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Jet Engines

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TOPIC: Jet Engines

 

GOAL: To learn about the benefit of jet engines and how they operate

Level: Medium

 

Exercise #1: Read about jet engines and then proceed to Exercise #2

Have you ever looked at a large airplane takeoff and sail into sky and wondered how much power it must take to get that vehicle off the ground? Many people have. Jet engines move the airplane forward with a great force that is produced by a tremendous thrust and causes the plane to fly very fast. All jet engines, which are also called gas turbines, work on the same principle. The engine sucks air in at the front with a fan. A compressor raises the pressure of the air. The compressor is made up of fans with many blades and attached to a shaft. The blades compress the air. The compressed air is then sprayed with fuel and an electric spark lights the mixture. The burning gases expand and blast out through the nozzle, at the back of the engine. As the jets of gas shoot backward, the engine and the aircraft are thrust forward. The air goes through the core of the engine as well as around the core. This causes some of the air to be very hot and some to be cooler. The cooler air then mixes with the hot air at the engine exit area.

What is Thrust?

Thrust is the forward force that pushes the engine and, therefore, the airplane forward. Sir Isaac Newton discovered that for "every action there is an equal and opposite reaction." An engine uses this principle. The engine takes in a large volume of air. The air is heated and compressed and slowed down. The air is forced through many spinning blades. By mixing this air with jet fuel, the temperature of the air can be as high as three thousand degrees. The power of the air is used to turn the turbine. Finally, when the air leaves, it pushes backward out of the engine. This causes the plane to move forward.

Parts of a Jet Engine

Fan – The fan is the first component in a turbofan. The large spinning fan sucks in large quantities of air. Most blades of the fan are made of titanium. It then speeds this air up and splits it into two parts. One part continues through the "core" or center of the engine, where it is acted upon by the other engine components. The second part "bypasses" the core of the engine. It goes through a duct that surrounds the core to the back of the engine where it produces much of the force that propels the airplane forward. This cooler air helps to quiet the engine as well as adding thrust to the engine.

Compressor – The compressor is the first component in the engine core. The compressor is made up of fans with many blades and attached to a shaft. The compressor squeezes the air that enters it into progressively smaller areas, resulting in an increase in the air pressure. This results in an increase in the energy potential of the air. The squashed air is forced into the combustion chamber.

Combustor – In the combustor the air is mixed with fuel and then ignited. There are as many as 20 nozzles to spray fuel into the airstream. The mixture of air and fuel catches fire. This provides a high temperature, high-energy airflow. The fuel burns with the oxygen in the compressed air, producing hot expanding gases. The inside of the combustor is often made of ceramic materials to provide a heat-resistant chamber. The heat can reach 2700°.

Turbine – The high-energy airflow coming out of the combustor goes into the turbine, causing the turbine blades to rotate. The turbines are linked by a shaft to turn the blades in the compressor and to spin the intake fan at the front. This rotation takes some energy from the high-energy flow that is used to drive the fan and the compressor. The gases produced in the combustion chamber move through the turbine and spin its blades. The turbines of the jet spin around thousands of times. They are fixed on shafts which have several sets of ball-bearing in between them.

Nozzle – The nozzle is the exhaust duct of the engine. This is the engine part which actually produces the thrust for the plane. The energy depleted airflow that passed the turbine, in addition to the colder air that bypassed the engine core, produces a force when exiting the nozzle that acts to propel the engine, and therefore the airplane, forward. The combination of the hot air and cold air are expelled and produce an exhaust, which causes a forward thrust. The nozzle may be preceded by a mixer, which combines the high temperature air coming from the engine core with the lower temperature air that was bypassed in the fan. The mixer helps to make the engine quieter.

Types of Jet Engines

Turbojets

The basic idea of the turbojet engine is simple. Air taken in from an opening in the front of the engine is compressed to 3 to 12 times its original pressure in compressor. Fuel is added to the air and burned in a combustion chamber to raise the temperature of the fluid mixture to about 1,100°F to 1,300° F. The resulting hot air is passed through a turbine, which drives the compressor. If the turbine and compressor are efficient, the pressure at the turbine discharge will be nearly twice the atmospheric pressure, and this excess pressure is sent to the nozzle to produce a high-velocity stream of gas which produces a thrust. Substantial increases in thrust can be obtained by employing an afterburner. It is a second combustion chamber positioned after the turbine and before the nozzle. The afterburner increases the temperature of the gas ahead of the nozzle. The result of this increase in temperature is an increase of about 40 percent in thrust at takeoff and a much larger percentage at high speeds once the plane is in the air.

The turbojet engine is a reaction engine. In a reaction engine, expanding gases push hard against the front of the engine. The turbojet sucks in air and compresses or squeezes it. The gases flow through the turbine and make it spin. These gases bounce back and shoot out of the rear of the exhaust, pushing the plane forward.

Turboprops

A turboprop engine is a jet engine attached to a propeller. The turbine at the back is turned by the hot gases, and this turns a shaft that drives the propeller. Some small airliners and transport aircraft are powered by turboprops.

Like the turbojet, the turboprop engine consists of a compressor, combustion chamber, and turbine, the air and gas pressure is used to run the turbine, which then creates power to drive the compressor. Compared with a turbojet engine, the turboprop has better propulsion efficiency at flight speeds below about 500 miles per hour. Modern turboprop engines are equipped with propellers that have a smaller diameter but a larger number of blades for efficient operation at much higher flight speeds. To accommodate the higher flight speeds, the blades are scimitar-shaped with swept-back leading edges at the blade tips. Engines featuring such propellers are called propfans.

Turbofans

A turbofan engine has a large fan at the front, which sucks in air. Most of the air flows around the outside of the engine, making it quieter and giving more thrust at low speeds. Most of today's airliners are powered by turbofans. In a turbojet all the air entering the intake passes through the gas generator, which is composed of the compressor, combustion chamber, and turbine. In a turbofan engine only a portion of the incoming air goes into the combustion chamber. The remainder passes through a fan, or low-pressure compressor, and is ejected directly as a "cold" jet or mixed with the gas-generator exhaust to produce a "hot" jet. The objective of this sort of bypass system is to increase thrust without increasing fuel consumption. It achieves this by increasing the total air-mass flow and reducing the velocity within the same total energy supply.

Turboshafts

The turboshaft is another form of gas-turbine engine that operates much like a turboprop system. It does not drive a propeller. Instead, it provides power for a helicopter rotor. The turboshaft engine is designed so that the speed of the helicopter rotor is independent of the rotating speed of the gas generator. This permits the rotor speed to be kept constant even when the speed of the generator is varied to modulate the amount of power produced.

Ramjets

The simplest jet engine – the ramjet – has no moving parts. The speed of the jet "rams" or forces air into the engine. It is essentially a turbojet in which rotating machinery has been omitted. Its application is restricted by the fact that its compression ratio depends wholly on forward speed. The ramjet develops no static thrust and very little thrust in general below the speed of sound. As a consequence, a ramjet vehicle requires some form of assisted takeoff, such as another aircraft. It has been used primarily in guided-missile systems. Space vehicles use this type of jet.

 

 

 

 

Exercise #2: Watch two videos about CRM then continue to Exercise #3

 

This video provides a good briefing on how a jet engine operates.

    http://www.grc.nasa.gov/WWW/K12/UEET/StudentSite/images/engines/video/howitworks256.wmv

    Exercise #3: Matching: Using the following vocabulary list and diagram, match the lettered parts of the turbine engine with their appropriate terminology.

    Compressor Air intake Combustion chamber
    Fuel-burner Turbine Nozzle

    A___________

    B___________

    C___________

    D___________

    E___________

    F___________

    Answers: (A) compressor (B) combustion chamber (C) turbine (D) air intake (E) fuel burner (F) nozzle

    Find out more about other parts of the aircraft!

    www.globalaviationenglish.com

     

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