report

Turbofan engine
Table of content
Topics page no
Certificate
Declaration
Acknowledgement
Preface
Abstract
Chapter 1
Introducation 1
Turbofan 3
Chapter 2
Turbofan engine 4
turbofan configration 7
Chapter 3
Types of turbofan engines 8-11
Chapter 4
Bypass of turbofan engine 12-14
Chapter 5
Working of turbofan engine &
Conclusion 15-21
Chapter 6
Turbofan engine terminology & application & references 22-26

Preface
A turbofan engine has a large fan at the front, which sucks in air. Most of the air
flows around the outside of the engine, making it quieter and giving more thrust at
low speeds. Most of today's airliners are powered by turbofans. In a turbojet all the
air entering the intake passes through the gas generator, which is composed of the
compressor, combustion chamber, and turbine. In a turbofan engine only a portion
of the incoming air goes into the combustion chamber. The remainder passes
through a fan, or low-pressure compressor, and is ejected directly as a "cold" jet or
mixed with the gas-generator exhaust to produce a "hot" jet. The objective of this
sort of bypass system is to increase thrust without increasing fuel consumption. It
achieves this by increasing the total air-mass flow and reducing the velocity within
the same total energy supply.
Chapter 1…………………………introduction about turbofan .
Chapter 2…………………………tell about what is turbo fan engine.
Chapter 3………………………….different types of turbofan engine
Chapter 4………………………….bypass turbo fan engine.
Chapter 5………………………….working of turbofan.
Chapter 6………………………….application of turbofan engine & terminology.
Turbofan engine

Abstract
Jet Propulsion is the thrust imparting forward motion to an object, as a reaction to
the rearward expulsion of a high-velocity liquid or gaseous stream.A simple
example of jet propulsion is the motion of an inflated balloon when the air is
suddenly discharged.
While the opening is held closed, the air pressure within the balloon is equal in all
directions; when the stem is released, the internal pressure is less at the open end
than at the opposite end, causing the balloon to dart forward. Not the pressure of
the escaping air pushing against the outside atmosphere but the difference between
high and low pressures inside the balloon propels it.
An actual jet engine does not operate quite as simply as a balloon, although the
basic principle is the same. More important than pressure imbalance is the
acceleration due to high velocities of the jet leaving the engine. This is achieved by
forces in the engine that enable the gas to flow backward forming the jet. Newton's
second law shows that these forces are proportional to the rate at which the
momentum of the gas is increased.
For a jet engine, this is related to the rate of mass flow multiplied by the rearward-leaving
jet velocity. Newton's third law, which states that every force must have an
equal and opposite reaction, shows that the rearward force is balanced by a forward
reaction, known as thrust.
This thrusting action is similar to the recoil of a gun, which increases as both the
mass of the projectile and its muzzle velocity are increased. High-thrust engines,
therefore, require both large rates of mass flow and high jet-exit velocities, which
can only be achieved by increasing internal engine pressures and by increasing the
volume of the gas by means of combustion.
Turbofan engine

Chapter 1
Introducation
The turbofan or fanjet is a type of airbreathing jet engine that finds wide use
in aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and "fan",
the turbo portion refers to a gas turbine engine which takes mechanical energy
from combustion, and the fan, a ducted fan that uses the mechanical energy from
the gas turbine to accelerate air rearwards. The ratio of the mass-flow of air
bypassing the engine core compared to the mass-flow of air passing through the
core is referred to as the bypass ratio. The engine produces thrust through a
combination of these two portions working in concert; engines that use more jet
thrust relative to fan thrust are known as low bypass tubine, while those that have
considerably more fan thrust than jet are known as high bypass. Most commercial
aviation jet engines in use today are of the high-bypass type, and most modern
military fighter engines are low-bypass. Afterburners are not used on high-bypass
turbofan engines but may be used on either low-bypass turbofan
or turbojet engines.
Turbofan engine

Turbofan engine
fig.1turbofan
The turbofan is a type of aircraft jet engine based around a gas turbine
engine.[1] Turbofans providethrust using a combination of a ducted fan and a jet
exhaust nozzle. Part of the airstream from the ducted fan passes through the core,
providing oxygen to burn fuel to create power.
However, the rest of the air flow bypasses the engine core and mixes with the
faster stream from the core, significantly reducing exhaust noise.
The substantially slower bypass airflow produces thrust more efficiently than the
high-speed air from the core, and this reduces thespecific fuel consumption.
A few designs work slightly differently, having the fan blades as a radial extension
of an aft-mounted low-pressure turbine unit.
Turbofans have a net exhaust speed that is much lower than a turbojet. This makes
them much more efficient at subsonic speeds than turbojets, and somewhat more
efficient at supersonic speeds up to roughly Mach 1.6, but have also been found to
be efficient when used with continuous afterburner at Mach 3 and above.
However, the lower speed also reduces thrust at high speeds.

Turbofan engine
fig. 2 turbofan
Turbofan
Early turbojet engines were very fuel-inefficient, as their overall pressure ratio and
turbine inlet temperature were severely limited by the technology available at the
time. The very first running turbofan was the German (designated as the 109-007
by the RLM) which was operated on its testbed on April 1, 1943. The engine was
abandoned later while the war went on and problems could not be solved. The
British wartime MERTOVICK F3 axial flow jet was given a fan, as the Metrovick
F.3 in 1943, to create the first British turbofan.

Fig3. CFMF6 turbofan
Improved materials, and the introduction of twin compressors such as in
the olympus and the later PRratt & whitney engine, increased the overall pressure
ratio and thus the thermodynamics efficiency of engines, but they also led to a poor
propulsive efficiency, as pure turbojets have a high specific thrust/high velocity
exhaust better suited to supersonic flight.
The original low bypass turbofan engines were designed to improve propulsive
efficiency by reducing the exhaust velocity to a value closer to that of the aircraft.
The RRC, the world's first production turbofan, had a bypass ratio of 0.3, similar to
the modern GENERAL ELECTRIC fighter engine. Civilian turbofan engines of
the 1960s, such as the PRATT & WHITNEY had bypass ratios closer to 1, but
were not dissimilar to their military equivalents.
Chapter 2
Turbofan engine
The turbofan is a derivative of the turbojet engine. In a turbojet, air undergoes four
main phases, a process known as the Brayton cycle:
1. A compression phase in a compressor, approximately adiabatic, where its
pressure and temperature increase;
2. A heating phase in a combuster, where its temperature and volume increase
at approximately constant pressure;
Turbofan engine

3. An expansion phase in a turbine, approximately adiabatic, where mechanical
work is extracted from the air to power the compressor;
4. A further expansion phase in a nozzel where its speed increases as it returns
to inlet pressure.
Thrust is provided by the difference in speed between the outlet and inlet.
It is important to note that in a turbojet the compressor and turbine taken together
form a net-zero mechanical energy system, i.e. all the mechanical shaft power
produced by the turbine is consumed by the compressor. The net output of a
turbojet is not shaft power, instead it is the kinetic energy of the jet exhaust itself.
Although the expansion process in the turbine reduces the gas pressure (and
temperature), there remains considerable thermal energy and pressure in the gases
leaving the turbine. These energy forms are partly converted into kinetic energy by
expansion to ambient pressure through a propelling nozzel, forming a high-velocity
flow which provides reactive propulsion.
After World War II, two-spool (or two-shaft) turbojets were developed to make it
easier to throttle back compression systems with a high design overall pressure
ratio (i.e., combustor inlet pressure/intake delivery pressure). Adopting the two-spool
arrangement enables the compression system to be split in two, with a low
pressure (LP) compressor supercharging a high pressure (HP) compressor. Each
compressor is mounted on a separate (co-axial) shaft, driven by its own turbine
(i.e., the HP turbine and LP turbine). Otherwise, a two-spool turbojet is much like a
single-spool engine.
Modern turbofans evolved from the two-spool axial flow turbofan engine,
essentially by increasing the relative size of the low pressure (LP) compressor to
the point where some (if not most) of the air exiting the unit actually bypasses the
core (or gas-generator) stream passing through the main combustor. Civil-aviation
high-bypass turbofans usually have a single large fan disk, whereas most military-aviation
low-bypass turbofans (e.g. combat and trainer aircraft applications) have
multi-disk compressors as a compromise between greater power-to-weight ratios,
supersonic performance, and the capability of using afterburners, versus the higher
fuel economy of a high-bypass design. Modern military transport turbofan engines
are virtually identical to their civilian counterparts.
Turbofan engine

Fig4. Turbofan engine
Turbo prop engines are gas-turbine engines that deliver almost all of their power to
a shaft to drive a propeller. Turboprops remain popular on very small or slow
aircraft, such as small commuter airliners, for their fuel efficiency at lower speeds,
as well as on medium military
Turbofan configuration
Turbofan engines come in a variety of engine configurations. For a given engine
cycle (i.e., same airflow, bypass ratio, fan pressure ratio, overall pressure ratio and
HP turbine rotor inlet temperature), the choice of turbofan configuration has little
impact upon the design point performance (e.g., net thrust, SFC), as long as overall
component performance is maintained. Off-design performance and stability is,
however, affected by engine configuration.
Turbofan engine

As the design overall pressure ratio of an engine cycle increases, it becomes more
difficult to throttle the compression system, without encountering an instability
known as compressor surge.
1. Single-shaft turbofan
Although far from common, the single-shaft turbofan is probably the simplest
configuration, comprising a fan and high pressure compressor driven by a single
turbine unit, all on the same shaft. The SNECMA M53, which powers Mirage
fighter aircraft, is an example of a single-shaft turbofan. Despite the simplicity of
the turbo machinery configuration, the M53 requires a variable area mixer to
facilitate part-throttle operation.
2. Aft-fan turbofan
One of the earliest turbofans was a derivative of the GENERAL
ELECTRIC turbojet, known as the CJ805, which featured an integrated aft fan/low
pressure (LP) turbine unit located in the turbojet exhaust jetpipe. Hot gas from the
turbojet turbine exhaust expanded through the LP turbine, the fan blades being a
radial extension of the turbine blades. This aft-fan configuration was later
exploited in the General electric GE 36 UDF (propfan) Demonstrator of the early
80s. One of the problems with the aft fan configuration is hot gas leakage from the
LP turbine to the fan.
3. Basic two spool
Many turbofans have the basic two-spool configuration where both the fan and LP
turbine (i.e., LP spool) are mounted on a second (LP) shaft, running concentrically
with the HP spool (i.e., HP compressor driven by HP turbine). The BR710 is
typical of this configuration. At the smaller thrust sizes, instead of all-axial
blading, the HP compressor configuration may be axial-centrifugal (e.g., General
electric CFE 738), double-centrifugal or even diagonal/centrifugal (e.g., Pratt &
Whitney PW 600).
4. Boosted two spool
Higher overall pressure ratios can be achieved by either raising the HP compressor
pressure ratio or adding an intermediate-pressure (IP) Compressor between the fan
and HP compressor, to supercharge or boost the latter unit helping to raise
the overall pressure of the engine cycle to the very high levels employed today
Turbofan engine

(i.e., greater than 40:1, typically). All of the large American turbofans feature an
IP compressor mounted on the LP shaft and driven, like the fan, by the LP turbine,
the mechanical speed of which is dictated by the tip speed and diameter of the fan.
The Rolls-Royce BR715 is a non-American example of this. The high bypass
ratios (i.e., fan duct flow/core flow) used in modern civil turbofans tends to reduce
the relative diameter of the attached IP compressor, causing its mean tip speed to
decrease. Consequently more IPC stages are required to develop the necessary IPC
pressure rise.
5. Three spool
Rolls-Royce chose a three spool configuration for their large civil turbofans where
the intermediate pressure (IP) compressor is mounted on a separate (IP) shaft,
running concentrically with the LP and HP shafts, and is driven by a separate IP
turbine. The first three spool engine was the earlier of 1967.
Chapter 3
Types of turbofan engine
Turbofan engine

1.low bypass turbofan
A high specific thrust/low bypass ratio turbofan normally has a multi-stage fan,
developing a relatively high pressure ratio and, thus, yielding a high (mixed or
cold) exhaust velocity.
The core airflow needs to be large enough to give sufficient core power to drive the
fan. A smaller core flow/higher bypass ratio cycle can be achieved by raising the
(HP) turbine rotor inlet temperature.
Imagine a retrofit situation where a new low bypass ratio, mixed exhaust, turbofan
is replacing an old turbojet, in a particular military application. Say the new engine
is to have the same airflow and net thrust (i.e. same specific thrust) as the one it is
replacing.
A bypass flow can only be introduced if the turbine inlet temperature is allowed to
increase, to compensate for a correspondingly smaller core flow. Improvements in
turbine cooling/material technology would facilitate the use of a higher turbine
inlet temperature, despite increases in cooling air temperature, resulting from a
probable increase in overall pressure ratio.
Efficiently done, the resulting turbofan would probably operate at a higher nozzle
pressure ratio than the turbojet, but with a lower exhaust temperature to retain net
thrust. Since the temperature rise across the whole engine (intake to nozzle) would
be lower, the (dry power) fuel flow would also be reduced, resulting in a
better specific fuel consumption (SFC).
2.High bypass turbofan
The low specific thrust/high bypass ratio turbofans used in today's civil jetliners
(and some military transport aircraft) evolved from the high specific thrust/low
bypass ratio turbofans used in such [production] aircraft back in the 1960s.
Low specific thrust is achieved by replacing the multi-stage fan with a single stage
unit. Unlike some military engines, modern civil turbofans do not have any
stationary inlet guide vanes in front of the fan rotor. The fan is scaled to achieve
the desired net thrust.
The core (or gas generator) of the engine must generate sufficient core power to at
least drive the fan at its design flow and pressure ratio.
Turbofan engine

Through improvements in turbine cooling/material technology, a higher (HP)
turbine rotor inlet temperature can be used, thus facilitating a smaller (and lighter)
core and (potentially) improving the core thermal efficiency.
Reducing the core mass flow tends to increase the load on the LP turbine, so this
unit may require additional stages to reduce the average stage loading and to
maintain LP turbine efficiency. Reducing core flow also increases bypass ratio
(5:1, or more, is now common).
Further improvements in core thermal efficiency can be achieved by raising the
overall pressure ratio of the core. Improved blade aerodynamics reduces the
number of extra compressor stages required.
With multiple compressors (i.e., LPC, IPC, and HPC) dramatic increases in overall
pressure ratio have become possible. Variable geometry . enable high pressure
ratio compressors to work surge-free at all throttle settings.
the first high-bypass turbofan engine was the General electric , designed in mid
1960s to power the lockheed military transport aircraft. The civil General electric
CF56 engine used a derived design.
Other high-bypass turbofans are the Pratt & whitney the three-shaft Rolls
royse and the CFM; also the smaller TF34. More recent large high-bypass
turbofans include the PW 400, the three-shaft ROLL & ROYSE, the General
electric GE 34 , produced jointly by GE and P&W.
For reasons of fuel economy, and also of reduced noise, almost all of today's jet
airliners are powered by high-bypass turbofans. Although modern combat aircraft
tend to use low bypass ratio turbofans, military transport aircraft mainly use high
bypass ratio turbofans for fuel efficiency.
Because of the implied low mean jet velocity, a high bypass ratio/low specific
thrust turbofan has a high thrust lapse rate (with rising flight speed).
Consequently the engine must be over-sized to give sufficient thrust during
climb/cruise at high flight speeds (e.g., Mach 0.83). Because of the high thrust
lapse rate, the static (i.e., Mach 0) thrust is relatively high.
This enables heavily laden, wide body aircraft to accelerate quickly during take-off
and consequently lift-off within a reasonable runway length.
Turbofan engine

Fig .5 high & low bypass turbofan
The turbofans on twin engined airliners are further over-sized to cope with losing
one engine during take-off, which reduces the aircraft's net thrust by 50%. Modern
twin engined airliners normally climb very steeply immediately after take-off. If
one engine is lost, the climb-out is much shallower, but sufficient to clear obstacles
in the flightpath.
The Soviet Union's engine technology was less advanced than the West's and its
first wide-body aircraft, was powered by low-bypass engines. The yy 42 a
medium-range, rear-engine aircraft seating up to 120 passengers introduced in
1980 was the first Soviet aircraft to use high-bypass engines.
3.After burning turbofan
Turbofan engine

Since the 1970s, most jet fighter engines have been low/medium bypass turbofans
with a mixed exhaust, after burner and variable area final nozzle. An afterburner is
a combustor located downstream of the turbine blades and directly upstream of the
nozzle, which burns fuel from afterburner-specific fuel injectors.
When lit, prodigious amounts of fuel are burnt in the afterburner, raising the
temperature of exhaust gases by a significant degree, resulting in a higher exhaust
velocity/engine specific thrust.
The variable geometry nozzle must open to a larger throat area to accommodate the
extra volume flow when the afterburner is lit. Afterburning is often designed to
give a significant thrust boost for take off, transonic acceleration and combat
maneuvers, but is very fuel intensive. Consequently afterburning can only be used
for short portions of a mission.
Unlike the main combustor, where the downstream turbine blades must not be
damaged by high temperatures, an afterburner can operate at the ideal maximum
temperature (i.e., about 2100K/3780Ra/3320F).
At a fixed total applied fuel:air ratio, the total fuel flow for a given fan airflow will
be the same, regardless of the dry specific thrust of the engine. However, a high
specific thrust turbofan will, by definition, have a higher nozzle pressure ratio,
resulting in a higher afterburning net thrust and, therefore, a lower afterburning
specific fuel consumption (SFC).
However, high specific thrust engines have a high dry SFC. The situation is
reversed for a medium specific thrust afterburning turbofan: i.e., poor afterburning
SFC/good dry SFC. The former engine is suitable for a combat aircraft which must
remain in afterburning combat for a fairly long period, but only has to fight fairly
close to the airfield (e.g. cross border skirmishes)
The latter engine is better for an aircraft that has to fly some distance, or loiter for a
long time, before going into combat. However, the pilot can only afford to stay in
afterburning for a short period, before aircraft fuel reserves become dangerously
low. Modern low-bypass military turbofans include the Pratt & whitnye W2,
the Eurojet EW 100, the GE 199, the , and all of which feature a mixed exhaust,
afterburner and variable area propelling nozzle.
Chapter 4
Bypass of turbofan & bypass ratio
Turbofan engine

1) The bypass ratio (BPR) of a turbofan engine is the ratio between the mass
flow rate of air drawn through the fan disk that bypasses the engine core (un-combusted
Turbofan engine
air) to the mass flow rate passing through the engine core that is
involved in combustion to produce mechanical energy. For example, a 10:1
bypass ratio implies that 10 kg of air passes around the combustion chamber
through the ducted fan for every 1 kg of air passing through the combustion
chamber.
2) The ducted fan, rather than combustion gases expanding in a nozzle,
produces the vast majority of the thrust in high-bypass designs.
3) A high bypass ratio provides a lower thrust specific fuel
consumption (grams/sec fuel per unit of thrust in kN using SI units) for
reasons explained below, especially at zero velocity (at takeoff) and at the
cruise speed of most commercial jet aircraft; however, the lower exhaust
velocities of high-bypass designs also figure strongly in lower noise
output, which is a decided advantage over earlier low or zero bypass
designs.
4) High bypass designs are by far the dominant type for all commercial
passenger aircraft and both civilian and military jet transports.
5) Military combat aircraft usually use engines with low bypass ratios to
compromise between fuel economy and the requirements of combat:
high power-to-weight ratios, supersonic performance, and the ability to
use afterburners, all of which are more compatible with low bypass engines.
6) A good example of the differences between a pure jet engine and a low-bypass
turbofan may be seen in the Spey turbofan used in the f-5
Turbojet engines are relatively inefficient as Brayton cycle engines because they
directly convert thermal energy from combusting fuel into kinetic energy in the
form of a high-velocity reaction jet directed through an expansion nozzle instead of
producing mechanical power; therefore, pounds force or kilonewtons—not
horsepower or kilowatts, as in propeller or turboprop engines—measure the power
of a turbojet.
Turbofans, conversely, are very efficient Brayton cycle engines because their gas
turbine convert thermal energy from combustion into mechanical shaft power: the

essential difference between a turbojet and turbofan gas turbine is that the turbine
stage in a turbojet is designed to extract only a fraction of the available thermal
energy in the high pressure and temperature exhaust, producing only enough
mechanical energy to run the compressor stage as a net-zero mechanical energy
system (ignoring very small mechanical outputs to run auxiliary equipment such as
generators) and leaving a relatively high temperature and back pressure exhaust at
the turbine exit for effective reaction propulsion.
The gas turbine on a turbofan has additional turbine disks and stators, which are
sufficient to convert most of the available thermal energy into mechanical work
and leave an exhaust plume of greatly reduced temperature, pressure, and
velocity. The back pressure at the turbine exit of a high bypass turbofan that
maximally converts thermal energy into mechanical energy should be close
to ambient pressure because the increased thrust derived from the ducted fan more
than compensates the low direct jet propulsive efficiency of such an engine.In a
high-bypass turbine engine, the gas turbine uses thermal energy from combustion
to turn a ducted fan that slightly increases the velocity of a large amount of air.
Only the limitations of weight and materials (e.g., the strengths and melting points
of materials in the turbine) reduce the efficiency at which a turbofan gas turbine
converts this thermal energy into mechanical energy, for while the exhaust gases
may still have available energy to be extracted, each additional stator and turbine
disk retrieves progressively less mechanical energy per unit of weight, and
increasing the compression ratio of the system by adding to the compressor stage
to increase overall system efficiency increases temperatures at the turbine
face.Nevertheless, high-bypass engines have a high propulsive efficiency because
even slightly increasing the velocity of a very large volume and consequently mass
of air produces a very large change in momentum and thrust: thrust is the engine's
mass flow (the amount of air flowing through the engine) multiplied by the
difference between the inlet and exhaust velocities in—a linear relationship—
but the kinetic energy of the exhaust is the mass flow multiplied by one-half
the square of the difference in velocities.
The Rolls–Royce Conway turbofan engine, developed in the early 1950s, better
uses this energy. In the Conway, an otherwise normal axial-flow turbojet was
equipped with an oversized first compressor stage (the one closest to the front of
the engine) and centered inside a tubular nacelle(in effect, a ducted
fan arrangement): while the inner portions of the compressor worked "as normal"
and provided air into the core of the engine, the outer portion blew air around the
engine to provide extra thrust. The Conway had a very small bypass ratio of only
0.3, but the improvement in fuel economy was notable; as a result, it and its
Turbofan engine

derivatives like the Spey became some of the most popular jet engines in the
world.
The growth of bypass ratios grew during the 1960s gave jetliners fuel efficiency
that could compete with that of piston-powered planes. Pratt &
Whitney and General Electricdeveloped most of the very large high-bypass
engines in the United States, which for the first time was besting the United
Kingdom in engine design. Rolls-Royce also started the development of the high-bypass
turbofan, and although it caused considerable trouble at the time,
the RB.211 would go on to become one of their most successful products.
Today, almost all jet engines have some bypass. Modern engines in slower aircraft,
such as airliners, have bypass ratios up to 17:1; in higher-speed aircraft, such
as fighters, bypass ratios are much lower, around 1.5; and craft designed for speeds
up to Mach 2 and somewhat above have bypass ratios below 0.5
. Concorde and Tu-144 had no bypass to reduce inlet drag during extended
supersonic cruise at Mach 2.
Turbofan engine
Engine Major applications Bypass ratio
Rolls-Royce/Snecma Olympus 593 (turbojet) Concorde 0:1
SNECMA M88 Rafale 0.30:1
General Electric F404 F/A-18, T-50, F-117, X-29, X-31 0.34:1
Pratt & Whitney F100 F-16, F-15 0.36:1
Eurojet EJ200 Typhoon 0.4:1
Klimov RD-33 MiG-29, Il-102 0.49:1
Saturn AL-31 Su-27, Su-30, J-10 0.59:1

Pratt & Whitney JT8D DC-9, MD-80, 727, 737 0.96:1
Kuznetsov NK-321 Tu-160 1.4:1
Rolls-Royce Tay Gulfstream IV, F70, F100 3.1:1
PowerJet SaM146 SJ 100 4.43:1
Pratt & Whitney PW2000 757, C-17 5.9:1
Progress D-436 Yak-42, Be-200, An-148 6.2:1
General Electric GEnx 747, 787 8.5:1
Rolls-Royce Trent 900 A380 8.7:1
General Electric GE90 777 9:1
Rolls-Royce Trent 1000 787 10:1
Pratt & Whitney PW1000G Bombardier CSeries 12:1
Table1 .Engine different bypass ratios
Turbofan engine

Chapter 5
Working of turbofan engine
Figure 6 is a sketch of a turbofan cross section. In this side view, the
components or stations have been numbered. The air travels through the engine
from left to right, starting at the fan (number 1 on the figure) and progressing
towards the exhaust nozzle (number 5 in the figure). The components and what
occurs at each station are described in detail below.
Turbofan engine
fig.6 turbofan engine
1. Air Intake/Ingestion
The fan is responsible for producing the majority of the thrust generated by a
turbofan engine and is easily visible when looking at the front of the engine, as
seen in Figure 3. The fan is directly connected to the low pressure compressor
(LPC) and the low pressure turbine (LPT) by way of a shaft known as the low
pressure shaft.

Fig7. Turbofan engine viewed from front ,with the fan visible
The fan is station 1 in Figure 6 (above). Ambient air enters the engine by passing
through the fan. Most of the air that passes through the fan travels around the core
of the engine (the center of the engine where the compressor, combustor, turbine,
and exhaust nozzle are located). This air that travels around the core is known as
bypass air (it bypasses the core, as seen in Figure 6). Bypass air is accelerated out
of the back of the engine by the fan thereby creating thrust. It never interacts with
the compressor, combustor, turbine, or exhaust nozzle. The remaining air enters the
core of the engine. This air has been somewhat accelerated by the fan, and
immediately enters the low pressure compressor.
2. Compression
The purpose of compression is to prepare the air for combustion by adding energy
in the form of pressure and heat. The compressor is divided into two portions: the
low pressure compressor, mentioned above, and the high pressure compressor. The
compressor is station 2 in Figure 6. Both compressors function in a similar manner;
however, they interact with different parts of the turbofan engine.
Turbofan engine

The Low Pressure Compressor(LPC)
The LPC is directly connected to the fan and the low pressure turbine (LPT) by the
low pressure shaft. The LPC has rows of spinning blades which push the air further
back into the engine. As the air is being forced rearward, the LPC’s cross sectional
area decreases, causing the volume of air to decrease. From the ideal gas law, this
implies that the air is becoming pressurized and the temperature is increasing.
Immediately after the air passes through the LPC, it enters the high pressure
compressor. The High Pressure Compressor (HPC) The high pressure compressor,
or HPC, is located directly downstream of the LPC and directly upstream of the
combustor. The HPC is connected directly to the high pressure turbine by the high
pressure shaft. Like the LPC, the HPC has rows of spinning blades which force the
air flow rearward into a higher pressure and higher temperature state due to a
decrease in volume. The HPC typically has more rows of blades when compared to
the LPC. Air exiting the HPC has a high temperature and pressure and is now
ready for combustion
The High Pressure Compressor (HPC)
The high pressure compressor, or HPC, is located directly downstream of the LPC
and directly upstream of the combustor. The HPC is connected directly to the high
pressure turbine by the high pressure shaft. Like the LPC, the HPC has rows of
spinning blades which force the air flow rearward into a higher pressure and higher
temperature state due to a decrease in volume. The HPC typically has more rows of
blades when compared to the LPC. Air exiting the HPC has a high temperature and
pressure and is now ready for combustion.
Turbofan engine
fig8.turbofan compressors

2. Combustion
Combustion occurs within the combustor, a stationary chamber within the core
of the engine, which is station 3 in Figure 6. The combustor is directly
downstream of the HPC and directly upstream of the high pressure turbine. The
purpose of the combustor is to add even more energy to the air flow by way of
heat addition. Within the combustor, fuel is injected and mixed with the air.
This fuel-air mixture is then ignited, creating a dramatic increase in temperature
and energizing the flow, propelling it rearward towards the high pressure
turbine.
4. Expansion
Expansion occurs within the high pressure and low pressure turbines. The
turbines are station 4 in Figure 2. Similar in appearance to the compressors, the
turbines have rows of blades which spin (as seen in Figure 4). The purpose of
the turbines is to extract energy from the flow which is then used to spin the
compressors and the fan. The spinning fan draws more air through the core of
the engine which continues the entire process, and it pulls more bypass air
around the engine, generating continuous thrust.
The High Pressure Turbine(HPT)
The high pressure turbine, or HPT, is located directly downstream of the
combustor and directly upstream of the low pressure turbine. The HPT is driven
by the high pressure air that passes through it. The HPT’s cross sectional area is
initially small and then increases downstream. This change in area allows the
air to expand, increasing in volume thereby decreasing in pressure and
temperature. This decrease in pressure andtemperature, along with the energy
used to spin the turbine, correspond to a decrease in the overall energy in Figure
6: Rows of turbine blades the air flow. Air exiting the HPT is significantly
cooler and less pressurized than the air entering; however, it still has viable
Turbofan engine

energy which will be extracted by the low pressure turbine. As mentioned
earlier, the HPT is connected to the HPC by the high pressure shaft. The high
pressure shaft spins the HPC when the HPT is spun by the air passing through
it. This interaction ensures that the HPC will be pulling air into the combustor
continuously, thus feeding the HPT highly energized, combusted air
continuously.
The Low Pressure Turbine (LPT)
The low pressure turbine, or LPT, is located directly downstream of the HPT
and directly upstream of the exhaust nozzle. The LPT functions exactly as the
HPT does; however, it is connected to the LPC and the fan via the low pressure
shaft. Therefore, when the LPT is driven by the air passing through it, is also
drives the LPC and the fan. When the LPC is spinning, it provides the HPC
with air to feed to the combustor. When the fan is spinning, it provides the LPC
with air to feed to the HPC, and it produces thrust by accelerating bypass air out
of the engine. Air exiting the LPT is significantly cooler than when entering,
but it is still hotter than the ambient air. This hot air exits the LPT and
immediately enters the exhaust nozzle.
Fig9.rows of turbine blades
Turbofan engine

Exhaust
The exhaust nozzle is located directly downstream of the LPT and it is the last
component that the air flow touches before exiting the engine. An example of
an exhaust nozzle can be seen in Figure 5. The exhaust nozzle is at station 5 in
Figure 2 and it is stationary, like the combustion chamber. The purpose of the
exhaust nozzle is to propel the core flow out of the engine, providing additional
thrust. This is accomplished by way of its geometry or shape. The nozzle also
helps regulate pressures within the engine to keep the other components
functioning properly and efficiently.
Conclusion
This entire process can be broken down into steps as follows:
1. Air enters the engine through the fan, which is being driven by the low pressure
turbine.
2. Most of the air is accelerated out of the back of the engine, creating thrust.
3. A portion of the air enters the core of the engine where it travels through the low
pressure and high pressure compressors.
4. This compressed air is then mixed with fuel and ignited in the combustor where
heat energy is added.
5. Heat energy and pressure energy cause the air to expand through the turbines,
spinning the high pressure and low pressure turbines which in turn spin the fan and
the compressors.
6. Air exits the turbines and exits the engine through the exhaust nozzle, which
also generates thrust, propelling the engine and vehicle forward. This process is
continuous, with each component doing a specific task to keep the engine as a
whole running. Overall, the idea behind a turbofan engine is relatively simple: add
Turbofan engine

energy to the air flowing through the engine then extract this energy in order to
drive the fan and create thrust. Turbofans are the main powerplant propelling jet
liners across our country and around the world. With some general physics and
chemistry concepts, one can gain an understanding of these machines that have
revolutionized the way we travel and have a greater appreciation for the
technology behind them.
Turbofan engine

Chapter 6
Overall Terminology used in turbofan engine
Afterburner
Extra combustor immediately upstream of final nozzle (also called reheat)
Augmentor
Afterburner on low-bypass turbofan engines.
Average stage loading
Constant × (delta temperature)/[(blade speed) × (blade speed) × (number of
stages)]
Bypass
Airstream that completely bypasses the core compression system, combustor and
turbine system.
Bypass ratio
Bypass airflow /core compression inlet airflow.
Core
Turbomachinery handling the airstream that passes through the combustor.
Core power
Residual shaft power from ideal turbine expansion to ambient pressure after
deducting core compression power.
Core thermal efficiency
Core power/power equivalent of fuel flow.
Dry
Afterburner (if fitted) not lit.
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EGT
Exhaust Gas Temperature.
EPR
Engine Pressure Ratio.
Fan
Turbofan LP compressor.
Fan pressure ratio
Fan outlet total pressure/intake delivery total pressure.
Flex temp
Use of artificially high apparent air temperature to reduce engine wear.
Gas generator
At engine core.
HP compressor
High pressure compressor (also HPC).
HP turbine
High pressure turbine.
Intake ram drag
Penalty associated with jet engines picking up air from the atmosphere
(conventional rocket motors do not have this drag term, because the oxidiser
travels with the vehicle).
IEPR
Integrated engine pressure ratio.
IP compressor
Intermediate pressure compressor (also IPC).
Turbofan engine

IP turbine
Intermediate pressure turbine (also IPT).
LP compressor
Low pressure compressor (also LPC).
LP turbine
Low pressure turbine (also LPT).
Net thrust
Nozzle total gross thrust - intake ram drag (excluding nacelle drag, etc., this is the
basic thrust acting on the airframe).
Overall pressure ratio
Combustor inlet total pressure/intake delivery total pressure.
Overall thermal efficiency
Thermal efficiency * propulsive efficiency.
Propulsive efficiency
Propulsive power/rate of production of propulsive kinetic energy (maximum
propulsive efficiency occurs when jet velocity equals flight velocity, which implies
zero net thrust!).
Specific fuel consumption (SFC)
Total fuel flow/net thrust (proportional to flight velocity/overall thermal
efficiency).
Spooling up
accelerating, marked by a delay.
Turbofan engine

Application
Most modern jet engines are actually turbofans, where the low pressure
compressor acts as a fan, supplying supercharged air to not only the engine core,
but to a bypass duct.
The bypass airflow either passes to a separate 'cold nozzle' or mixes with low
pressure turbine exhaust gases, before expanding through a 'mixed flow nozzle'.
Forty years ago there was little difference between civil and military jet engines,
apart from the use of afterburning in some (supersonic) applications.
Civil turbofans today have a low specific thrust (net thrust divided by airflow) to
keep jet noise to a minimum and to improve fuel efficiency.
Consequently the bypass ratio (bypass flow divided by core flow) is relatively high
(ratios from 4:1 up to 8:1 are common). Only a single fan stage is required,
because a low specific thrust implies a low fan pressure ratio.Today's military
turbofans, however, have a relatively high specific thrust, to maximize the thrust
for a given frontal area, jet noise being of little consequence.
Multi-stage fans are normally required to achieve the relatively high fan pressure
ratio needed for a high specific thrust. Although high turbine inlet temperatures are
frequently employed, the bypass ratio tends to be low (usually significantly less
than 2.0).
Turbofan engine

REFERENCES
1. Marshall Brain. "How Gas Turbine Engines Work". howstuffworks.com.
Retrieved 2010-11-24.
2. "Turbofan Engine". www.grc.nasa.gov. Retrieved 2010-11-24.
3. Neumann, Gerhard(2004) [1984], Herman the German: Just Lucky I Guess,
Bloomington, IN, USA: Authorhouse, ISBN1-4184-7925-X. First published by
Morrow in 1984 as Herman the German: Enemy Alien U.S. Army Master
Sergeant. Republished with a new title in 2004 by Authorhouse, with minor
or no changes., pp. 228–230.
4. Spittle, Peter. "Gas turbine technology"Rolls-Royce plc, 2003. Retrieved: 21
July 2012.
5. "Metrovick F3 Cutaway - Pictures & Photos on FlightGlobal Airspace".
Flightglobal.com. 2007-11-07. Retrieved 2013-04-29. "1954 | 0985 | Flight
Archive". Flightglobal.com. 1954-04-09. Retrieved 2013-04-29.
6. PDF C. Riegler, C. Bichlmaier:, 1st CEAS European Air and Space
Conference, 10–13 September 2007, Berlin, Germany
7. PS-90A turbofan, Aviadvigatel, 2011-01-17
8. Turbofan Engine Family for Regional Jet, Aviadvigatel, 2011-01-17
Turbofan engine

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