inlet, after that the gas then passes through the
compressor. We assume that the compression is
performed isentropically. So, s1 = s2. The gas is then
heated in the combustor. (Point 3.) This is done
isobarically (at constant pressure). So, p2 = p3. Finally,
the gas is expanded in the turbine. (Point 4.) This is
again done isentropic ally. So, s3 = s4.
The whole process is visualized in the temperature-
entropy diagram as shown above. The cycle consists of
an isentropic compression of the gas from state 1 to
state 2; a constant pressure heat addition to state 3; an
isentropic expansion to state 4, in which work is done;
and an isobaric closure of the cycle back to state 1.
Above Figure shows, a compressor is connected to a
turbine by a rotating shaft. The shaft transmits the
power necessary to drive the compressor and delivers
the balance to a power-utilizing load, such as an electrical
When examining the gas turbine cycle, we do
make a few assumptions. We assume that the
working fluid is a perfect gas with constant
specific heats cp and cv. Also, the specific heat
ratio k (sometimes also denoted by ) is constant.
We also assume that the kinetic/potential energy
of the working fluid does not vary along the gas
turbine. Finally, pressure losses, mechanical
losses and other kinds of losses are ignored.
The gas turbine can be classified into two
categories, i.e. impulse gas turbine and reaction
gas turbine. If the entire pressure drop of the
turbine occurs across the fixed blades, the
design is impulse type, while if the drop is taken
place in the moving blades, the fixed blades
serving only as deflectors, the design is called
The advantage of the impulse design is that there is no
pressure force tending to move the wheel in the axial
direction and no special thrust balancing arrangement is
There being no tendency for gas to leak over the tips of
the moving blades. A purely reaction turbine is not
generally used. In a small multi-stage construction the
velocity change in the moving and fixed blades is about
the same, the design being 50% reaction types.
GAS TURBINE POWER PLANT
The simple gas turbine power plant mainly consists of a
gas turbine coupled to a rotary type air compressor and
a combustor or combustion chamber which is placed
between the compressor and turbine in the fuel circuit.
Auxillaries, such as cooling fan, water pumps, etc. and
the generator itself, are also driven by the turbine.
Other auxillaries are starting device, lubrication system,
duct system, etc. A modified plant may have in
addition to the above, an inter-cooler, a regenerator and
a reheater. The arrangement of a simple gas turbine
power plant is shown in Figure in next slide
Schematic Arrangement of a Simple
Gas Turbine Power Plant
The basic construction of a gas turbine employs vanes
or blades mounted on a shaft and enclosed in a casing.
The flow of fluid through turbine in most designs is
axial and tangential to the rotor at a nearly constant or
increasing radius. There are two types of blades used in
all turbines : those that are fixed on the rotor and
move with the shaft and those that are fixed to the
casing and help to guide and accelerate or decelerate
the flow of fluid, being called fixed blades or vanes.
The power of the turbine depends upon the size, shape
and the speed of the blades used.
Multi-staging is employed to increase the power output
of the turbine by placing additional sets of fixed and
moving blades in series. To prevent leakage of gas
along the shaft gas seals or glands are provided where
the shaft emerges from the turbine casing. The
extending lengths of the shaft on the two sides of the
turbine are supported on journal bearings which also
maintain it’s proper alignment.
There are several accessories fitted to the turbine.
These are : a tachometer driven through a gear box, an
over speed governor, a lubricating oil pump and a fuel
regulator. The starting gear is mounted on the shaft at
one end. The tachometer shows the speed of the
machine and also actuates the fuel regulator in case of
speed rises above or fall below the regulated speed, so
that the fuel regulator admits less fuel or more fuel into
the combustor and varies the turbine power according
to demand of load.
The governor back off fuel feed, if the exhaust
temperature from turbine exceeds the safe limit, thermal
switches at the turbine exhaust acting on fuel control to
maintain present maximum temperature. The lubricating
pump supplies oil to bearing under pressure. Other
auxillaries used on the turbine plant include the starting
motor or engine with starting gear, oil coolers, filters and
inlet and exhaust mufflers. The turbine (and with it the
compressors) is driven by the starting motor through a
clutch and set-up gearing. A standby motor driven pump
is also provided for emergency service. A failure of
lubricating pump system results in stopping of the unit
A compressor is a device that is used to supply
compressed air to the combustion chamber.
Compressors are broadly classified as positive
displacement type and rotodynamic type and may
be of single stage or multi-stage design.
In the positive displacement machine, successive
volumes of air are pressurized within a closed space.
These may be of reciprocating type or rotary type. In
reciprocating type machines, air is compressed by a
piston in a cylinder, while in the rotary type, this is
accomplished by positive action of rotating elements.
The roto-dynamic compressors may be of radial
flow, axial flow or mixed flow type. In these
machines, compression takes place by dynamic
action of rotating vanes or impellers which
impart velocity and pressure to the air as it flows
through the compressor. Roto-dynamic type
compressors include the centrifugal, axial and mixed
flow compressors which are all high speed machines
running at as high as 3,000 to 4,000 RPM driven by
turbines. These are designed to have high value of air
discharge capacity at moderate pressure. These types of
compressors are usually employed for gas turbine
A combustor is a device inside which the combustion
of fuel takes place. For an efficient operation of gas
turbine plant, it is necessary to ensure good combustor
performance. A good combustor should achieve
completeness of fuel combustion and the lowest
possible pressure drop in the gas, besides being
compact, reliable and easy to control. Complete
combustion of fuel depends upon three factors, viz.
temperature, time and turbulence. Temperature in the
combustor directly affects combustion and high
temperature is conductive to rapid combustion.
It is a device that generates electricity. It is
coupled to the same shaft of turbine and runs
at same speed to that of the turbine. The
capacity of generators depends on installed
capacity of the plant. The types of generators to
be used depend on the purpose for which
electrical energy is to be produced.
TYPES OF GAS TURBINE
The gas turbine power plants can be classified mainly
into two categories. These are :open cycle gas turbine
power plant and closed cycle gas turbine power plant.
Open Cycle Gas Turbine Power Plant In this type
of plant the atmospheric air is charged into the
combustor through a compressor and the exhaust of
the turbine also discharge to the atmosphere.
Closed Cycle Gas Turbine Power Plant In this type
of power plant, the mass of air is constant or another
suitable gas used as working medium, circulates
through the cycle over and over again.
OPEN CYCLE GAS TURBINE
POWER PLANTAND ITS
The schematic arrangement of a simple open
cycle gas turbine power plant is shown in Figure
in next slide
In the process shown the cycles are :
2-3: Isentropic compression
3-4: Heat addition at constant pressure
4-1: Isentropic expansion
1-2: Heat rejection at constant pressure
The ideal thermal efficiency for the cycle,ç
t, is given by, Heat supplied - Heat rejected/Heat
where, r is the compression ratio=V2/V3and k is
the ratio of specific heat of the gas.
In actual operation the processes along 2-3 and 4-1 are
never isentropic and the degree of irreversibility of these
processes and the mechanical efficiencies of the machine
components greatly reduce the ideal value of thermal
efficiencies of the cycle. If the air entering the combustor
is preheated by the heat of exhaust gases escaping from
the turbine, some heat can be recovered resulting into an
increase in the efficiency of the cycle improved. Such
heating of combustion air is known as regeneration and
the heat exchanger transferring heat from gas to air is
Since most of the output of turbine is consumed by
the compressor, the actual efficiency of the cycle
greatly depends upon an efficient working of the
compressor. To attain higher compression ratios, it
is necessary to use multi-stage compression with
inter-cooling. In actual practice, all these
modifications, viz. regeneration, reheating and inter-
cooling are combined in a simple modified cycle
and a substantial gain in the overall plant efficiency
5 CLOSED CYCLE GAS
TURBINE POWER PLANTAND
In the closed cycle, quantity of air is constant, or
another suitable gas used as working medium,
circulates through the cycle over and over again.
Combustion products do not come in contact with
the working fluid and, thus, remain closed.
A development in the basic gas turbine cycle is the
use of the closed cycle which permits a great deal
of flexibility in the use of fuels. Moreover, working
medium of the plant could be any suitable
substance other than air which would give higher
efficiency. An arrangement of closed gas turbine
cycle is shown in Figure in next slide. In this cycle,
working fluid is compressed through the requisite
pressure ratio in the compressor, and fed into the
heater, where it is heated up to the temperature of
Arrangement of Closed Cycle Gas
The fluid is then expanded in the turbine and the exhaust is
cooled to the original temperature in the pre-cooler. It then
re-enter the compressor to begin the next cycle. Thus, the
same working fluid circulates through the working parts of
the system. The heater burns any suitable fuel and provides
the heat for heating the working fluid. In fact, this
combustor is akin to an ordinary boiler furnace, working at
the atmosphere pressure and discharging the gaseous
products to the atmosphere. There is, thus, a great deal of
flexibility in respect of furnace design and use of fuel,
allowing low cost fuel to be used.
Another advantages in use of closed cycle is the
choice of selecting a convenient pressure range,
once the pressure ratio has been selected. The
volume of the air or the working fluid in the cycle
depends upon the pressure range which, in turn,
affects the sizes of the air heater, compressor,
turbine, etc. In a closed cycle, there is no restriction
to keep the pressure low and this could be kept at
any suitable value say 7.03 kg/cm2(68.9 N/cm )
The pre-cooler in a closed cycle plant is an important
equipment and corresponds to the condenser of a steam
plant. However, unlike the condenser, cooling water in
the pre-cooler could be heated to a fairly high
temperature depending upon temperature of exit gas
from the turbine, and then used elsewhere in the plant.
The design of pre-cooler is commonly of the shell and
tube type, and water is the coolant commonly used. The
air heater of the closed cycle corresponds to the water
heaters of the steam plant, but with one important
difference that it has very small heat storage capacity .
FUEL FOR GAS TURBNE POWER
Natural gas is the ideal fuel for gas turbines, but this is
not available everywhere. Blast furnace and producer gas
may also be used for these plants. However, liquid fuels
of petroleum origin, such as, distillate oils or residual oils
are most commonly used for gas turbine power plants.
The essential qualities of these fuels include proper
volatility, viscosity and calorific value. At the same time,
the fuel should be free from any content of moisture and
suspended impurities that may clog the small passages of
the nozzles and damage valves and plungers of the fuel
However, liquid fuels of petroleum origin, such
distillate oils or residual oils are most commonly used
for gas turbine plants. Residual oils burns with less ease
than distillate oils and the heaters are often used to start
the unit from cold, after which the residual oils are red
into the combustor. Pre-heating of residual oils may be
necessary in cold climates. Use of solid fuel, such as
coal in pulverized form in gas turbines presents several
difficulties, most of which have been only partially
Types of Gas Turbines
Airbreathing jet engines are gas turbines optimized to
produce thrust from the exhaust gases, or from ducted
fans connected to the gas turbines. Jet engines that
produce thrust from the direct impulse of exhaust gases
are often called turbojets, whereas those that generate
thrust with the addition of a ducted fan are often called
turbofans or (rarely) fan-jets.
Gas turbines are also used in many liquid propellant
rockets, the gas turbines are used to power a
turbopump to permit the use of lightweight, low
pressure tanks, which saves considerable dry mass.
A turboprop engine is a type of turbine engine
which drives an external aircraft propeller using
a reduction gear. Turboprop engines are
generally used on small subsonic aircraft, but
some large military and civil aircraft, such as the
Airbus A400M, Lockheed L-188 Electra and
Tupolev Tu-95, have also used turboprop
Aeroderivative gas turbines
Aeroderivatives are also used in electrical power
generation due to their ability to be shut down,
and handle load changes more quickly than
industrial machines. They are also used in the
marine industry to reduce weight. The General
Electric LM2500, General Electric LM6000,
Rolls-Royce RB211 and Rolls-Royce Avon are
common models of this type of machine.
Amateur gas turbines
In its most straightforward form, these are
commercial turbines acquired through military
surplus or scrapyard sales, then operated for
display as part of the hobby of engine collecting.
In its most extreme form, amateurs have even
rebuilt engines beyond professional repair and
then used them to compete for the Land Speed
Auxiliary power units
APUs are small gas turbines designed to supply auxiliary
power to larger, mobile, machines such as an aircraft.
compressed air for air conditioning and ventilation,
compressed air start-up power for larger jet engines,
mechanical (shaft) power to a gearbox to drive shafted
accessories or to start large jet engines, and
electrical, hydraulic and other power-transmission sources
to consuming devices remote from the APU.
Industrial gas turbines for power
Industrial gas turbines differ from aeronautical
designs in that the frames, bearings, and blading are
of heavier construction. They are also much more
closely integrated with the devices they power—
electric generator—and the secondary-energy
equipment that is used to recover residual energy
They range in size from man-portable mobile plants
to enormous, complex systems weighing more than
a hundred tonnes housed in block-sized buildings.
Turboshaft engines are often used to drive
compression trains (for example in gas pumping
stations or natural gas liquefaction plants) and are
used to power almost all modern helicopters. The
primary shaft bears the compressor and the high
speed turbine (often referred to as the Gas
Generator), while a second shaft bears the low-
speed turbine (a power turbine or free-wheeling
turbine on helicopters, especially, because the gas
generator turbine spins separately from the power
Microturbines are one of the most promising
technologies for powering hybrid electric
vehicles. They range from hand held units
producing less than a kilowatt, to commercial
sized systems that produce tens or hundreds of
kilowatts. Basic principles of microturbine are
based on micro combustion.
Microturbine systems have many claimed advantages
over reciprocating engine generators, such as higher
power-to-weight ratio, low emissions and few, or just
one, moving part. Nevertheless reciprocating engines
overall are still cheaper when all factors are considered.
Microturbines also have a further advantage of having
the majority of the waste heat contained in the relatively
high temperature exhaust making it simpler to capture,
whereas the waste heat of reciprocating engines is split
between its exhaust and cooling system.
Most gas turbines are internal combustion
engines but it is also possible to manufacture an
external combustion gas turbine which is,
effectively, a turbine version of a hot air engine.
Those systems are usually indicated as EFGT
(Externally Fired Gas Turbine) or IFGT
(Indirectly Fired Gas Turbine).
External combustion has been used for the
purpose of using pulverized coal or finely
ground biomass (such as sawdust) as a fuel.
Gas turbines in surface vehicles
Gas turbines are often used on ships, locomotives,
helicopters, tanks, and to a lesser extent, on cars, buses, and
Gas turbines offer a high-powered engine in a very small
and light package. However, they are not as responsive and
efficient as small piston engines over the wide range of
RPMs and powers needed in vehicle applications. Turbines
have historically been more expensive to produce than
piston engines, though this is partly because piston engines
have been mass-produced in huge quantities for decades,
while small gas turbine engines are rarities; however,
turbines are mass-produced in the closely related form of
The turbocharger is basically a compact and simple
free shaft radial gas turbine which is driven by the
piston engine's exhaust gas. The centripetal turbine
wheel drives a centrifugal compressor wheel through a
common rotating shaft. This wheel supercharges the
engine air intake to a degree that can be controlled by
means of a wastegate or by dynamically modifying the
turbine housing's geometry (as in a VGT turbocharger).
It mainly serves as a power recovery device which
converts a great deal of otherwise wasted thermal and
kinetic energy into engine boost.
The first serious investigation of using a gas turbine in cars
was in 1946 when two engineers, Robert Kafka and
Robert Engerstein of Carney Associates, a New York
engineering firm, came up with the concept where a
unique compact turbine engine design would provide
power for a rear wheel drive car. The original General
Motors Firebird was a series of concept cars developed for
the 1953, 1956 and 1959 Motorama auto shows, powered
by gas turbines.
Toyota demonstrated several gas turbine powered concept
cars such as the Century gas turbine hybrid in 1975, the
Sports 800 Gas Turbine Hybrid in 1979 and the GTV in
The first race car (in concept only) fitted with a
turbine was in 1955 by a US Air Force group as
a hobby project with a turbine loaned them by
Boeing and a race car owned by Firestone Tire
& Rubber company. The first race car fitted
with a turbine for the goal of actual racing was
by Rover and the BRM Formula One team
joined forces to produce the Rover-BRM, a gas
turbine powered coupe, which entered the 1963
24 Hours of Le Mans, driven by Graham Hill
and Richie Ginther.
The arrival of the Capstone Microturbine has led
to several hybrid bus designs, starting with
HEV-1 by AVS of Chattanooga, Tennessee in
1999, and closely followed by Ebus and ISE
Research in California, and Design Line
Corporation in New Zealand (and later the
The MTT Turbine SUPERBIKE appeared in 2000
(hence the designation of Y2K Superbike by MTT) and
is the first production motorcycle powered by a turbine
engine - specifically, a Rolls-Royce Allison model 250
turboshaft engine, producing about 283 kW (380 bhp).
Speed-tested to 365 km/h or 227 mph (according to
some stories, the testing team ran out of road during
the test), it holds the Guinness World Records for most
powerful production motorcycle and most expensive
production motorcycle, with a price tag of US$185,000.
Several locomotive classes have been powered
by gas turbines, the most recent incarnation being
The German Army's development division, studied a
number of gas turbine engines for use in tanks starting in
mid-1944. The first gas turbine engines used for armoured
fighting vehicle GT 101 was installed in the Panther tank.
A turbine is theoretically more reliable and easier to
maintain than a piston engine, since it has a simpler
construction with fewer moving parts but in practice
turbine parts experience a higher wear rate due to their
higher working speeds. The turbine blades are highly
sensitive to dust and fine sand, so that in desert operations
air filters have to be fitted and changed several times daily.
Like most modern diesel engines used in tanks, gas turbines
are usually multi-fuel engines.
Gas turbines are used in many naval vessels, where
they are valued for their high power-to-weight ratio
and their ships' resulting acceleration and ability to
get underway quickly.
The first gas-turbine-powered naval vessel was the
Royal Navy's Motor Gun Boat MGB 2009 (formerly
MGB 509) converted in 1947.
Advances in technology
Gas turbine technology has steadily advanced since its
inception and continues to evolve. Development is
active in producing both smaller gas turbines and more
powerful and efficient engines. Main drivers are
computer design (specifically CFD and finite element
analysis) and development of advanced materials: Base
materials with superior high temperature strength (e.g.,
single-crystal superalloys that exhibt yield strength
anomaly) or thermal barrier coatings that protect the
structural material underneath from ever higher
temperatures. These advances allowed higher
compression ratios and turbine inlet temperatures,
more efficient combustion and better cooling of engine
AIRCRAFT GAS TURBINE ENGINES
ENGINE TYPES and APPLICATIONS
Most of modern passenger and military aircraft are powered by gas
turbine engines, which are also called jet engines. There are several
types of jet engines, but all jet engines have some parts in common
. Aircraft gas turbine engines can be classified according to (1) the
type of compressor used and (2) power usage produces by the
Compressor types are as follows:
1. Centrifugal flow
2. Axial flow
3. Centrifugal-Axial flow.
Power usage produced are as follows:
1. Turbojet engines
2. Turbofan engines.
3. Turboshaft engines.
Centrifugal Compressor Engines
Centrifugal flow engines compress the air by
accelerating air outward perpendicular to the
longitudinal axis of the machine. Centrifugal
compressor engines are divided into Single-Stage
and Two-Stage compressor. The amount of
thrust is limited because the maximum
Principal Advantages of Centrifugal Compressor
1. Light Weight
3. Low cost.
Axial Flow Compressor Engines
Axial flow compressor engines may incorporate
one , two , or three spools (Spool is defined as a
group of compressor stages rotating at the same
speed) . Two spool engine , the two rotors
operate independently of one another. The
turbine assembly for the low pressure
compressor is the rear turbine unit .
Centrifugal compressor engine were used in
many early jet engines , the efficiency level of
single stage centrifugal compressor is relatively
low . The multi-stage compressors are some
what better , but still do not match with axial
Characteristics and Applications
The turbojet engine : Turbojet engine derives its
thrust by highly accelerating a mass of air , all of
which goes through the engine. Since a high " jet
" velocity is required to obtain an acceptable of
thrust, the turbine of turbo jet is designed to
extract only enough power from the hot gas
stream to drive the compressor and accessories .
All of the propulsive force (100% of thrust )
produced by a jet engine derived from exhaust
The turboprop engine : Turboprop engine derives its
propulsion by the conversion of the majority of gas stream
energy into mechanical power to drive the compressor ,
accessories , and the propeller load. The shaft on which the
turbine is mounted drives the propeller through the
propeller reduction gear system . Approximately 90% of
thrust comes from propeller and about only 10% comes
from exhaust gas.
The turbofan engine : Turbofan engine has a duct enclosed
fan mounted at the front of the engine and driven either
mechanically at the same speed as the compressor , or by an
independent turbine located to the rear of the compressor
drive turbine . The fan air can exit seperately from the
primary engine air , or it can be ducted back to mix with the
primary's air at the rear .
The turboshaft engine : Turboshaft engine derives its
propulsion by the conversion of the majority of gas
stream energy into mechanical power to drive the
compressor , accessories , just like the turboprop engine
but The shaft on which the turbine is mounted drives
something other than an aircraft propeller such as the
rotor of a helicopter through the reduction gearbox .
The engine is called turboshaft.
There are two big advantages:
Gas turbine engines have a great power-to-weight ratio
compared to reciprocating engines. That is, the amount
of power you get out of the engine compared to the
weight of the engine itself is very good.
Gas turbine engines are also smaller than their
reciprocating counterparts of the same power.
The Gas Turbine Plant is simple in Design and
Construction. It has few Reciprocating Parts and is
lighter in weight.
The Gas Turbine is quite useful in the regions where
due to scarcity it is not possible to supply water in
abundance for raising steam.
Other advantages include:
Moves in one direction only, with far less vibration than a
Fewer moving parts than reciprocating engines.
Greater reliability, particularly in applications where
sustained high power output is required
Waste heat is dissipated almost entirely in the exhaust.
This results in a high temperature exhaust stream that is
very usable for boiling water in a combined cycle, or for
Low operating pressures.
High operation speeds.
Low lubricating oil cost and consumption.
Can run on a wide variety of fuels.
Very low toxic emissions of CO and HC due to
excess air, complete combustion and no
"quench" of the flame on cold surfaces
The main disadvantage of gas turbines is that,
compared to a reciprocating engine of the same size,
they are expensive. Because they spin at such high
speeds and because of the high operating temperatures,
designing and manufacturing gas turbines is a tough
problem from both the engineering and materials
Gas turbines also tend to use more fuel when they are
idling and they prefer a constant load rather than a
fluctuating load. That makes gas turbines great for
things like trans-continental jet aircraft and power
So far we have read about:
History of gas turbines
Gas Turbine Power Plant
Open type gas turbine power plant
Closed type gas turbine power plant
Fuel used in gas turbine power plant
Types of gas turbines
Aircraft Gas Turbine Engines