AIRCRAFT JET ENGINE
AIRCRAFT JET ENGINE
An aircraft engine is a
propulsion system for an aircraft. The key part of a jet engine is the
exhaust nozzle. This is the part which produces thrust for the jet; the
hot airflow from the engine is accelerated when exiting the nozzle,
creating thrust, which, in conjunction with the pressures acting inside
the engine which are maintained and increased by the constriction of the
nozzle, pushes the aircraft forward.
The
most common jet propulsion engines flown are turbojet, turbofan and
rocket. Other types such as pulsejets, ramjets, scramjets and Pulse
Detonation Engines have also flown.
TURBOPROP
Turboprop
engines are a type of aircraft powerplant that use a gas turbine to
drive a propeller. The gas turbine is designed specifically for this
application, with almost all of its output being used to drive the
propeller. The engine's exhaust gases contain little energy compared to a
jet engine and play a minor role in the propulsion of the aircraft.
The propeller is coupled to the turbine through a reduction gear
that converts the high RPM, low torque output to low RPM, high torque.
The propeller itself is normally a constant speed (variable pitch) type
similar to that used with larger reciprocating aircraft engines.
Turboprop
engines are generally used on small subsonic aircraft, but some
aircraft outfitted with turboprops have cruising speeds in excess of 500
kt (926 km/h, 575 mph). Large military and civil aircraft, such as the
Lockheed L-188 Electra and the Tupolev Tu-95, have also used turboprop
power. The Airbus A400M is powered by four Europrop TP400 engines, which
are the third most powerful turboprop engines ever produced, after the
Kuznetsov NK-12 and Progress D-27.
Turboprops are very
efficient at modest flight speeds (below 450 mph) because the jet
velocity of the propeller (and exhaust) is relatively low. Due to the
high price of turboprop engines, they are mostly used where
high-performance short-takeoff and landing (STOL) capability and
efficiency at modest flight speeds are required. The most common
application of turboprop engines in civilian aviation is in small
commuter aircraft, where their greater reliability than reciprocating
engines offsets their higher initial cost.
TURBOSHAFT
A
turboshaft engine is a form of gas turbine which is optimized to
produce shaft power, rather than jet thrust. In principle, a turboshaft
engine is similar to a turbojet, except the former features additional
turbine expansion to extract heat energy from the exhaust and convert it
into output shaft power.
Turboshaft
engines are commonly used in applications which require a sustained
high power output, high reliability, small size and light weight. These
include helicopters, auxiliary power units, boats and ships, tanks,
hovercraft, and stationary equipment.
The general
layout of a turboshaft is similar to that of a turboprop. The main
difference is that a turboprop is structurally designed to support the
loads created by a rotating propeller, as the propeller is not attached
to anything but the engine itself. In contrast, turboshaft engines
usually drive a transmission which is not structurally attached to the
engine. The transmission is attached to the vehicle structure and
supports the loads created instead of the engine. However, in practice
many of the same engines are built in both turboprop and turboshaft
versions, with only minor differences.
TURBOJET
Turbojets
are the oldest kind of general-purpose jet engines. Turbojets consist
of an air inlet, an air compressor, a combustion chamber, a gas turbine
(that drives the air compressor) and a nozzle. The air is compressed
into the chamber, heated and expanded by the fuel combustion and then
allowed to expand out through the turbine into the nozzle where it is
accelerated to high speed to provide propulsion.
Turbojets
are quite inefficient (if flown below about Mach 2) and very noisy.
Most modern aircraft use turbofans instead for economic reasons.
Turbojets are still very common in medium range cruise missiles, due to
their high exhaust speed, low frontal area and relative simplicity.
TURBOFAN
A
turbofan is a type of aircraft gas turbine engine that provides thrust
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 rather slower bypass airflow
produces thrust more efficiently than the high-speed air from the core,
and this reduces the specific fuel consumption.
A
few designs work slightly differently and have 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.
All of the jet engines used in currently
manufactured commercial jet aircraft are turbofans. They are used
commercially mainly because they are highly efficient and relatively
quiet in operation.
PULSEJET
A
pulse jet engine (or pulsejet) is a very simple type of jet engine in
which combustion occurs in pulses. Pulsejet engines can be made with few
or no moving parts, and are capable of running statically.
Pulsejet
engines are a unique type of jet engine, able to operate statically
with few or no moving parts. There are two main types of pulsejet
engines, both types use resonant combustion and harness the expanding
combustion products to form a pulsating exhaust jet, which produces
thrust intermittently.
RAMJET
A
ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a
form of jet engine using the engine's forward motion to compress
incoming air, without a rotary compressor. Ramjets cannot produce thrust
at zero airspeed and thus cannot move an aircraft from a standstill.
Ramjets
require considerable forward speed to operate well, and as a class work
most efficiently at speeds around Mach 3. This type of jet can operate
up to speeds of at least Mach 6.
Ramjets can be
particularly useful in applications requiring a small and simple engine
for high speed use; such as missiles, while weapon designers are looking
to use ramjet technology in artillery shells to give added range; it is
anticipated that a 120-mm mortar shell, if assisted by a ramjet, could
attain a range of 22 mi (35 km). They have also been used successfully,
though not efficiently, as tip jets on helicopter rotors.
SCRAMJET
A
scramjet (supersonic combustion ramjet) is a variation of a ramjet
distinguished by supersonic combustion. A scramjet, like a ramjet,
essentially consists of a constricted tube through which inlet air is
compressed by the high speed of the vehicle, a combustion chamber where
fuel is combusted, and a nozzle through which the exhaust jet leaves at
higher speed than the inlet air.
Most
jet engines use a fan-style compressor to squeeze air into the engine,
then spray fuel into the compressed air and ignite it to produce thrust
as it exits the engine through an expansion nozzle. A ramjet uses the
speed of the aircraft to compress the air, so very few moving parts are
needed to operate it. In particular there is no high-speed turbine, as
in a turbofan or turbojet engine, that is expensive to produce and can
be a major point of failure.
Most ramjets decelerate
the incoming air to subsonic speeds (relative to the engine) so that
combustion can be more easily sustained. However, higher vehicle speeds
cause higher increases in air pressure and temperature during this
deceleration at the air intake. The properties of known materials impose
effective limits on what pressures and temperatures an engine can
withstand, thus limiting ramjet airspeed to about Mach 5—if the air must
be decelerated inside the engine to below Mach 1. If combustion can be
sustained in supersonic air inside the engine, then the vehicle can fly
at higher speeds while engine inlet air deceleration pressures and
temperatures remain at tolerable levels.
In order to
achieve sufficient internal air pressure and temperature to function, a
ramjet must be accelerated by some other means of propulsion. A
scramjet, designed for higher speeds, requires acceleration to
hypersonic speed before it can become active. A scramjet requires
supersonic airflow through the engine; thus, similar to a ramjet,
scramjets have a minimum functional speed, about Mach 4.5 for current
models.
Projections for the top speed of a scramjet
engine (without additional oxidiser input) vary between Mach 12 and Mach
24 (orbital velocity).
