Jet engine


A jet engine is a type of engine that uses a "jet" of air for thrust.

The basis of most jet engines is the same. Air is drawn in at the front and
compressed. Fuel is then added and the resulting mixture is combusted. The
combustion greatly increases the volume of the gases which are then
exhausted out of the rear of the engine. The efficiency of the process, like
any heat engine, is defined by the ratio of the compressed air's volume to
the exhaust volume.

The advantage of the jet engine is its efficiency at high speeds (especially
supersonic) and high altitudes. On slower aircraft, a propeller (powered by
a gas turbine), commonly known as a turboprop is more common.

History

The earliest attempts at jet engines were hybrid designs, where the
compression was supplied by an external power source. In this system (called
a thermojet by Secondo Campini) the air is compressed by a fan driven by a
conventional gasoline engine, mixed with fuel, and then burned for jet
thrust. Three known examples of this sort of design were the Henri Coanda's
Coanda-1910 aircraft, 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 a traditional design with the same engine.

The key to the useful jet engine was the gas turbine, used to extract energy
to drive the compressor from the engine itself. Work on such a
"self-contained" design started in England in 1930 when Frank Whittle
submitted patents for such an engine (granted in 1932) using a single
turbine stage in the exhaust to drive a centrifugal compressor. In 1935 Hans
von Ohain started work on a similar design in Germany, seemingly unaware of
Whittle's work.

Whittle had significant problems in getting anyone to fund research into the
design, and the Air Ministry largely ignored it while they concentrated on
more pressing issues. Using private funds he was able to get a test engine
running in 1937, but this was very large and unsuitable for use in an
aicraft. By 1939 work had progressed to the point where the engine was
starting to look useful, and Whittle's Power Jets Ltd. started receiving Air
Ministry money. In 1941 a flyable version of the engine called the W.1,
developing 1000 lbs of thrust, was fitted to the Gloster E28/39 airframe,
and flew in May 1941.

Ohain had fewer problems, which is notable considering the highly political
nature of the German aircraft industry at the time. He 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 1937. Unlike Whittle's design, Ohain used hydrogen
as a fuel, which he credits for the early success. Their follow-on designs
culminated in the HeS-3 of 1,100lbs, which was fitted to Heinkel's simple He
178 airframe and flew in August 1939, an impressively short time for
development.

One problem with both of these early designs was that the compressor works
by "throwing" air outward from the intake to the sides of the engine, where
it is compressed by being "crushed" up against the side. This leads to a
very large cross section for the engine, as well as having the air flowing
the wrong way after compression - it has to be collected up and "bent" to
flow to the rear of the engine where the turbine is located.

Anselm Franz of Junkers' engine division (Jumo for Junkers Motoren)
addressed this problem 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 to the rear of the engine by a fan, where it is crushed
against a set of non-rotating blades called stators. 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. Jumo
was assigned the next engine number, 4, and the result was the Jumo 004
engine. This would be the first jet engine to see service, when it powered
the Me 262 in 1944.

By the end of the war the British designs were generally much better than
their German counterparts. Their main advantage was Britain's long history
of working with high-heat metals. Their engines were licensed widely in the
US, whose own designs wouldn't come fully into their own until the 1960s.
Their most famous design, the Nene, would also power the USSR's jet aircraft
after a particularly stupid technology exchange.

Types

There are a number of different types of jet engines:

Turbojet

Whittle's and von Ohain's designs are now classified as turbojets, mostly to
distinguish them from some of the types outlined below. Generally turbojets
are arranged around a central shaft running the length of the engine, with
the compressor and turbine connected to the shaft at either end. In the
middle is a combustion area, typically in the form of a number of individual
"flame cans" which are used to stabilize the combustion.

Like all heat engines, the effeciency of a jet engine is strongly dependant
upon the temperature of the exhaust gas -- higher temperature means more
energy from the fuel. Due to the physics of gasses, where temperature and
pressure are inversely related, a simplification is to compare the pressure
of gas taken in to when it is burned, the so-called compression ratio. Early
jet engines had compression ratios as low as 5 to 1, compared to a normal
otto cycle engine at anywhere from 6 to 1, to 9 to 1. The limiting factor is
the temperature at the front of the turbine; increasing the compression
ratio means that there is considerably more fuel/air mixture (the charge)
burning in the flame cans, and a higher temperature. This is primarily a
problem when taking off; as the aircraft climbs the ambient pressure drops
and the compressor can be run at higher ratios.

German engines had serious problems in this regard. Their early engines
averaged only 10 hours of operation before failing--often with chunks of
metal flying out the back of the engine when the turbine overheated. British
engines tended to fair much better due to better metals. For a time some US
jet engines included the ability to inject water onto the engine to cool the
exhaust in these cases. This was particularly notable because of the huge
amounts of smoke that would pour out of the engine when it was turned on
(typically for takeoff).

Today this problem is no longer a concern. Better materials have increased
the critical temperature, and automatic throttle controls have made it
basically impossible to overheat the engine. However the real solution was
to bleed off some of the air from the compressor, run it down the shaft, and
blow it through the middle of hollow turbine blades. This made the blades
quite expensive to build, which is why jet engines never became as universal
as it was first believed. However the quality of these bleed systems has
continued to improve to the point where the latest Rolls-Royce Trent designs
operate at a compression ratio of 44:1, considerably better than piston
engines.

The compressor uses up about 60 to 65% of all of the power generated by a
jet engine. This explains why they aren't used in cars: you would be burning
the fuel needed for a race while sitting still at a red light. Every bit of
efficiency in running the compressor is needed, so one common design
technique is to use more than one turbine to drive the compressors at
various speeds. Most such designs that use two stages are are known as "two
spool" engines. A few have used three stages.

Given that 60% of the engine's power is being used up for driving the
compressor, one option for better efficiency is to do less compression -
that is, make a smaller engine. This seems self-defeating, but it's not the
case. If you instead use some of that energy not to compress the air, but
simply push it, you can get thrust without compression. This leads to....

Turboprop or turboshaft

By adding another turbine stage to the engine, all of the jet exhaust can be
used for rotary force rather than jet thrust. Coupling this second (or
third) turbine to a propeller makes for a very efficient engine due to the
inherent efficiency of a propeller at low speeds. This is called a
turboprop, and can be found on many smaller commuter planes, cargo planes,
and helicopters (where it is often known as a turboshaft, largely for
academic reasons). Propellers lose efficiency as aircraft speed increases,
which is why they are not used on higher-speed aircraft.

Similar engines are "hidden" in many places. Connected to a generator , they
make excellent light-weight and very reliable power sources. In fact almost
all large aircraft include a much smaller engine to provide power while
parked at the airport, called an APU. You can often see small pop-up doors
near the tail used to feed them air.

Larger versions of the same design are found in many industrial
applications, peak-demand power generation stations, and military ships.

Turbofan

If the propeller is better at low speeds, and the turbojet is better at high
speeds, you might imagine that at some speed range in the middle a mixture
of the two is best. Such an engine is the turbofan (originally termed bypass
turbojet by the inventors at Rolls Royce). Turbofans essentially increase
the size of the first-stage compressor to the point where they act as a
ducted propeller (or fan) blowing air past the "core" of the engine.

In fact the speed range where this type of engine is best turns out to be
everything from about 250mph to 650mph, which is why the turbofan is by far
the most used type of engine for aviation use.

The bypass ratio (the ratio of bypassed air to combustor air) is an
important parameter for turbofans. Early turbofans (and most modern jet
fighter engines) are low-bypass turbofans with bypass ratios less than 1.
However, the "large mouthed" engines you have seen on almost all modern
civilian jet aircraft are high-bypass turbofans which generally have bypass
ratios of 3 or more.

Turbofans (especially high bypass engines) have another nice feature, they
are fairly quiet. The noise of a jet engine is strongly related to the
temperature of the air coming out the back. In the turbofan this hot air is
mixed with the cold air bypassing the engine, so the result is a much lower
temperature. You might think that jet aircraft are actually quite noisy, but
if you stop to consider that the engines are delivering several tens of
thousands of horsepower, you can see that a conventional engine of the same
power would be much louder.

Propfan

The reason propeller engines lose efficiency at high speed is the same
reason that airplanes find it difficult to fly at supersonic speeds: an
effect known as wave drag significantly increases drag just below the speed
of sound, and led to the concept of the sound barrier.

In the case of a propeller this effect can happen any time the prop is spun
fast enough that the tips of the prop start travelling near the speed of
sound, even if the plane is sitting still. This can be controlled to a large
degree by adding more blades to the prop, using up more power at a lower
speed. This is why most WWII fighters started with two-blade props and were
using five-blade designs by the end of the war as their engines increased in
power, they couldn't just spin the prop faster. However this solution does
not help as the plane itself accelerates; at some point the forward speed of
the plane combined with the rotational speed of the propeller will once
again result in wave drag problems.

The solution to decreasing wave drag was discovered by German researchers in
WWII: it was to sweep the wing backwards at a strong angle. Today almost all
aircraft designed to fly much above 450km use a swept-wing. In the 1970s
NASA started researching propellers with similar sweep. Since the inside of
the prop is turning slower than the outside, the blade became progressively
more swept toward the outside, leading to a curved shape.

Although in reality such designs remained turboprops, the name propfan was
picked to make them sound more interesting. However the ducting of the
normal turbofan has the side effect of containing the sonic boom of the fan
inside the engine where it is largely muted. Such is not the case on a
propfan. Propfans were at one time thought to be the next logical step in
engine development for subsonic aircraft, but their very high noise levels
made them unattractive, and work on them has since stopped.

Propfans are also known as ultra high by-pass (UHB) engines.

Ramjet

At the other end of the scale from the increasing complexity of the fans is
the ramjet. When air enters a jet engine its speed decreases and its
pressure increases, called the ram compression effect. At high speeds this
process can be fairly effective, and can compress enough oxygen to
efficiently burn the fuel for the engine all on its own. Typically the speed
needed to make this process work effectively is above 600mph, and doesn't
outperform traditional designs until supersonic.

Ramjets are built to utilize this compression effect through a careful inlet
design. Beyond that the engine is largely nothing more than a well-designed
tube. A ramjet thus contains no (major) moving parts and is particularly
useful in cases where you need small and simple engine for high speed use.
On the downside they need to be flying at high speed to start with, making
them less than useful for general tasks. As you might expect they have found
use almost exclusively in missiles, where they are boosted to operating
speeds by a rocket motor, or by being attached to another aircraft
(typically a fighter). Today ramjets have been generally replaced by small
turbofans, or rockets.

See also: Ram accelerator

Pulsejet

The pulsejet was invented in the first half of the 20th century and was the
power-plant that propelled the world's first cruise missile, the German V1.

Like most jet engines, the pulsejet is very simple in design -- consisting
primarily of a long tube into which air enters and is mixed with fuel to
create a combustible (stoichiometric) mixture. Where the pulsejet differs
from other engines such as the Turbojet or Ramjet is that the combustion
inside the engine is not continuous but occurs in the form of repeated
explosions, hence the name "pulsejet".

There are two basic types of pulsejets. The first is known as a valved or
traditional pulsejet and it has a set of one-way valves through which the
incoming air passes. When the air/fuel is ignited, these valves slam shut
which means that the hot gases can only leave through the engine's tailpipe,
thus creating thrust in the opposite direction.

The second type of pulsejet is the valveless. These engines have no valves;
indeed they have no moving parts at all and in that respect they are even
simpler than a ramjet. With these engines, the intake and exhaust pipes are
usually both faced in the same direction. This often necessitates bending
the engine in half (the Lockwood design is made this way) or placing a 180
degree bend in the intake tube. This is necessary because when the air/fuel
inside the engine ignites, hot gases will rush out both the intake tube and
the exhaust tube, there being no valves to stop them. If both tubes weren't
facing in the same direction, little or no thrust would be generated because
the reactions from the intake and exhaust tubes would cancel each other out.

The advantage of the valveless pulsejet is simple and obvious, there are no
moving parts to wear out so they are far more reliable and a lot simpler to build.

However, despite this advantage, pulsejets are seldom considered to be
practical power plant due to their high fuel consumption, noise, and
significant vibration levels. Today, they survive as a powerplant for model aeroplanes.

Scramjet

When the air inside a ramjet exceeds the speed of sound (meaning an aircraft
speed of around Mach 5+) combustion fails to occur properly. This is
overcome in a scramjet (supersonic combusting ramjet): the inlet is much
wider (typically the entire underside of the craft) so the compression is
less and the air remains at supersonic speeds. But conventional fuels are
unusable at these speeds, so reactive chemicals or gases are used and the
design of the jet is much more complex. Like a ramjet the scramjet must
already be moving extremely fast before it will start working, but
theoretically, speeds in excess of Mach 20 are possible.

turbo-rocket

Rocket engines need to carry both their fuel and air, which makes them carry
around much more weight than a jet for the same amount of fuel burned. The
turborocket is an attempt to reduce the amount of air (or to be exact,
oxidizer) that needs to be carried by extracting some from the air the
rocket flies through. Typical designs use a compressor similar to that of a
traditional jet engine, but mix that along with additional oxidizer from the
tanks. The compressor is turned off when reaching altitudes where there is
no longer enough air to make this practical. Note that there are several
other systems for extracting oxider from the air as well, designs known as LACE.