| Jet Engine |
Types
There are a large number of different types of jet engines, all of which
achieve propulsion from a high speed exhaust jet.
| Type |
Description |
Advantages |
Disadvantages |
|
Water jet |
For propelling boats; squirts water out the back through a nozzle |
Can run in shallow water, high acceleration, no risk of engine
overload (unlike propellers), less noise and vibration, highly
manoeuvrable at all boat speeds, high speed efficiency, less vulnerable
to damage from debris, very reliable, more load flexibility, less
harmful to wildlife |
Can be less efficient than a propeller at low speed, more expensive,
higher weight in boat due to entrained water, will not perform well if
boat is heavier than the jet is sized for |
|
Motorjet |
Most primitive airbreathing jet engine. Essentially a
supercharged piston engine with a jet exhaust. |
Higher exhaust velocity than a propeller, offering better thrust at
high speed |
Heavy, inefficient and underpowered |
|
Turbojet |
Generic term for simple turbine engine |
Simplicity of design, efficient at supersonic speeds (~M2) |
A basic design, misses many improvements in efficiency and power for
subsonic flight, relatively noisy. |
| Low-bypass Turbofan |
One- or two-stage fan added in front bypasses a proportion of the
air through a bypass chamber surrounding the core. Compared with its
turbojet ancestor, this allows for more efficient operation with
somewhat less noise. This is the engine of high-speed military aircraft,
some smaller private jets, and older civilian airliners such as the
Boeing 707, the
McDonnell Douglas DC-8, and their derivatives. |
As with the turbojet, the design is aerodynamic, with only a modest
increase in diameter over the turbojet required to accommodate the
bypass fan and chamber. It is capable of supersonic speeds with minimal
thrust drop-off at high speeds and altitudes yet still more efficient
than the turbojet at subsonic operation. |
Noisier and less efficient than high-bypass turbofan, with less
static (Mach 0) thrust. Added complexity to accommodate dual shaft
designs. More inefficient than a turbojet around M2 due to higher
cross-sectional area. |
| High-bypass Turbofan |
First stage compressor drastically enlarged to provide bypass
airflow around engine core, and it provides significant amounts of
thrust. Compared to the low-bypass turbofan and no-bypass turbojet, the
high-bypass turbfan works on the principle of moving a great deal of air
somewhat faster, rather than a small amount extremely fast. This
translates into less noise. Most common form of jet engine in civilian
use today- used in airliners like the Boeing 747, most 737s, and all
Airbus aircraft. |
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.
High bypass variants exhibit good fuel economy. |
Greater complexity (additional ducting, usually multiple shafts) and
the need to contain heavy blades. Fan diameter can be extremely large,
especially in high bypass turbofans such as the
GE90. More subject to
FOD and ice damage. Top speed is limited due to the potential for
shockwaves to damage engine. Thrust lapse at higher speeds, which
necessitates huge diameters and introduces additional drag. |
| Rocket |
Carries all propellants and oxidants on-board, 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, fairly small surface area to keep cool, and no
turbine in hot exhaust stream. |
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 on-board 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 air-breathing jets (thrust/weight
ratio up to 30 at optimum speed), cooling much easier than turbojets as
no turbine blades to cool. |
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, thrust/weight ratio is similar to ramjets. |
|
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 maximum 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. Much heavier than simple rockets. |
|
Precooled jets/LACE |
Intake air is chilled to very low temperatures at inlet in a heat
exchanger before passing through a ramjet or turbojet engine. Can be
combined with a rocket engine for orbital insertion. |
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, very long distance intercontinental
travel. |
Exists only at the lab prototyping stage. Examples include
RB545,
SABRE,
ATREX. Requires liquid hydrogen fuel which has very low density and
heavily insulated tankage. |
All jet engines are reaction engines that generate thrust by emitting a
jet
of fluid rearwards at relatively high speed. The forces on the inside of the
engine needed to create this jet give a strong thrust on the engine which pushes
the craft forwards.
Jet engines make their jet from propellant from tankage that is attached to
the engine (as in a 'rocket') or from sucking in external fluid (very typically
air) and expelling it at higher speed; or more commonly, a combination of the
two sources.
Thrust
The motion impulse of the engine is equal to the fluid mass multiplied by the
speed at which the engine emits this mass:
- I = m c
where m is the fluid mass per second and c is the exhaust speed. In other
words, a vehicle gets the same thrust if it outputs a lot of exhaust very
slowly, or a little exhaust very quickly.
However, when an vehicle moves with certain velocity v, the fluid moves
towards it, creating an opposing ram drag at the intake:
- m v
Most types of jet engine have an intake, which provides the bulk of the fluid
exiting the exhaust. Conventional rocket motors, however, do not have an intake,
the oxidizer and fuel both being carried within the vehicle. 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 jet engine with an intake is only useful if the velocity of the gas from
the engine, c, is greater than the vehicle velocity, v, as the net engine thrust
is the same as if the gas were emitted with the velocity c-v. So the thrust is
actually equal to
- S = m (c-v)
Energy efficiency
For all jet engines the
propulsive efficiency (essentially
energy efficiency) is highest when the engine emits an exhaust jet at a
speed that is the same as, or nearly the same as, the vehicle velocity. The
exact formula for air-breathing engines as given in the literature,
Noise
Noise is due to shockwaves that form when the exhaust jet interacts with the
external air.
The intensity of the noise is proportional to the thrust as well as
proportional to the fourth power of the jet velocity.
Generally then, the lower speed exhaust jets emitted from engines such as
high bypass turbofans are the quietest, whereas the fastest jets are the
loudest.
Although some variation in jet speed can often be arranged from a jet engine
(such as by throttling back and adjusting the nozzle) it is difficult to vary
the jet speed from an engine over a very wide range. Therefore since engines for
supersonic vehicles such as Concorde, military jets and rockets inherently need
to have supersonic exhaust at top speed, so these vehicles are especially noisy
even at low speeds.
Common types
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 and volume of the air. Hot
combustion products leaving the combustor expand through a gas
turbine,
where power is extracted to drive the compressor. This expansion process reduces
both the gas temperature and pressure but sufficient fuel is burnt so that both
parameters are usually still well above ambient conditions at exit from the
turbine. The gas stream is then expanded to ambient pressure via a propelling
nozzle, producing a high velocity jet as the exhaust. 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. Two spool designs have two
concentric turbine-compressor systems, that spin independently with the turbine
and compressors for each section connected from opposite ends of the engine via
concentric
shafts. This allows for a higher compression ratio as well as improved
compressor stability during engine throttle movements. Three spool designs also
exist.
Turbofan engines
Most modern jet engines are actually turbofans, where the low pressure
compressor acts as a fan, supplying supercharged air not only to 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'.
Turbofans are used for airliners because they give an exhaust speed that is
better matched to subsonic airliner's flight speed, conventional turbojet
engines generate an exhaust that ends up travelling very fast backwards, and
this wastes energy. By emitting the exhaust so that it ends up travelling more
slowly, better fuel consumption is achieved. In addition, the lower exhaust
speed gives much lower noise.
In the 1960s 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 exhaust speed (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:
where:
intake mass
flow rate
fully expanded jet velocity (in the exhaust plume)
aircraft flight velocity
While the
term represents the gross thrust of the nozzle, the
term represents the ram drag of the intake.
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