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Jet Engines

Last Update:

09/06/2016

Introduction

Traveling across the world would never be as quick or easy if it was not for innovation of powerful Jet Engines that drive the modern aircraft. Jet engines actually changed the equation during the World War II, which almost re-wrote the destiny of some of the nations. Every time you see a Boeing or an Airbus fly across the sky, you see a jet leaving a vapor trail, etching its name in clear skies. 

History and Evolution:

  • Frank Whittle, an English RAF engineer and air force officer, is credited with fathering the turbojet engine.
  • He had gained honors as a pilot and engineer during his RAF tenure, when he submitted the designs for Jet Engines which were far in advance of the age.
  • Turned down by the RAF, he obtained a patent in 1930 which unfortunately expired in 1935.
  • The Germans recognized his advanced designs and developed it.
  • With the help of friends Whittle set up Power Jets Ltd in April 1937 and developed the world’s first experimental Jet with his engine designs.
  • In 1941 a Jet developed by Whittle equipped Gloster E28  took to the skies in England.
  • However, the Germans had gone ahead and Hans Von Ohain built the first jet the Heinkel He-178 in 1939.
  • Gyorgy Jendrassik, a Hungarian, is credited with developing the first turboprop engine in 1938, which is seen powering most small air-crafts today.
  • German Government nationalized the Power Jets in 1944. Whittle was working on developing a supersonic plane at the time.
  • Whittle’s jet powered Spitfires changed the fortunes of the War countries, but his work did not get him the recognition he deserved. His designs found their way to the US and to the Russians who developed the MiGs. Ironically, it was Ohain who migrated to the US and was honored by the US National Academy of Engineering with an award as the pioneer of the jet age.

Early Jet Engines were notorious for high fuel consumption. It was Pratt & Whitney, who put two engines in tandem with two compressors to deliver high compression with axially stacked turbines resulting in lower fuel consumption and higher power. This early age Prat & Whitney engines were also a part of Boeing 707 and Douglas DC8

Types

The underlying principle of jet engines is propulsion. The basic jet design has a rotary air compressor working in tandem with a compressor. Over time several versions came into being.

  • Turboprop: This type of engine makes use of an attached propeller. Hot gases drive the turbine that in turn drives the shaft and the attached propeller. The engine chain comprises of turbine, compressor and combustion chamber. Combustion chamber delivers the pressure to run the turbine that drives the compressor and the shaft. Small aircrafts equipped with these engines can achieve speeds up to about 500 miles/hour.
  • Turbofan: This type of jet engine features a fan at the front to draw the air in. Large part of air sucked in, flows outside the engine and a smaller part goes to the traditional combustion chamber. The result is increased thrust without increasing fuel consumption.
  • Turbojet: The basic jet engine in which air is sucked into a compressor where fuel is added and the mix ignites in a combustion chamber. The burnt gas passes a turbine at a much higher pressure than the intake that in turn is connected to the compressor. The higher pressure exhaust gases pass through a nozzle resulting in the thrust that provides propulsion power.
  • Turboshaft: Similar to the turboprop, the difference here is that it powers a helicopter rotor. The speed of gas generator can vary, but it does not influence the rotor speed, which can remain constant. 
  • Ramjet: The speed of the aircraft pushes air into the engine. It does not have a compressor and since compression is in proportion to speed, the vehicle can travel above speed of sound but the vehicle needs liftoff by another aircraft. 

Main Parts

The main parts of the jet engine are the cold section and the hot section.

The cold section components include

  • Air intake duct – It ensures smooth airflow in the engine which is also unidirectional to safeguard against the cross winds and inflight situation on the ground. The air enters the compressor at half the speed of the sound.
  • Fan or multi-stage rotary compressor – This is made of various multiple rotating blades aligned at various stages. As the air moves between them the pressure and the temperature increases.
  • Bypass ducts – It delivers the air from fan to the bypass propelling nozzle
  • Axial shaft connecting compressor, fan and turbine – There are sometimes as many as three consecutive shafts that runs across the length of the engine.  
  • Diffuser section -  It reduces the air flow losses into a combustion chamber by slowing the delivery process.

The hot section components include

  • Combustion chamber where fuel burns
  • Turbine made up of discs with blades that are driven by exhausting gases.
  • Turbofan, turboprop and turboshaft engines may have more than one turbine stage.
  • Afterburner to burn fuel to produce extra thrust, usually in military aircraft
  • Exhaust nozzle at the back
  • These are at the core of the engine. Additional components comprise of thrust reversers, cooling systems, air systems, fuel system, fuel control unit, propellant pump, engine starting system, ignition system, control system and lubrication system.

 

How It Works

The basic principle of operation of a jet engine is the same, varying only slightly across engine types.

  • A fan at the front helps suck in air into the engine. Part of it flows into the compressor and part of it flows over and around it to mix with burnt exhaust gases.
  • The compressor compresses incoming air and pushes it forward where fuel is sprayed to provide a fuel-air mixture.
  • From here the air-fuel mix flows into the combustion chamber
  • Ignition takes place in the combustion chamber
  • Burnt gases exit at a much higher pressure through the nozzle at the rear. The pressure differential provides thrust to propel the aircraft forward.

On the way out the burnt gases pass through a turbine, which, in turn, drive the compressor and intake fan through a common central shaft or axle. Unlike two or four stroke engines, combustion is continuous in jet engines.

Effectiveness, Pros and Challenges

Pros:

  • Jet engines can achieve the cruising speeds that is just impossible for others to achieve.
  • Jet Engines use aviation fuel, a type of kerosene which is relatively less risky rather than petrol.
  • The maintenance is relatively cheaper as it has lesser parts to maintain given the power to weight ratio

Cons

  • Jet engine poses the problem of air pollution that can deplete the ozone layer as a result of flying at higher altitudes
  • Jet engines are restricted to Airplanes as they are noisy in excess of 130dB which is very harmful.
  • Jet engines are heavy fuel guzzlers.

Some of the other challenges of Jet engines are compressor stall or surge, bursts of fuel flows etc that can result in visible flames in the exhaust or oil bleeding.

Maintenance

  • Strangely, jet engine life is measured by flight cycles and flight hours. One flight cycle includes one take-off and one landing. A regular, highly trained and professional crew routinely checks each engine of each aircraft at each flight cycle for engine condition, lubricant level and sound that could indicate anomalies. Hot and cold sections receive visual and other checks to detect cracks and abnormalities.
  • Minor maintenance is required once in 200 cycles or 400 cycles and major maintenance is required every 7300 cycles for modern jet engines.

 

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