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Jet engine

2007 Schools Wikipedia Selection. Related subjects: Engineering

   A Pratt & Whitney F100 turbofan engine for the F-15 Eagle is tested at
   Robins Air Force Base, Georgia, USA. The tunnel behind the engine
   muffles noise and allows exhaust to escape. The mesh cover at the front
   of the engine (left of photo) prevents foreign objects (including
   people) from being pulled into the engine by the huge volume of air
   rushing into the inlet.
   Enlarge
   A Pratt & Whitney F100 turbofan engine for the F-15 Eagle is tested at
   Robins Air Force Base, Georgia, USA. The tunnel behind the engine
   muffles noise and allows exhaust to escape. The mesh cover at the front
   of the engine (left of photo) prevents foreign objects (including
   people) from being pulled into the engine by the huge volume of air
   rushing into the inlet.

   A jet engine is an engine that discharges a fast moving jet of fluid to
   generate thrust in accordance with Newton's third law of motion. This
   broad definition of jet engines includes turbojets, turbofans, rockets
   and ramjets and water jets, but in common usage, the term generally
   refers to a gas turbine Brayton cycle engine used to produce a jet of
   high speed exhaust gases for special propulsive purposes. Jet engines
   are so familiar to the modern world that gas turbines are sometimes
   mistakenly referred to as a particular application of a jet engine,
   rather than the other way around.

History

   Jet engines can be dated back to the first century AD, when Hero of
   Alexandria invented the aeolipile. This used steam power directed
   through two jet nozzles so as to cause a sphere to spin rapidly on its
   axis. So far as is known, it was never used for supplying mechanical
   power, and the potential practical applications of Hero's invention of
   the jet engine were not recognized. It was simply considered a
   curiosity.

   Jet propulsion only literally and figuratively took off with the
   invention of the rocket by the Chinese in the 11th century. Rocket
   exhaust was initially used in a modest way for fireworks but gradually
   progressed to propel some quite fearsome weaponry; and there the
   technology stalled for hundreds of years.

   The problem was that rockets are simply too inefficient to be useful
   for general aviation. Instead, by the 1930s, the piston engine in its
   many different forms (rotary and static radial, aircooled and
   liquid-cooled inline) was the only type of powerplant available to
   aircraft designers. This was acceptable as long as only low performance
   aircraft were required, and indeed all that were available.

   However, engineers were beginning to realize conceptually that the
   piston engine was self-limiting in terms of the maximum performance
   which could be attained; the limit was essentially one of propeller
   efficiency. This seemed to peak as blade tips approached the speed of
   sound. If engine, and thus aircraft, performance were ever to increase
   beyond such a barrier, a way would have to be found to radically
   improve the design of the piston engine, or a wholly new type of
   powerplant would have to be developed. This was the motivation behind
   the development of the gas turbine engine, commonly called a "jet"
   engine, which would become almost as revolutionary to aviation as the
   Wright brothers' first flight.

   The earliest attempts at jet engines were hybrid designs in which an
   external power source supplied the compression. In this system (called
   a thermojet by Secondo Campini) the air is first compressed by a fan
   driven by a conventional piston engine, then it is mixed with fuel and
   burned for jet thrust. The examples of this type of design were the
   Henri Coandă's Coandă-1910 aircraft, and the much later Campini Caproni
   CC.2, and the Japanese Tsu-11 engine intended to power Ohka kamikaze
   planes towards the end of World War II. None were entirely successful
   and the CC.2 ended up being slower than the same design with a
   traditional engine and propeller combination.
   Jet engine airflow simulation
   Enlarge
   Jet engine airflow simulation

   The key to a practical jet engine was the gas turbine, used to extract
   energy to drive the compressor from the engine itself. The gas turbine
   was not an idea developed in the 1930s: the patent for a stationary
   turbine was granted to John Barber in England in 1791. The first gas
   turbine to successfully run self-sustaining was built in 1903 by
   Norwegian engineer Ægidius Elling. The first patents for jet propulsion
   were issued in 1917. Limitations in design and practical engineering
   and metallurgy prevented such engines reaching manufacture. The main
   problems were safety, reliability, weight and, especially, sustained
   operation.
   The W2/700 engine flew in the Gloster E.28/39, the first British
   aircraft to fly with a turbojet engine, and the Gloster Meteor.
   Enlarge
   The W2/700 engine flew in the Gloster E.28/39, the first British
   aircraft to fly with a turbojet engine, and the Gloster Meteor.

   In 1929, Aircraft apprentice Frank Whittle formally submitted his ideas
   for a turbo-jet to his superiors. On 16 January 1930 in England,
   Whittle submitted his first patent (granted in 1932). The patent showed
   a two-stage axial compressor feeding a single-sided centrifugal
   compressor. Whittle would later concentrate on the simpler centrifugal
   compressor only, for a variety of practical reasons.

   In 1935 Hans von Ohain started work on a similar design in Germany,
   seemingly unaware of Whittle's work.

   Whittle had his first engine running in April 1937. It was
   liquid-fuelled, and included a self-contained fuel pump. Von Ohain's
   engine, as well as being 5 months behind Whittle's, relied on gas
   supplied under external pressure, so was not self-contained. Whittle's
   team experienced near-panic when the engine would not stop, even after
   the fuel was switched off. It turned out that fuel had leaked into the
   engine and accumulated in pools. So the engine would not stop till all
   the leaked fuel had burned off. Whittle unfortunately failed to secure
   proper backing for his project, and so fell behind Von Ohain in the
   race to get a jet engine into the air.

   Ohain approached Ernst Heinkel, one of the larger aircraft
   industrialists of the day, who immediately saw the promise of the
   design. Heinkel had recently purchased the Hirth engine company, and
   Ohain and his master machinist Max Hahn were set up there as a new
   division of the Hirth company. They had their first HeS 1 engine
   running by September 1937. Unlike Whittle's design, Ohain used hydrogen
   as fuel, supplied under external pressure. Their subsequent designs
   culminated in the gasoline-fuelled HeS 3 of 1,100 lbf (5 kN), which was
   fitted to Heinkel's simple and compact He 178 airframe and flown by
   Erich Warsitz in the early morning of August 27, 1939, from Marienehe
   aerodrome, an impressively short time for development. The He 178 was
   the world's first jet plane.

   Meanwhile, Whittle's engine was starting to look useful, and his Power
   Jets Ltd. started receiving Air Ministry money. In 1941 a flyable
   version of the engine called the W.1, capable of 1000 lbf (4 kN) of
   thrust, was fitted to the Gloster E28/39 airframe specially built for
   it, and first flew on May 15, 1941 at RAF Cranwell.
   A picture of an early centrifugal engine (the DH Goblin II) sectioned
   to show its internal components
   Enlarge
   A picture of an early centrifugal engine (the DH Goblin II) sectioned
   to show its internal components

   One problem with both of these early designs, which are called
   centrifugal-flow engines, was that the compressor worked by "throwing"
   (accelerating) air outward from the central intake to the outer
   periphery of the engine, where the air was then compressed by a
   divergent duct setup, converting its velocity into pressure. An
   advantage of this design was that it was already well understood,
   having been implemented in centrifugal superchargers. However, given
   the early technological limitations on the shaft speed of the engine,
   the compressor needed to have a very large diameter to produce the
   power required. A further disadvantage was that the air flow had to be
   "bent" to flow rearwards through the combustion section and to the
   turbine and tailpipe.

   Austrian Anselm Franz of Junkers' engine division (Junkers Motoren or
   Jumo) addressed these problems with the introduction of the axial-flow
   compressor. Essentially, this is a turbine in reverse. Air coming in
   the front of the engine is blown towards the rear of the engine by a
   fan stage (convergent ducts), where it is crushed against a set of
   non-rotating blades called stators (divergent ducts). The process is
   nowhere near as powerful as the centrifugal compressor, so a number of
   these pairs of fans and stators are placed in series to get the needed
   compression. Even with all the added complexity, the resulting engine
   is much smaller in diameter and thus, more aerodynamic. Jumo was
   assigned the next engine number, 4, and the result was the Jumo 004
   engine. After many lesser technical difficulties were solved, mass
   production of this engine started in 1944 as a powerplant for the
   world's first jet-fighter aircraft, the Messerschmitt Me 262 (and later
   the worlds first jet-bomber aircraft, the Arado Ar 234). Because Hitler
   insisted the Me 262 be designated a bomber, this delay caused the
   fighter version to arrive too late to decisively impact Germany's
   position in World War II. Nonetheless, it will be remembered as the
   first use of jet engines in service. Following the end of the war the
   German jet aircraft and jet engines were extensively studied by the
   victorious allies and contributed to work on early Soviet and US jet
   fighters. The legacy of the axial-flow engine is seen in the fact that
   practically all jet engines on fixed wing aircraft have had some
   inspiration from this design.
   A cutaway of the Junkers Jumo 004 engine.
   Enlarge
   A cutaway of the Junkers Jumo 004 engine.

   Centrifugal-flow engines have improved since their introduction. With
   improvements in bearing technology, the shaft speed of the engine was
   increased, greatly reducing the diameter of the centrifugal compressor.
   The short engine length remains an advantage of this design,
   patricularly for use in helicopters. Also, its engine components are
   robust; axial-flow compressors are more liable to foreign object
   damage.

   British engines also were licensed widely in the US (see Tizard
   Mission). Their most famous design, the Nene would also power the
   USSR's jet aircraft after a technology exchange. American designs would
   not come fully into their own until the 1960s.

Types

   There are a large number of different types of jet engines, all of
   which get propulsion from a high speed exhaust jet.
   Type Description Advantages Disadvantages
   Water jet Squirts water out the back of a boat Can run in shallow
   water, powerful, less harmful to wildlife Can be less efficient than a
   propeller, more vulnerable to debris
   Thermojet Most primitive airbreathing jet engine. Essentially a
   supercharged piston engine with a jet exhaust. Heavy, inefficient and
   underpowered
   Turbojet Generic term for simple turbine engine Simplicity of design,
   efficient at supersonic speeds (~M2) Basic design, misses many
   improvements in efficiency and power for subsonic flight, relatively
   noisy.
   Turbofan First stage compressor greatly enlarged to provide bypass
   airflow around engine core Quieter due to greater mass flow and lower
   total exhaust speed, more efficient for a useful range of subsonic
   airspeeds for same reason, cooler exhaust temperature Greater
   complexity (additional ducting, usually multiple shafts), large
   diameter engine, need to contain heavy blades. More subject to FOD and
   ice damage. Top speed is limited due to the potential for shockwaves to
   damage engine. Most common form of jet engine in use today. Used in
   airliners like the Boeing 747 and military jets, where an afterburner
   is often added for supersonic flight.
   Rocket Carries all propellants onboard, emits jet for propulsion Very
   few moving parts, Mach 0 to Mach 25+, efficient at very high speed (>
   Mach 10.0 or so), thrust/weight ratio over 100, no complex air inlet,
   high compression ratio, very high speed ( hypersonic) exhaust, good
   cost/thrust ratio, fairly easy to test, works in a vacuum-indeed works
   best exoatmospheric which is kinder on vehicle structure at high speed.
   Needs lots of propellant- very low specific impulse — typically 100-450
   seconds. Extreme thermal stresses of combustion chamber can make reuse
   harder. Typically requires carrying oxidiser onboard which increases
   risks. Extraordinarily noisy.
   Ramjet Intake air is compressed entirely by speed of oncoming air and
   duct shape (divergent) Very few moving parts, Mach 0.8 to Mach 5+,
   efficient at high speed (> Mach 2.0 or so), lightest of all
   airbreathing jets (thrust/weight ratio up to 30 at optimum speed) Must
   have a high initial speed to function, inefficient at slow speeds due
   to poor compression ratio, difficult to arrange shaft power for
   accessories, usually limited to a small range of speeds, intake flow
   must be slowed to subsonic speeds, noisy, fairly difficult to test,
   finicky to keep lit.
   Turboprop ( Turboshaft similar) Strictly not a jet at all — a gas
   turbine engine is used as powerplant to drive propeller shaft (or Rotor
   in the case of a Helicopter) High efficiency at lower subsonic
   airspeeds(300 knots plus), high shaft power to weight Limited top speed
   (aeroplanes), somewhat noisy, complex transmission
   Propfan/Unducted Fan Turboprop engine drives one or more propellers.
   Similar to a turbofan without the fan cowling. Higher fuel efficiency,
   potentially less noisy than turbofans, could lead to higher-speed
   commercial aircraft, popular in the 1980s during fuel shortages
   Development of propfan engines has been very limited, typically more
   noisy than turbofans, complexity
   Pulsejet Air is compressed and combusted intermittently instead of
   continuously. Some designs use valves. Very simple design, commonly
   used on model aircraft Noisy, inefficient (low compression ratio),
   works poorly on a large scale, valves on valved designs wear out
   quickly
   Pulse detonation engine Similar to a pulsejet, but combustion occurs as
   a detonation instead of a deflagration, may or may not need valves
   Maximum theoretical engine efficiency Extremely noisy, parts subject to
   extreme mechanical fatigue, hard to start detonation, not practical for
   current use
   Air-augmented rocket Essentially a ramjet where intake air is
   compressed and burnt with the exhaust from a rocket Mach 0 to Mach 4.5+
   (can also run exoatmospheric), good efficiency at Mach 2 to 4 Similar
   efficiency to rockets at low speed or exoatmospheric, inlet
   difficulties, a relatively undeveloped and unexplored type, cooling
   difficulties, very noisy.
   Scramjet Similar to a ramjet without a diffuser; airflow through the
   entire engine remains supersonic Few mechanical parts, can operate at
   very high Mach numbers (Mach 8 to 15) with good efficiencies Still in
   development stages, must have a very high initial speed to function
   (Mach >6), cooling difficulties, very poor thrust/weight ratio (~2),
   extreme aerodynamic complexity, airframe difficulties, testing
   difficulties/expense
   Turborocket A turbojet where an additional oxidizer such as oxygen is
   added to the airstream to increase max altitude Very close to existing
   designs, operates in very high altitude, wide range of altitude and
   airspeed Airspeed limited to same range as turbojet engine, carrying
   oxidizer like LOX can be dangerous
   Precooled jets / LACE Intake air is chilled to very low temperatures at
   inlet before passing through a ramjet or turbojet engine Easily tested
   on ground. Very high thrust/weight ratios are possible (~14) together
   with good fuel efficiency over a wide range of airspeeds, mach 0-5.5+;
   this combination of efficiencies may permit launching to orbit, single
   stage, or very rapid intercontinental travel. Exists only at the lab
   prototyping stage. Examples include RB545, SABRE, ATREX

Type comparison

   Comparative suitability for (left to right) turboshaft, low bypass and
   turbojet to fly at 10 km attitude in various speeds. Horizontal axis -
   speed, m/s. Vertical axis carries only logical meaning.
   Enlarge
   Comparative suitability for (left to right) turboshaft, low bypass and
   turbojet to fly at 10 km attitude in various speeds. Horizontal axis -
   speed, m/s. Vertical axis carries only logical meaning.
   Efficiency as a function of speed of different Jet types. Although
   efficiency plummets with speed, greater distances are covered, it turns
   out that efficiency per unit distance (per km or mile) is roughly
   independent of speed for Jet engines as a group; however airframes
   become inefficient at supersonic speeds
   Enlarge
   Efficiency as a function of speed of different Jet types. Although
   efficiency plummets with speed, greater distances are covered, it turns
   out that efficiency per unit distance (per km or mile) is roughly
   independent of speed for Jet engines as a group; however airframes
   become inefficient at supersonic speeds
   Dependence of the energy eficiency (η) from the exhaust speed/airplane
   speed ratio (c/v)
   Enlarge
   Dependence of the energy eficiency (η) from the exhaust speed/airplane
   speed ratio (c/v)

   The motion impulse of the engine is equal to the air mass, multiplied
   by the speed that the engine emits this mass:

          I = m c

   where m is the air mass per second and c is the exhaust speed. In other
   words, the plane will fly faster if the engine emits the air mass with
   a higher speed or if it emits more air per second with the same speed.
   However, when the plane flies with certain velocity v, the air moves
   towards it, creating the opposing ram drag at the air intake:

          m v

   Most types of jet engine have an air intake, which provides the bulk of
   the gas exiting the exhaust. Conventional rocket motors, however, do
   not have an air intake, the oxidizer and fuel both being carried within
   the airframe. Therefore, rocket motors do not have ram drag; the gross
   thrust of the nozzle is the net thrust of the engine. Consequently, the
   thrust characteristics of a rocket motor are completely different from
   that of an air breathing jet engine.

   The air breathing engine is only useful if the velocity of the gas from
   the engine, c, is greater than the airplane velocity, v. The net engine
   thrust is the same as if the gas were emitted with the velocity c-v. So
   the pushing moment is actually equal to

          S = m (c-v)

   The turboprop has a wide rotating fan that takes and accelerates the
   large mass of air but only till the limited speed of any propeller
   driven airplane. When the plane speed exceeds this limit, propellers no
   longer provide any thrust (c-v < 0).

   The turbojets and other similar engines accelerate much smaller mass of
   the air and burned fuel, but they emit it at the much higher speeds
   possible with a de Laval nozzle. This is why they are suitable for
   supersonic and higher speeds.

   From the other side, the energy efficiency is higher when the engine
   pushes as large as possible mass of air at the speed, comparable to the
   airplane velocity. The exact formula, given in the literature, is

          \eta = \frac{2}{1 + \frac{c}{v}}

   The low bypass turbofans have the mixed exhaust of the two air flows,
   running at different speeds (c1 and c2). The pushing moment of such
   engine is

          S = m1 (c1 - v) + m2 (c2 - v)

   where m1 and m2 are the air masses, being blown from the both exhausts.
   Such engines are effective at lower speeds, than the pure jets, but at
   higher speeds than the turboshafts and propellers in general. For
   instance, at the 10 km attitude, turboshafts are most effective at
   about 0.4 mach, low bypass turbofans become more effective at about
   0.75 mach and true jets become more effective as mixed exaust engines
   when the speed approaches 1 mach - the speed of sound.

   Rocket engines are best suited for high speeds and altitudes. At any
   given throttle, the thrust and efficiency of a rocket motor improves
   slightly with increasing altitude (because the back-pressure falls thus
   increasing net thrust at the nozzle exit plane), whereas with a
   turbojet (or turbofan) the falling density of the air entering the
   intake (and the hot gases leaving the nozzle) causes the net thrust to
   decrease with increasing altitude.

Turbojet engines

   A turbojet engine, in its simplest form is simply a gas turbine with a
   nozzle attached
   Enlarge
   A turbojet engine, in its simplest form is simply a gas turbine with a
   nozzle attached

   A turbojet engine is a type of internal combustion engine often used to
   propel aircraft. Air is drawn into the rotating compressor via the
   intake and is compressed, through successive stages, to a higher
   pressure before entering the combustion chamber. Fuel is mixed with the
   compressed air and ignited by flame in the eddy of a flame holder. This
   combustion process significantly raises the temperature of the gas. Hot
   combustion products leaving the combustor expand through the turbine,
   where power is extracted to drive the compressor. Although this
   expansion process reduces both the gas temperature and pressure at exit
   from the turbine, both parameters are usually still well above ambient
   conditions. The gas stream exiting the turbine expands to ambient
   pressure via the propelling nozzle, producing a high velocity jet in
   the exhaust plume. If the jet velocity exceeds the aircraft flight
   velocity, there is a net forward thrust upon the airframe.

   Under normal circumstances, the pumping action of the compressor
   prevents any backflow, thus facilitating the continuous-flow process of
   the engine. Indeed, the entire process is similar to a four-stroke
   cycle, but with induction, compression, ignition, expansion and exhaust
   taking place simultaneously, but in different sections of the engine.
   The efficiency of a jet engine is strongly dependent upon the overall
   pressure ratio (combustor entry pressure/intake delivery pressure) and
   the turbine inlet temperature of the cycle.

   It is also perhaps instructive to compare turbojet engines with
   propeller engines. Turbojet engines take a relatively small mass of air
   and accelerate it by a large amount, whereas a propeller takes a large
   mass of air and accelerates it by a small amount. The high-speed
   exhaust of a turbojet engine makes it efficient at high speeds
   (especially supersonic speeds) and high altitudes. On slower aircraft
   and those required to fly short stages, a gas turbine-powered propeller
   engine, commonly known as a turboprop, is more common and much more
   efficient. Very small aircraft generally use conventional piston
   engines to drive a propeller but small turboprops are getting smaller
   as engineering technology improves.

   The turbojet described above is a single-spool design, in which a
   single shaft connects the turbine to the compressor. Higher overall
   pressure ratio designs often have two concentric shafts, to improve
   compressor stability during engine throttle movements. The outer high
   pressure (HP) shaft connects the HP compressor to the HP turbine. This
   HP Spool, with the combustor, forms the core or gas generator of the
   engine. The inner shaft connects the low pressure (LP) compressor to
   the LP Turbine to create the LP Spool. Both spools are free to operate
   at their optimum shaft speed. (Concorde used this type).

Turbofan engines

   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 less concern in military uses relative to civil uses.
   Multistage fans are normally needed to reach the relatively high fan
   pressure ratio needed for high specific thrust. Although high turbine
   inlet temperatures are often employed, the bypass ratio tends to be
   low, usually significantly less than 2.0.

   An approximate equation for calculating the net thrust of a jet engine,
   be it a turbojet or a mixed turbofan, is:

   F_n = \dot{m}(V_{jfe} - V_a)\,

   where:

   \dot{m} = \, intake mass flow rate

   V_{jfe} =\, fully expanded jet velocity (in the exhaust plume)

   V_a =\, aircraft flight velocity

   While the \dot{m}.V_{jfe}\, term represents the gross thrust of the
   nozzle, the \dot{m}. V_a\, term represents the ram drag of the intake.

Major components

   The major components of a jet engine are similar across the major
   different types of engines, although not all engine types have all
   components. The major parts include:
     * Air intake (Inlet)
       The standard reference frame for a jet engine is the aircraft
       itself. For subsonic aircraft, the air intake to a jet engine
       presents no special difficulties, and consists essentially of an
       opening which is designed to minimise drag, as with any other
       aircraft component. However, the air reaching the compressor of a
       normal jet engine must be travelling below the speed of sound, even
       for supersonic aircraft, to sustain the flow mechanics of the
       compressor and turbine blades. At supersonic flight speeds,
       shockwaves form in the intake system and reduce the recovered
       pressure at inlet to the compressor. So some supersonic intakes use
       devices, such as a cone or ramp, to increase pressure recovery, by
       making more efficient use of the shock wave system.
     * Compressor or Fan
       The compressor is made up of stages. Each stage consists of vanes
       which rotate, and stators which remain stationary. As air is drawn
       deeper through the compressor, its heat and pressure increases.
       Energy is derived from the turbine (see below), passed along the
       shaft.
     * Shaft
       This carries power from the turbine to the compressor, and runs
       most of the length of the engine. There may be as many as three
       concentric shafts, rotating at independent speeds, with as many
       sets of turbines and compressors. Other services, like a bleed of
       cool air, may also run down the shaft.
     * Combustor or Can or Flameholders or Combustion Chamber
       This is a chamber where fuel is continuously burned in the
       compressed air.
     * Turbine
       The turbine acts like a windmill, extracting energy from the hot
       gases leaving the combustor. This energy is used to drive the
       compressor through the shaft, or bypass fans, or props, or even
       (for a gas turbine-powered helicopter) converted entirely to
       rotational energy for use elsewhere. Relatively cool air, bled from
       the compressor, may be used to cool the turbine blades and vanes,
       to prevent them from melting.
     * Afterburner or reheat (chiefly UK)
       (mainly military) Produces extra thrust by burning extra fuel,
       usually inefficiently, to significantly raise Nozzle Entry
       Temperature at the exhaust. Owing to a larger volume flow (i.e.
       lower density) at exit from the afterburner, an increased nozzle
       flow area is required, to maintain satisfactory engine matching,
       when the afterburner is alight.
     * Exhaust or Nozzle
       Hot gases leaving the engine exhaust to atmospheric pressure via a
       nozzle, the objective being to produce a high velocity jet. In most
       cases, the nozzle is convergent and of fixed flow area.
     * Supersonic nozzle
       If the Nozzle Pressure Ratio (Nozzle Entry Pressure/Ambient
       Pressure) is very high, to maximize thrust it may be worthwhile,
       despite the additional weight, to fit a convergent-divergent (de
       Laval) nozzle. As the name suggests, initially this type of nozzle
       is convergent, but beyond the throat (smallest flow area), the flow
       area starts to increase to form the divergent portion. The
       expansion to atmospheric pressure and supersonic gas velocity
       continues downstream of the throat, whereas in a convergent nozzle
       the expansion beyond sonic velocity occurs externally, in the
       exhaust plume. The former process is more efficient than the
       latter.

   The various components named above have constraints on how they are put
   together to generate the most efficiency or performance. The
   performance and efficiency of an engine can never be taken in
   isolation; for example fuel/distance efficiency of a supersonic jet
   engine maximises at about mach 2, whereas the drag for the vehicle
   carrying it is increasing as a square law and has much extra drag in
   the transonic region. The highest fuel efficiency for the overall
   vehicle is thus typically at Mach ~0.85.

   For the engine optimisation for its intended use, important here is air
   intake design, overall size, number of compressor stages (sets of
   blades), fuel type, number of exhaust stages, metallurgy of components,
   amount of bypass air used, where the bypass air is introduced, and many
   other factors. For instance, let us consider design of the air intake.

Air intakes

Subsonic inlets

   Pitot intake operating modes
   Enlarge
   Pitot intake operating modes

   Pitot intakes are the dominant type for subsonic applications. A
   subsonic pitot inlet is little more than a tube with an aerodynamic
   fairing around it.

   At zero airspeed (i.e., rest), air approaches the intake from a
   multitude of directions: from directly ahead, radially, or even from
   behind the plane of the intake lip.

   At low airspeeds, the streamtube approaching the lip is larger in
   cross-section than the lip flow area, whereas at the intake design
   flight Mach number the two flow areas are equal. At high flight speeds
   the streamtube is smaller, with excess air spilling over the lip.

   Beginning around 0.85 Mach, shock waves can occur as the air
   accelerates through the intake throat.

   Careful radiusing of the lip region is required to optimize intake
   pressure recovery (and distortion) throughout the flight envelope.

Supersonic inlets

   Supersonic intakes exploit shock waves to decelerate the airflow to a
   subsonic condition at compressor entry.

   There are basically two forms of shock waves:

   1) Normal shock waves lie perpendicular to the direction of the flow.
   These form sharp fronts and shock the flow to subsonic speeds.
   Microscopically the air molecules smash into the subsonic crowd of
   molecules like alpha rays. Normal shock waves tend to cause a large
   drop in stagnation pressure. Basically, the higher the supersonic entry
   Mach number to a normal shock wave, the lower the subsonic exit Mach
   number and the stronger the shock (i.e. the greater the loss in
   stagnation pressure across the shock wave).

   2) Conical (3-dimensional) and oblique shock waves (2D) are angled
   rearwards, like the bow wave on a ship or boat, and radiate from a flow
   disturbance such as a cone or a ramp. For a given inlet Mach number,
   they are weaker than the equivalent normal shock wave and, although the
   flow slows down, it remains supersonic throughout. Conical and oblique
   shock waves turn the flow, which continues in the new direction, until
   another flow disturbance is encountered downstream.

   Note: Comments made regarding 3 dimensional conical shock waves,
   generally also apply to 2D oblique shock waves.

   A sharp-lipped version of the pitot intake, described above for
   subsonic applications, performs quite well at moderate supersonic
   flight speeds. A detached normal shock wave forms just ahead of the
   intake lip and 'shocks' the flow down to a subsonic velocity. However,
   as flight speed increases, the shock wave becomes stronger, causing a
   larger percentage decrease in stagnation pressure (i.e. poorer pressure
   recovery). An early US supersonic fighter, the F-100 Super Sabre, used
   such an intake.
   An unswept lip generate a shock wave, which is reflected multiple times
   in the inlet. The more reflections before the flow gets subsonic, the
   better pressure recovery
   Enlarge
   An unswept lip generate a shock wave, which is reflected multiple times
   in the inlet. The more reflections before the flow gets subsonic, the
   better pressure recovery

   More advanced supersonic intakes, excluding pitots:

   a) exploit a combination of conical shock wave/s and a normal shock
   wave to improve pressure recovery at high supersonic flight speeds.
   Conical shock wave/s are used to reduce the supersonic Mach number at
   entry to the normal shock wave, thereby reducing the resultant overall
   shock losses.

   b) have a design shock-on-lip flight Mach number, where the
   conical/oblique shock wave/s intercept the cowl lip, thus enabling the
   streamtube capture area to equal the intake lip area. However, below
   the shock-on-lip flight Mach number, the shock wave angle/s are less
   oblique, causing the streamline approaching the lip to be deflected by
   the presence of the cone/ramp. Consequently, the intake capture area is
   less than the intake lip area, which reduces the intake airflow.
   Depending on the airflow characteristics of the engine, it may be
   desirable to lower the ramp angle or move the cone rearwards to refocus
   the shockwaves onto the cowl lip to maximise intake airflow.

   c) are designed to have a normal shock in the ducting downstream of
   intake lip, so that the flow at compressor/fan entry is always
   subsonic. However, if the engine is throttled back, there is a
   reduction in the corrected airflow of the LP compressor/fan, but (at
   supersonic conditions) the corrected airflow at the intake lip remains
   constant, because it is determined by the flight Mach number and intake
   incidence/yaw. This discontinuity is overcome by the normal shock
   moving to a lower cross-sectional area in the ducting, to decrease the
   Mach number at entry to the shockwave. This weakens the shockwave,
   improving the overall intake pressure recovery. So, the absolute
   airflow stays constant, whilst the corrected airflow at compressor
   entry falls (because of a higher entry pressure). Excess intake airflow
   may also be dumped overboard or into the exhaust system, to prevent the
   conical/oblique shock waves being disturbed by the normal shock being
   forced too far forward by engine throttling.

   Many second generation supersonic fighter aircraft featured an inlet
   cone, which was used to form the conical shock wave. This type of inlet
   cone is clearly seen at the very front of the English Electric
   Lightning and MiG-21 aircraft, for example.

   The same approach can be used for air intakes mounted at the side of
   the fuselage, where a half cone serves the same purpose with a
   semicircular air intake, as seen on the F-104 Starfighter and BAC
   TSR-2.

   Some intakes are biconic; that is they feature two conical surfaces:
   the first cone is supplemented by a second, less oblique, conical
   surface, which generates an extra conical shockwave, radiating from the
   junction between the two cones. A biconic intake is usually more
   efficient than the equivalent conical intake, because the entry Mach
   number to the normal shock is reduced by the presence of the second
   conical shock wave.

   A very sophisticated conical intake was featured on the SR-71's Pratt &
   Whitney J58s that could move a conical spike fore and aft within the
   engine nacelle, preventing the shockwave formed on the spike from
   entering the engine and stalling the engine, while keeping it close
   enough to give good compression. Movable cones are uncommon.

   A more sophisticated design than cones is to angle the intake so that
   one of its edges forms a ramp. An oblique shockwave will form at the
   start of the ramp. The Century series of US jets featured several
   variants of this approach, usually with the ramp at the outer vertical
   edge of the intake, which was then angled back inward towards the
   fuselage. Typical examples include the Republic F-105 Thunderchief and
   F-4 Phantom.
   Concorde intake operating modes
   Enlarge
   Concorde intake operating modes

   Later this evolved so that the ramp was at the top horizontal edge
   rather than the outer vertical edge, with a pronounced angle downwards
   and rearwards. This design simplified the construction of intakes and
   allowed use of variable ramps to control airflow into the engine. Most
   designs since the early 1960s now feature this style of intake, for
   example the F-14 Tomcat, Panavia Tornado and Concorde.

   From another point of view, like in a supersonic nozzle the corrected
   (or non-dimensional) flow has to be the same at the intake lip, at the
   intake throat and at the turbine. One of this three can be fixed. For
   inlets the throat is made variable and some air is bypassed around the
   turbine and directly fed into the afterburner. Unlike in a nozzle the
   inlet is either instable or ineffiecient, because a normal shock wave
   in the throat will suddenly move to the lip, thereby increasing the
   pressure at the lip, leading to drag and reducing the pressure
   recovery, leading to turbine surge and the loss of one SR-71.

Compressors

   Compressor stage GE J79
   Enlarge
   Compressor stage GE J79

   Axial compressors rely on spinning blades that have aerofoil sections,
   similar to aeroplane wings. As with aeroplane wings in some conditions
   the blades can stall. If this happens, the airflow around the stalled
   compressor can reverse direction violently. Each design of a compressor
   has an associated operating map of airflow versus rotational speed for
   characteristics peculiar to that type (see compressor map).

   At a given throttle condition, the compressor operates somewhere along
   the steady state running line. Unfortunately, this operating line is
   displaced during transients. Many compressors are fitted with
   anti-stall systems in the form of bleed bands or variable geometry
   stators to decrease the likelihood of surge. Another method is to split
   the compressor into two or more units, operating on separate concentric
   shafts.

   Another design consideration is the average stage loading. This can be
   kept at a sensible level either by increasing the number of compression
   stages (more weight/cost) or the mean blade speed (more blade/disc
   stress).

   Although large flow compressors are usually all-axial, the rear stages
   on smaller units are too small to be robust. Consequently, these stages
   are often replaced by a single centrifugal unit. Very small flow
   compressors often employ two centrifugal compressors, connected in
   series. Although in isolation centrifugal compressors are capable of
   running at quite high pressure ratios (e.g. 10:1), impeller stress
   considerations (i.e. T3, NH implications) limit the pressure ratio that
   can be employed in high overall pressure ratio engine cycles.

   Increasing overall pressure ratio implies raising the high pressure
   compressor exit temperature (i.e. T3). This implies a higher high
   pressure shaft speed, to maintain the datum blade tip Mach number on
   the rear compressor stage. Stress considerations, however, may limit
   the shaft speed increase, causing the original compressor to
   throttle-back aerodynamically to a lower pressure ratio than datum.
   Combustion chamber GE J79
   Enlarge
   Combustion chamber GE J79

Combustors

   Great care must be taken to keep the flame burning in a moderately fast
   moving airstream, at all throttle conditions, as efficiently as
   possible. Since the turbine cannot withstand stoichiometric
   temperatures, resulting from the optimum combustion process, some of
   the compressor air is used to quench the exit temperature of the
   combustor to an acceptable level. Air used for combustion is considered
   to be primary airflow, while excess air used for cooling is called
   secondary airflow. Combustor configurations include can, annular, and
   can-annular.

Turbines

   Turbine Stage GE J79
   Enlarge
   Turbine Stage GE J79

   Because a turbine expands from high to low pressure, there is no such
   thing as turbine surge or stall. The turbine needs fewer stages than
   the compressor, mainly because the higher inlet temperature reduces the
   deltaT/T (and thereby the pressure ratio) of the expansion process. The
   blades have more curvature and the gas stream velocities are higher.

   Designers must, however, prevent the turbine blades and vanes from
   melting in a very high temperature and stress environment. Consequently
   bleed air extracted from the compression system is often used to cool
   the turbine blades/vanes internally. Other solutions are improved
   materials and/or special insulating coatings. The discs must be
   specially shaped to withstand the huge stresses imposed by the rotating
   blades. They take the form of impulse, reaction, or combination
   impulse-reaction shapes. Improved materials help to keep disc weight
   down.

Turbopumps

   Turbopumps are used to raise the fuel pressure above the pressure in
   the combustion chamber so that it can be injected. Turbopumps are very
   commonly used with rockets, but ramjets also have been known to use
   them. The turbopump is usually driven by a gas turbine.

Nozzles

   Afterburner GE J79
   Enlarge
   Afterburner GE J79

   The primary object of a nozzle is to expand the exhaust stream to
   atmospheric pressure, thereby producing a high velocity jet, relative
   to the vehicle. If the fully expanded jet velocity exceeds the flight
   velocity, there will be a forward thrust on the airframe.

   Simple convergent nozzles are used on many jet engines. If the nozzle
   pressure ratio is above the critical value (about 1.8:1) a convergent
   nozzle will choke, resulting in some of the expansion to atmospheric
   pressure taking place downstream of the throat (i.e. smallest flow
   area), in the jet wake. Although much of the gross thrust produced will
   still be from the jet momentum, additional (pressure) thrust will come
   from the imbalance between the throat static pressure and atmospheric
   pressure.

   Many military combat engines incorporate an afterburner (or reheat) in
   the engine exhaust system. When the system is lit, the nozzle throat
   area must be increased, to accommodate the extra exhaust volume flow,
   so that the turbomachinery is unaware that the afterburner is lit. A
   variable throat area is achieved by moving a series of overlapping
   petals, which approximate the circular nozzle cross-section.

   At high nozzle pressure ratios, much of the expansion will take place
   downstream of a convergent nozzle, which is somewhat inefficient.
   Consequently, some jet engines incorporate a convergent-divergent
   nozzle, to allow most of the expansion to take place within the nozzle
   to maximise thrust. However, unlike the con-di nozzle used on a
   conventional rocket motor, when such a device is used on a jet engine
   it has to be a complex variable geometry device, to cope with the wide
   variation in nozzle pressure ratio encountered in flight and engine
   throttling. This further increases the weight and cost of such an
   installation.
   Afterburner nozzle
   Enlarge
   Afterburner nozzle

   The simpler of the two is the ejector nozzle, which creates an
   effective nozzle through a secondary airflow and spring-loaded petals.
   At subsonic speeds, the airflow constricts the exhaust to a convergent
   shape. As the aircraft speeds up, the two nozzles dilate, which allows
   the exhaust to form a convergent-divergent shape, speeding the exhaust
   gasses past Mach 1. More complex engines can actually use a tertiary
   airflow to reduce exit area at very low speeds. Advantages of the
   ejector nozzle are relative simplicity and reliability. Disadvantages
   are average performance (compared to the other nozzle type) and
   relatively high drag due to the secondary airflow. Notable aircraft to
   have utilized this type of nozzle include the SR-71, Concorde, F-111,
   and Saab Viggen

   For higher performance, it is necessary to use an iris nozzle. This
   type uses overlapping "petals" which mechanically adjusts the petals
   with hydraulics. Although more complex than the ejector nozzle, it has
   significantly higher performance and smoother airflow. As such, it is
   employed primarily on high-performance fighters such as the F-14, F-15,
   F-16, though is also used in high-speed bombers such as the B-1B. Some
   modern iris nozzle additionally have the ability to change the angle of
   the thrust.
   Iris vectored thrust nozzle
   Enlarge
   Iris vectored thrust nozzle

   Rocket motors also employ convergent-divergent nozzles, but these are
   usually of fixed geometry, to minimize weight. Because of the much
   higher nozzle pressure ratios experienced, rocket motor con-di nozzles
   have a much greater area ratio (exit/throat) than those fitted to jet
   engines.

   At the other extreme, some high bypass ratio civil turbofans use an
   extremely low area ratio (less than 1.01 area ratio),
   convergent-divergent, nozzle on the bypass (or mixed exhaust) stream,
   to control the fan working line. The nozzle acts as if it has variable
   geometry. At low flight speeds the nozzle is unchoked (less than a Mach
   number of unity), so the exhaust gas speeds up as it approaches the
   throat and then slows down slightly as it reaches the divergent
   section. Consequently, the nozzle exit area controls the fan match and,
   being larger than the throat, pulls the fan working line slightly away
   from surge. At higher flight speeds, the ram rise in the intake
   increases nozzle pressure ratio to the point where the throat becomes
   choked (M=1.0). Under these circumstances, the throat area dictates the
   fan match and being smaller than the exit pushes the fan working line
   slightly towards surge. This is not a problem, since fan surge margin
   is much better at high flight speeds.

Cooling systems

   All jet engines require high temperature gas for good efficiency,
   typically achieved by combusting hydrocarbon or hydrogen fuel.
   Combustion temperatures can be as high as 3500K (5000F), above the
   melting point of most materials.

   Cooling systems are employed to keep the temperature of the solid parts
   below the failure temperature.

Air systems

   A complex air system is built into most turbine based jet engines,
   primarily to cool the turbine blades, vanes and discs.

   Air, bled from the compressor exit, passes around combustor and is
   injected into the rim of the rotating turbine disc. The cooling air
   then passes through complex passages within the turbine blades. After
   removing heat from the blade material, the air (now fairly hot) is
   vented, via cooling holes, into the main gas stream. Cooling air for
   the turbine vanes undergoes a similar process.

   Cooling the leading edge of the blade can be difficult, because the
   pressure of the cooling air just inside the cooling hole may not be
   much different from that of the oncoming gas stream. One solution is to
   incorporate a cover plate on the disc. This acts as a centrifugal
   compressor to pressurize the cooling air before it enters the blade.
   Another solution is to use an ultra-efficient turbine rim seal to
   pressurize the area where the cooling air passes across to the rotating
   disc.

   Seals are used to prevent oil leakage, control air for cooling and
   prevent stray air flows into turbine cavities.

   A series of (e.g. labyrinth) seals allow a small flow of bleed air to
   wash the turbine disc to extract heat and, at the same time, pressurize
   the turbine rim seal, to prevent hot gases entering the inner part of
   the engine. Other types of seals are hydraulic, brush, carbon etc.

   Small quantities of compressor bleed air are also used to cool the
   shaft, turbine shrouds, etc. Some air is also used to keep the
   temperature of the combustion chamber walls below critical. This is
   done using primary and secondary airholes which allow a thin layer of
   air to cover the inner walls of the chamber preventing excessive
   heating.

   Exit temperature is dependent on the turbine upper temperature limit
   depending on the material. Reducing the temperature will also prevent
   thermal fatigue and hence failure. Accesories may also need their own
   cooling systems using air from the compressor or outside air.

   Air from compressor stages is also used for heating of the fan,
   airframe anti-icing and for cabin heat. Which stage is bled from
   depends on the atmospheric conditions at that altitude.

Rocket engines

   Rocket engines have extreme cooling requirements, due to the
   simultaneous combination of both high pressures (typically 10-200 bar)
   and high temperatures (~3500 K) typically found in the combustion
   chamber.

   Rocket engines often use liquid coolant, typically the propellant is
   passed around the hot parts of the engine ( regenerative cooling); but
   other techniques such as radiative cooling or dump cooling can be used.

   In addition, the chamber is normally designed so that the injectors
   provide for cooler gas at the circumference (curtain cooling) or cool
   liquid: film cooling however these techniques reduce performance
   somewhat due to incompletely burnt propellant being ejected, but are
   nevertherless used by many engines.

Fuel system

   Apart from providing fuel to the engine, the fuel system is also used
   to control propeller speeds, compressor airflow and cool lubrication
   oil. Fuel is usually introduced by an atomized spray, the amount of
   which is controlled automatically depending on the rate of airflow.

   So the sequence of events for increasing thrust is, the throtttle opens
   and fuel spray pressure is increased, increasing the amount of fuel
   being burned. This means that exhaust gases are hotter and so are
   ejected at higher acceleration, which means they exert higher forces
   and therefore increase the engine thrust directly. It also increases
   the energy extracted by the turbine which drives the compressor even
   faster and so there is an increase in air flowing into the engine as
   well.

   Obviously, it is the rate of the mass of the airflow that matters since
   it is the change in momentum (mass x velocity) that produces the force.
   However, density varies with altitude and hence inflow of mass will
   also vary with altitude, temperature etc. which means that throttle
   values will vary according to all these parameters without changing
   them manually.

   This is why fuel flow is controlled automatically. Usually there are 2
   systems, one to control the pressure and the other to control the flow.
   The inputs are usually from pressure and temperature probes from the
   intake and at various points through the engine. Also throttle inputs,
   engine speed etc are required. These affect the high pressure fuel
   pump.

Fuel control unit (FCU)

   This element is something like a mechanical computer. It determines the
   output of the fuel pump by a system of valves which can change the
   pressure used to cause the pump stroke, thereby varying the amount of
   flow.

   Take the possibility of increased altitude where there will be reduced
   air intake pressure. In this case, the chamber within the FCU will
   expand which causes the spill valve to bleed more fuel. This causes the
   pump to deliver less fuel until the opposing chamber pressure is
   equivalent to the air pressure and the spill valve goes back to its
   position.

   And when the throttle is opened, it releases i.e. lessens the pressure
   which lets the throttle valve fall. The pressure is transmitted
   (because of a back-pressure valve i.e. no air gaps in fuel flow) which
   closes the FCU spill valves (as they are commonly called) which then
   increases the pressure and causes a higher flow rate.

   The engine speed governor is used to prevent the engine from
   over-speeding. It has the capability of disregarding the FCU control.
   It does this by use of a diaphragm which senses the engine speed in
   terms of the centrifugal pressure caused by the rotating rotor of the
   pump. At a critical value, this diaphragm causes another spill valve to
   open and bleed away the fuel flow.

   There are other ways of controlling fuel flow for example with the
   dash-pot throttle lever. The throttle has a gear which meshes with the
   control valve (like a rack and pinion) causing it to slide along a
   cylinder which has ports at various positions. Moving the throttle and
   hence sliding the valve along the cylinder, opens and closes these
   ports as designed. There are actually 2 valves viz. the throttle and
   the control valve. The control valve is used to control pressure on one
   side of the throttle valve such that it gives the right opposition to
   the throttle control pressure. It does this by controlling the fuel
   outlet from within the cylinder.

   So for example, if the throttle valve is moved up to let more fuel in,
   it will mean that the throttle valve has moved into a position which
   allows more fuel to flow through and on the other side, the required
   pressure ports are opened to keep the pressure balance so that the
   throttle lever stays where it is.

   At initial acceleration, more fuel is required and the unit is adapted
   to allow more fuel to flow by opening other ports at a particular
   throttle position. Changes in pressure of outside air i.e. altitude,
   speed of aircraft etc are sensed by an air capsule.

Fuel pump

   Fuel pumps are used to raise the fuel pressure above the pressure in
   the combustion chamber so that the fuel can be injected. Fuel pumps are
   usually driven by the main shaft, via gearing.

   Turbopumps are very commonly used with liquid-fuelled rockets and rely
   on the expansion of an onboard gas through a turbine.

   Ramjet turbopumps use ram air expanding through a turbine.

Engine starting system

   The fuel system as explained above, is one of the 2 systems required
   for starting the engine. The other is the actual ignition of the
   air/fuel mixture in the chamber. Usually, an auxiliary power unit is
   used to start the engines. It has a starter motor which has a high
   torque transmitted to the compressor unit. When the optimum speed is
   reached, i.e. the flow of gas through the turbine is sufficient, the
   turbines take over. There are a number of different starting methods
   such as electric, hydraulic, pneumatic etc.

   The electric starter works with gears and clutch plate linking the
   motor and the engine. The clutch is used to disengage when optimum
   speed is achieved. This is usually done automatically. The electric
   supply is used to start the motor as well as for ignition. The voltage
   is usually built up slowly as starter gains speed.

   Some military aircraft need to be started quicker than the electric
   method permits and hence they use other methods such as a turbine
   starter. This is an impulse turbine impacted by burning gases from a
   cartridge. It is geared to rotate the engine and also connected to an
   automatic disconnect system. The cartidge is set alight electrically
   and used to turn the turbine.

   Another turbine starter system is almost exactly like a little engine.
   Again the turbine is connected to the engine via gears. However, the
   turbine is turned by burning gases - usually the fuel is
   iso-propyl-nitrate stored in a tank and sprayed into a combustion
   chamber. Again, it is ignited with a spark plug. Everything is
   electrically controlled, such as speed etc.

   Commercial aircraft usually use what is called an auxiliary power unit
   or APU. It is normally a small gas turbine. Thus, one could say that
   using such an APU is using a small jet engine to start a larger one.
   High pressure air from the compressor section of the APU is bled off
   through a system of pipes to the engines where it is directed into the
   starting system. This "bleed air" is directed into a mechanism to start
   the engine turning and begin pulling in air. When the rotating speed of
   the engine is sufficient to pull in enough air to support combustion,
   fuel is introduced and ignited. Once the engine ignites and reaches
   idle speed, the bleed air is shut off.

   The APUs on Boeing or Airbus aircraft such as (respectively) the 737
   and A320 can be seen at the extreme rear of the aircraft. This is the
   typical location for an APU on most commercial airliners although some
   may be within the wing root ( 727) or the aft fuselage ( DC-9/ MD80) as
   examples and some military transports carry their APU's in one of the
   main landing gear pods ( C-141).

   The APUs also provide enough power to keep the cabin lights, pressure
   and other systems on while the engines are off. The valves used to
   control the airflow are usually electrically controlled. They
   automatically close at a pre-determined speed. As part of the starting
   sequence on some engines fuel is combined with the supplied air and
   burned instead of using just air. This usually produces more power per
   unit weight.

   Usually an APU is started by its own electric starter motor which is
   switched off at the proper speed automatically. When the main engine
   starts up and reaches the right conditions, this auxiliary unit is then
   switched off and disengages slowly.

   Hydraulic pumps can also be used to start some engines through gears.
   The pumps are electrically controlled on the ground.

Ignition

   Usually there are 2 igniter plugs in different positions in the
   combustion system. A high voltage spark is used to ignite the gases.
   The voltage is stored up from a low voltage supply provided by the
   starter system. It builds up to the right value and is then released as
   a high energy spark. Depending on various conditions, the igniter
   continues to provide sparks to prevent combustion from failing if the
   flame inside goes out.

   Of course, in the event that the flame does go out, there must be
   provision to relight. There is a limit of altitude and air speed at
   which an engine can obtain a satisfactory relight.

Lubrication system

   A lubrication system serves to ensure lubrication of the bearings and
   to maintain sufficiently cool temperatures, mostly by eliminating
   friction.

   The lubrication system as a whole should be able to prevent foreign
   material from entering the plane, and reaching the bearings, gears, and
   other moving parts. The lubricant must be able to flow easily at
   relatively low temperatures and not disintegrate or break down at very
   high temperatures.

   Usually the lubrication system has subsystems that deal individually
   with the pressure of an engine, scavenging, and a breather.

   The pressure system components are an oil tank and de-aerator, main oil
   pump, main oil filter/filter bypass valve, pressure regulating valve
   (PRV), oil cooler/by pass valve and tubing/jets.
   Usually the flow is from the tank to the pump inlet and PRV, pumped to
   main oil filter or it's bypass valve and oil cooler, then through some
   more filters to jets in the bearings.

   Using the PRV method of control, means that the pressure of the feed
   oil must be below a critical value (usually controlled by other valves
   which can leak out excess oil back to tank if it exceeds the critical
   value). The valve opens at a certain pressure and oil is kept moving at
   a constant rate into the bearing chamber.

   If the engine speed increases, the pressure within the bearing chamber
   also increases, which means the pressure difference between the
   lubricant feed and the chamber reduces which could reduce slow rate of
   oil when it is needed even more. As a result, some PRVs can adjust
   their spring force values using this pressure change in the bearing
   chamber proportionally to keep the lubricant flow constant.

Advanced designs

J-58 combined ramjet/turbojet

   The SR-71's Pratt & Whitney J58 engines were rather unusual. They could
   convert in flight from being largely a turbojet to being largely a
   compressor-assisted ramjet. At high speeds (above Mach 2.4), the engine
   used variable geometry vanes to direct excess air through 6 bypass
   pipes from downstream of the fourth compressor stage into the
   afterburner. 80% of the SR-71's thrust at high speed was generated in
   this way, giving much higher thrust, improving specific impulse by
   10-15%, and permitting continuous operation at Mach 3.2. The name
   coined for this configuration is turbo-ramjet.

Pre-cooled turbojets

   Engines that may need to operate at low hypersonic speeds could
   theoretically have much higher performance if a heat exchanger is used
   to cool the incoming air. The low temperature allows lighter materials
   to be used and combustors to inject more fuel (ordinarily, fuel flow
   must be reduced at high speed to prevent the turbines from melting, but
   doing so greatly reduces thrust- precooling the air avoids this.)

   This idea leads to plausible designs like SABRE, that might permit
   single-stage-to-orbit, and ATREX, that might permit jet engines to be
   used as boosters for space vehicles.

Nuclear-powered ramjet

   Project Pluto was a nuclear-powered ramjet, intended for use in a
   cruise missile. Rather than combusting fuel as in regular jet engines,
   air was heated using a high-temperature, unshielded nuclear reactor.
   This raised the specific impulse of the engine by stupendous amounts,
   and the ramjet was predicted to be able to fly for months at supersonic
   speeds (Mach 3 at tree-top height). However, there was no obvious way
   to stop it once it had taken off, which is a great disadvantage.
   Unfortunately, because the reactor was unshielded, it was dangerous to
   be in or around the flight path of the vehicle (although the exhaust
   itself wasn't radioactive).

Scramjets

   Main article: scramjet

   Scramjets are an evolution of the ramjet that are able to operate at
   much higher speeds than ramjets (or any other kind of airbreathing
   engine) are capable of reaching. They share a similar structure with
   ramjets, being a specially-shaped tube that compresses air with no
   moving parts through ram-air compression. Scramjets, however, operate
   with supersonic airflow through the entire engine. Thus, scramjets do
   not have the diffuser required by ramjets to slow the incoming airflow
   to subsonic speeds.

   Scramjets start working at speeds of at least Mach 4, and have a
   theoretical maximum speed of Mach 17. Due to aerodynamic heating at
   these high speeds, scramjets are expected to be very heavy. Cooling, as
   a result, poses a challenge to engineers.

Trivia

     * A Scrapheap Challenge team once made a big truck's turbocharger
       into a crude but working turbojet engine.
     * The J-58 engines were believed to be capable of about Mach 4.5, but
       the SR-71 airframe they were attached to got too hot to exceed Mach
       3.2.
     * Jet-engined aircraft appear to be cooling the earth down slightly (
       global dimming) in the short term, but heating it up in the long
       term (global warming).
     * In the severe winter of 1946-1947 in Britain, there were instances
       of jet engines (blowing forwards) mounted on railway trucks being
       used for snow clearance.

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