Jet Engines

A jet engine is an engine that discharges a fast moving jet of fluid to generate thrust in accordance with Newton's third law of motion. This broad definition of jet engines includes turbojets, turbofans, rockets and ramjets and water jets, but in common usage, the term generally refers to a gas turbine used to produce a jet of high speed exhaust gases for special propulsive purposes.

 

 Gas Turbine engines :

Turbojet 

This is the Basic gas turbine engine. All other types of Gas turbine engines have been derived from its design. A gas turbine power plant used to propel aircraft, where the thrust is derived within the turbo-machinery in the process of accelerating the air and products of combustion out an exhaust jet nozzle.

In its most elementary form (see illustration), the turbojet operates on the gas turbine or Brayton thermodynamic cycle. The working fluid, air drawn into the inlet of the engine, is first compressed in a turbo-compressor with a pressure ratio of typically 10:1 to 20:1. The high-pressure air then enters a combustion chamber, where a steady flow of a hydrocarbon fuel is introduced in either spray or vapor form and burned continuously at constant pressure. The exiting stream of hot high-pressure air, at an average temperature whose maximum value may range typically from 1800 to 2800F (980 to 1540C), is then expanded through a turbine, where energy is extracted to power the compressor. Because heat had been added to the air at high pressure, there is a surplus of energy left in the stream of combustion products that exits from the turbine and that can be harnessed for propulsion.

Advantages : This engine is known for its simplicity of design. Found to be  efficient at supersonic speeds (~M2)

Disadvantages Basic design, misses many improvements in efficiency and power for subsonic flight, relatively noisy.

Uses : Used mainly in aircraft where supersonic flights need to be performed. Fitted in most of the high speed fighter jets and also was fitted in concorde.

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Turbofan

An air-breathing aircraft gas turbine engine with operational characteristics between those of the turbojet and the turboprop. Like the turboprop, the turbofan consists of a compressor-combustor-turbine unit, called a core or gas generator, and a power turbine. This power turbine drives a low- or medium-pressure-ratio compressor, called a fan, some or most of whose discharge bypasses the core.

The gas generator produces useful energy in the form of hot gas under pressure. Part of this energy is converted by the power turbine and the fan it drives into increased pressure of the fan airflow. This airflow is accelerated to ambient pressure through a fan jet nozzle and is thereby converted into kinetic energy. The residual core energy is converted into kinetic energy by being accelerated to ambient pressure through a separate core jet nozzle. The reaction in the turbomachinery in producing both streams produces useful thrust.

Advantages : 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.

Disadvantages : Greater complexity (additional ducting, usually multiple shafts), large diameter engine, need to contain heavy blades. More subject to foreign object damage and ice damage. Top speed is limited due to the potential for shockwaves to damage engine. Most common form of jet engine in use today. 

Uses : Used in airliners like the Boeing 747 and military jets, where an afterburner is often added for supersonic flight.

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Turboprop (similar to turboshaft)

Strictly speaking its 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). It is a hybrid engine that provides jet thrust and also drives a propeller. It is similar to the turbojet except that an added turbine, behind the combustion chamber, works through a shaft and speed-reducing gears to turn a propeller at the front of the engine. Because of improvements in turbojet design, the turboprop, which is less efficient at high speeds, lost much of its importance in the 1960s, though it is still used for relatively short-range aircraft.

Advantages : High efficiency at lower subsonic airspeeds(300 knots plus), high shaft power to weight

Disadvantages : Limited top speed (aeroplanes), somewhat noisy, complex transmission

Uses : Commonly used in low speed aircrafts, Commonly seen on ATR's and other business jet categories of aircrafts

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Other Types of Jet Engines :

Pulsejet

A pulse jet engine (or pulsejet) is a very simple form of internal combustion engine wherein the combustion occurs in pulses and the propulsive effort is a jet, this is, a reaction to the rearward flow of hot gases.

A typical pulsejet comprises an air intake fitted with a one-way valve, a combustion chamber, and an acoustically resonant exhaust pipe. The valving is accomplished though the use of reed valves or, in a valveless pulse jet engine, through aerodynamics. Fuel in the form of a gas or liquid aerosol is either mixed with the air in the intake or injected into the combustion chamber. Once the engine is running it requires only an input of fuel, but it usually requires forced air and an ignition method for the fuel-air mix. Once running, the engine is self-sustaining.

Advantages : Very simple design, no rotating compressor or turbine sections, economical design

Disadvantages : Noisy, inefficient (low compression ratio), works poorly on a large scale, valves on valved designs wear out quickly

Uses : Used on earlier generation of fighter aircrafts. Nowadays used on model aircrafts

 

Ramjet

Air-breathing jet engine that operates with no major moving parts. It relies on the craft's forward motion to draw in air and on a specially shaped intake passage to compress the air for combustion. After fuel sprayed into the engine has been ignited, combustion is self-sustaining. As in other jet engines, forward thrust is obtained as a reaction to the rearward rush of hot exhaust gases. Ramjets work best at speeds of Mach 2 (twice the speed of sound) and higher.

Advantages : Very few moving parts, Mach 0.8 to Mach 5+, efficient at high speed (> Mach 2.0 or so), lightest of all airbreathing jets (thrust/weight ratio up to 30 at optimum speed)

Disadvantages : 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.

Uses : Long Range Missiles, etc

 

Scramjet

A scramjet (supersonic combustion ramjet) is a variation of a ramjet where the flow of the air and combustion of the fuel air mixture through the engine happen at supersonic speeds. This allows the scramjet to achieve greater speeds than a conventional ramjet which slows the incoming air to subsonic speeds before entering the combustion chamber.

Advantages : Few mechanical parts, can operate at very high Mach numbers (Mach 8 to 15) with good efficiencies

Disadvantages : 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

Uses : Still no practical use as such. Has been tested on unmanned vehicle by NASA.

 





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