EXPLAINED : HOW JET ENGINES WORK

[Major Shaitan Singh]

Jet engines revolutionized air travel. They allowed designers to make aircraft which could fly faster than the propeller-driven, petrol engine powered aircraft of the day. Jet engines were first made in the 1930s, but didn’t enter service or large scale production till the 1940s. They are basically air-breathing engines which depend on the input of air to propel the aircraft. During WW2, Germany was the only country which possessed jet powered aircraft. But soon after the war ended, other European nations, Americans and the Russians got hold of this technology and adopted jet propulsion for their aircraft in a big way. Their use spread rapidly and many improvements were made in jet propulsion which made it economical and affordable to be used on civilian aircraft on a large scale.

Presently, almost every aircraft in the world is powered by a turbojet, turbofan or a turboprop engine. These engines have made air travel faster and more economical than ever before. There are several other types of jet engines like ramjet, scramjet etc. In this article, I will give a brief yet thorough explanation about how these jet engines work from an engineering point of view.

BASIC PRINCIPLE
All jet engines work on the same principle, production of thrust to propel the aircraft forward. All jet engines have an air intake through which air enters. This air is burnt in the combustion chamber with fuel and the hot exhaust gas comes out of a nozzle, forming jet thrust. The actual working of these engines involves additional components and stages which will be explained below.

TURBOJET ENGINE

The stages of a turbojet engine. Wikimedia image.
This is one of the oldest types of jet engines in existence and it equipped the earliest jet fighters. It is very efficient at flight speeds above 800 km/hr. Its working depends on the following stages.

Diffuser: This is the first stage of the engine. Here, the atmospheric air enters with a velocity equal to the velocity of the aircraft and it is slowed down in the diffuser.
Compressor: The air leaving the diffuser has negligible velocity and it enters the compressor. Here, the air is compressed to high pressure with the help of an axial compressor.
Combustion Chamber: After compression, the air enters the combustion chamber where fuel is sprayed on it and combustion takes place.

Turbine: The products of the combustion chamber are at high pressure and temperature. They drive the turbine blades, which in turn drives the compressor and thereby allows it to suck in more air. The turbine is also coupled to a generator unit on aircraft to produce electricity.

Exit nozzle: The gases coming out of the turbine expand and leave the nozzle at a high velocity. This produces the required thrust and propels the aircraft in accordance with Newton’s third law.
Afterburner (optional):This component is present only on military aircraft. It basically injects fuel into the exhaust gases coming out of the turbine and the resulting combustion produces additional thrust by increasing the velocity and temperature of the exhaust. This extra thrust is very useful while the aircraft is taking off or to fly at supersonic speeds. The afterburner is used only for short durations (2-3 minutes) as it consumes a very large amount of fuel, and the increased exhaust temperature could damage the nozzle if used for longer periods.

USES: Turbojets were used extensively in military and civilian aircraft from the late 1940s to the 1970s. Then they were gradually replaced by turbofans which were more fuel efficient. Turbojets continue to be used to this day, but are very rare.

The B-52 bomber had 8 turbojet engines which have now been replaced by turbofans

An F-14 Tomcat with its engines on full afterburner during takeoff
TURBOFAN ENGINE

The stages of a turbofan engine. Wikimedia image.
A turbofan is almost identical to a turbojet and consists of just one additional stage, a fan.

• This fan is present ahead of the diffuser and connected to the same shaft which drives the compressor and turbine in a turbojet.
• The fan sucks in air at a faster rate into the turbojet stage and also provides additional bypass thrust as some of the air sucked in by the fan, exits the engines from outside the turbojet stage and supplements the jet thrust coming out of the nozzles of the turbojet.
• Since a turbofan is basically a turbojet with a fan for producing bypass thrust, it is also called as a bypass turbojet.
• These engines are highly efficient at medium and high speeds. Hence they have almost completely replaced turbojets in civilian and military applications. Turbofans have also replaced turboprops in some military aircraft.

Turbofan engine on the Airbus A380
USES: Almost all modern fighter aircraft use high power turbofans with afterburners. Cruise missiles and UAVs also use turbofans. Most of the commercial airliners have switched over to turbofan powered aircraft.

GE-90-115B The world’s largest turbofan engine

TURBOPROP ENGINE

The stages of a turboprop engine. Wikimedia image.

The main reason for the development of the Turboprop engine was the inefficiency of turbojets at flight speeds below 800km/hr. A turboprop engine is very efficient even at low flight speeds.

• It basically consists of a geared propeller connected to a turbojet engine. Hence the working principle remains almost similar. The additional stages will be explained here.
• The turbine in a turboprop engine is bigger than the one on a turbojet. This is because the turbine on a turboprop has to drive the propeller in addition to the compressor and the auxiliary systems like generators, whereas the turbine on a turbojet has to drive only the compressor and the auxiliaries.
• 80-90% of the net power from the turbine is consumed by the propeller and the remaining is left to produce jet thrust. The propeller produces thrust by changing the momentum of the air around it.
• The rotation of the propeller causes a reduction of pressure in front of it (upstream). The air in this region accelerates towards the propeller and flows over it and the pressure increases.
• Thus the air behind the propeller (downstream) is at a higher velocity and constitutes the thrust. This thrust is combined with the small amount of jet thrust exiting from the nozzle and propels the aircraft forward.

The thrust produced by a turboprop at lower flight velocities is considerably higher than turbojets. Hence they find wide application in small to medium size aircraft like civilian and military transports which normally fly at speeds of 400-600 km/hr.

 

[Major Shaitan Singh]

The Airbus A400M is one of the largest aircraft to be powered by turboprops
USES: The most famous turboprop powered aircraft is the C-130 transporter. The fastest turboprop powered aircraft is the Russian Tu-95 which is powered by 4 contra-rotating turboprops rotating at supersonic speeds which enable the aircraft to fly close to 1000 km/hr., which is almost unheard of for a turboprop powered aircraft.
Nowadays many military transports are using turboprops because of the large thrust produced at low speeds. Small civilian aircraft which are economy conscious also use turboprop powered aircraft. Interestingly, turboprops find application in hovercrafts as well.

The Tu-95 is the fastest turboprop powered aircraft. Each engine has 2 sets of contra-rotating propellers
TURBOSHAFT ENGINE

The stages of a turboshaft engine. Wikimedia image.
This engine is used to power every helicopter in the world. The working principle is the same as a turbojet engine, but the exhaust gases aren’t used to propel the helicopter forward.

• The compressed air is burnt in the combustion chamber and it is used to drive a turbine. The exhaust gases exit from the sides and diffuse into the atmosphere and provide negligible thrust.
• The turbine rotates the central shaft as usual, which in turn rotates the compresser. But the shaft is extended in the opposite direction as well and is called the power shaft.
• The rotation of the turbine blades rotates this shaft as well. The power shaft is connected to the helicopter rotor blade shaft via a gearbox.
• Thus rotors of a helicopter rotate

A graphic showing the connection between the power shaft and the rotor shaft with the help of gears©Deargruadher

The Turboshaft engines on a CH-47 Chinook
RAMJET ENGINE

The stages of a ramjet engine. Wikimedia image.
A ramjet engine is used when you need to achieve supersonic speeds in the range of 2-4 times the speed of sound. This is the simplest air-breathing engine in existence as it has no moving parts like compressors or turbines.

• It consists of a diffuser which compresses air by the principle of ‘ram compression’. Ram compression is s type of compression where the kinetic energy of the inlet air is converted into pressure energy with the help of a diffuser, thereby compressing it.
• The air which enters at supersonic speeds, is reduced to subsonic speeds before entering the combustion chamber. Here, fuel is sprayed and burnt in a manner similar to turbojets.
• But the hot exhaust has no turbine to run and the entire exhaust comes out of the nozzle as jet thrust.
• The interesting thing is that this engine cannot be started from zero speed and it needs to be moving at a high speed to start working, hence it is often attached to a turbojet or a rocket booster to propel it to the required speeds.
• A solid fuel rocket booster is the most common attachment for ramjet powered missiles. A turbojet attached to a ramjet is called a turboramjet and is used in military aircraft.

BrahMos missile. Note the conical diffusers in the nose for compressing the inlet air.
USES: This engine is restricted only for military applications and is almost exclusively used on missiles. The popular modern missiles using a ramjet engine are the BrahMos and Meteor.

SCRAMJET ENGINE

The stages of a scramjet engine. Wikimedia image.
A scramjet is a Supersonic Combusting Ramjet engine, named so because it is basically a ramjet engine where the combustion of air happens at supersonic rather than subsonic speeds. Its working is similar to a ramjet engine. A scramjet powered missile or aircraft must be accelerated to 4 times the speed of sound by an external source like a rocket engine before the scramjet can start working.

 

[Major Shaitan Singh]

Currently, countries like USA, Russia and India are working on scramjet powered missiles which can travel faster than 6 times the speed of sound. Theoretically a scramjet engine can achieve speeds of 12 times the speed of sound.

The X-51 waverider is a scramjet demonstration vehicle.
MISCELLANEOUS FACTS

Mach Diamonds
Mach diamonds are formed when the pressure of the gases exiting the nozzle is different from the ambient air pressure.

• When an aircraft is close to the ground and the atmospheric pressure is very high, the exhaust gas coming out of the engine nozzle is at a lower pressure than the surrounding air.
• The high-pressure air presses in on the gas from all sides and compresses it.
• The diamonds are a series of shock-waves and expansions and compression of the exhaust gas which continue until the pressure of the exhaust becomes equal to the pressure of the surrounding atmosphere.
• When the exhaust gas is being compressed, the glowing diamonds formed in the shock-waves are a result of excess fuel being ignited by the afterburner.
• The fuel is trapped in the compression and expansion shockwaves and hence when it ignites, it appears like a string of glowing balls.

SR-71 Blackbird showing mach diamonds formed during takeoff

Mach diamonds formed under laboratory conditions. Photo : Swiss Propulsion laboratory
Thrust Vectoring
Thrust vectoring is a method of manipulating the thrust from the engine of an aircraft to achieve additional directional or altitude control.
It basically directs the thrust in the required direction so that the aircraft can move in the opposite direction. Such a system can allow the aircraft to turn in a very short radius and impart excellent maneuverability. The reason that the Su-30 family of aircraft are very popular in airshows in because of their thrust vectoring nozzles which allows them to perform very complicated maneuvers.

The Harrier was the world’s first operational fighter with thrust vectoring

The thrust vectoring nozzle on a Su-35S

CONCLUSION
The working of jet engines appear simple enough and they have been around from 70 years, yet only a handful on nations have the ability to successfully design jet engines and produce them. Country which can make top quality fighter jets also need to import engines from countries like USA and Russia. Why is it so?
It is because jet engines are simple to understand, but incredibly complex to design and build. It is the heart of every man-made flying object. An aircraft can fly without navigational systems or radars, but it wont even be an aircraft without the engine. The turbine blades of a jet engine function at temperatures over 1000°C for hundreds of hours over its lifetime. It has to be made with the perfect composition of metals so that no fatigue or creep is induced with the associated temperature and physical stresses. A single fault will cause the aircraft to crash.

Currently, American and European jet engines have the highest reliability rate, followed by Russian engines. China has developed its own engines, but is hesitant to use them on a large scale and still imports from Russia as they aren’t confidence about its reliability and performance. India tried its hand at developing a jet engine, but soon abandoned the project as it didn’t meet the required performance parameters even after years of testing.

In the future, we will see turbofans becoming more efficient, ramjet and scramjet powered missiles becoming more popular and new types of engines could emerge. But currently, turbofan is the king and turboprop is the queen for propelling an aircraft forward and turboshafts rule the helicopter world.

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