Inside It: Jet Engine

Jet Engine

Sources: Thermodynamics by Cengel and Boles.

Jet - Propulsion Cycle
A Jet engine works on the principle of Newton's Third Law of Motion, which states "Each action has an equal and opposite reaction"
Gas turbine cycles are used to power aircrafts because:

They are light
Compact
Have a high power to weight ratio

Aircraft turbines operate on an open cycle jet-propulsion cycle
The ideal jet propulsion cycle differs from an ideal Brayton Cylce (more about Brayton Cycle here #-Link-Snipped-#).
In the turbine the gases are not expanded to ambient pressure.
They are expanded to a pressure such that the power generated by turbine is sufficient to drive compressor and some auxiliary equipment. The net work output is almost zero. The exhaust gases are at a relatively high pressure and these are accelerated in the Nozzle and thus provide the thrust to propel the craft.

Air is compressed by a compressor. Mixed with fuel in combustion chamber and the mixture is burnt at a constant pressure. High pressure and temperature gases expand partially in the turbine. Gases then expand to ambient pressure in the nozzle and leave at a high velocity.

Thrust developed is the unbalanced force due to the difference in the momentum of low velocity air at the inlet and high velocity air at the exit.
This is determined by Newton's Second Law:
F = (mV)exit - (mV)inlet = m(Vexit -Vinlet)

Power developed by the thrust is called Propulsion Power.
W = FV

Propulsion efficiency is the ratio of Propulsion Power with Energy Input.

Replies

gohm

Good one Patty! Tell us about the differences between turbofan and turbojet?
raj87verma88

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Photo: www.aerospaceweb.org

The picture shows the diagram of a Jet Engine.
raj87verma88

Modifications to a Turbojet Engine

Earlier aircrafts i.e. before jets, were propeller driven. Both propeller and jet have their pros and cons. Efforts were made to combine the desirable characteristics of both.
Turboprop and Turbofan are two such modifications.

Turbofan: It is the most widely used engine in aircraft propulsion. A large fan driven by the turbine forces a large amount of air through a duct surrounding the engine. The fan exhaust leaves engine at a high velocity thereby increasing the total thrust. It's working principle is that for the same power, large volume of slower moving air produces more thrust than a small volume of fast moving air.

Turboprop: If the cowl from a turbofan engine is removed it results in a turboprop engine. Generally propellers are more efficient than jet engines, but are limited to low speeds and altitude because their efficiency decreases with speed and altitude.

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durga ch

OK. as far as I can see, I can see 2 different type of jets.
So what type is used where? I mean the application part fo having three different types?
raj87verma88
Application
- Turbojets have a lower efficiency and produce a lot of noise, but the speed of the exhaust gases is very high. Due to this they are mostly used in Medium range cruise missiles.
- Turbofans are the most widely used engines in jet aircrafts. All the jet planes for civil use are powered by turbofans.
- Turboprops are used for commercial planes of medium size and range.
raj87verma88

Pictorial Examples of Planes powered by Turboprop Engines

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Photo: Aerospace Technology | Aircraft News & Views Updated Daily

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Photo: cdn-www.airliners.net
raj87verma88

Airplanes with Turbofan Engines

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Photo: widebodyaircraft.nl
Harshad Italiya

Superb Information there Raj... with Attractive Format.
din_Power Engg

Excellent one.. i got a nice brush up to my favorite subject....Also, there are some additional infos, turbofans requires a long takeoff runway and short landing runway and Turboprops the vice versa. This is due to the obvious reason that propeller can produce a large amount of Thrust than that of a fan, facilitating in easier takeoff. This is the reason why in small aircrafts turboprops are preferred so that they are well sophisticated with shorter takeoff......all the commercial Airbus made aircraft are operated by Turbofan engines and most the earlier versions of Boeing Aircraft are operated by turboprops (some are still in use)........
raj87verma88

Other Modifications

The jet engine was further modified to perform specific tasks. Some fo the modification are:

Afterburners
Popular in military aircrafts. Placed between turbine and the nozzle they are used to provide extra thrust for short takeoffs or during combat situations. Additional fuel is injected into the Oxygen rich combustion gases leaving the turbine. Due to this added energy, exhaust gases leave at a higher velocity and therefore a greater thrust.

Afterburners in action
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Afterburners On
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Photo: AVSIM - The AVSIM Community
raj87verma88

A Plane takes off from an Aircraft Carrier using Afterburners
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Photo: i56.photobucket.com

Cruising. No use of Afterburners in this picture.
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Photo: : Bharat-Rakshak.com - The Consortium of Indian Military and Defence Websites :
raj87verma88

Ramjet
This engine is a properly shaped duct with no compressor or turbine. It is used for high speed propulsion of missiles and airplanes. Pressure rise n engine is provided by the ram effect of incoming high speed air being rammed against a barrier. That is why this engine has to be brought to a sufficiently high speed by an external source before it can be fired.
It best performs when the craft is flying above Mach 2 or 3.
In the ramjet, air is slowed down to near about Mach 0.2, fuel added to air and burned at this low velocity. Exhaust gases are expanded and accelerated in the nozzle.

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raj87verma88

Scramjet

It is essentially a ramjet in which air flows through at supersonic speeds. The combustion takes place above the sped of sound. At higher speeds, it is necessary to combust supersonically to maximize the efficiency of the combustion process.Ramjets that convert to scramjets at speeds above Mach 6 are successfully tested at speeds of about Mach 8.

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raj87verma88

Note: Both ramjet and scramjet contain no moving parts.

Rocket

Solid or liquid fuel and an oxidizer react in the combustion chamber. The high pressure combustion gases expand in the nozzle. They leave the rocket at a very high velocity, producing the thrust to propel the rocket.
raj87verma88

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Photo: www.grc.nasa.gov

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Photo: www.grc.nasa.gov
raj87verma88

Thanks everyone for the feedback.

Special thanks to din-Power Engg for improving on the thread by adding his own knowledge.
raj87verma88

For a more detailed knowledge about the cycle that gave birth to this engine go to the following link.
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You are reading an archived discussion.

Inside It: Jet Engine

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