Aeronautics, Energy, Mechanics, Temperature, Thermodynamics, Transport

Other jet engines

In addition to the turbofan, exist other jet engines. In this post it’s shown the operation and applications on aviation of turbojet, turboprop, and turboshaft.

Turbojet

jet engines turbojet
The turbojet J58 of Pratt & Whitney. Source: Hill Air Force Base.

This is the oldest and simplest jet engine. Does not have a big fan in front, which makes it less efficient than the engine used in commercial planes. The operation of jet engines by stages have already been explained in the post about the turbofan.

TurbofanClick here

This type of engine is used in many military aircraft, that prioritizes higher speed and less weight, in addition that can go to higher altitudes, where air pressure is lower. However, they are much more noisy.

The afterburner

To achieve supersonic speeds, above speed of sound produced by the airplane, many jet fighter use engines with afterburner. This component was introduced in the second generation jet fighters.

The PW-F100 engine with afterburner turned on. Source: Softpedia.

This component is at the rear of jet engine, between turbine and nozzle. Inject fuel directly to make combustion with the remaining oxygen, creating an additional impulse and increasing temperature. Consists in fuel injectors, tube to transport kerosene, flame holder and an adjustable nozzle.

jet engines with afterburner
The adjustable nozzle serves to engine to keep working with the afterburner turned on or off.

With the afterburner, it’s possible to increase thrust without increase much weight and complexity. The use of afterburner consumes huge energy, reducing a lot plane’s autonomy.

Turboprop

The propeller and motor’s shaft are connected by a reduction gearbox, to avoid the propeller’s tips to achieve supersonic speeds and consequently, generate small shockwaves that would destroy the propeller. In turbines part, there is an additional blade stage in airfoil shape, to add more power to engine shaft, reducing the exhaustion speed on output. Most of airplane’s thrust is produced by the propeller.

turboprop schematics
Source: pinterest.

This type of engine is very efficient for small low speed airplanes. Some turboprops have engines with reverse flux, where atmospheric air goes to the back and the exhaust duct is close to reduction gearbox.

PT6 turboprop
The models of Pratt & Whitney’s PT6 family are examples of engines with reverse flux. Source: StackExchange.

The advantage of reverse flux engine is that air flux can be used to limit high temperature in the combustion chamber, avoiding chamber’s fusion. 

Turboshaft

engine types turboshaft

This engine is exclusively for helicopters. Also priories power on shaft. The engine’s shaft is mechanically linked to the main propeller’s shaft. The turbines and compressors must spin in opposite directions to eliminate total angular momentum, because it becomes more difficult the helicopter’s control.

angular momentum on jet engines
Angular momentum (orange vector parallel to rotation axis) is produced by an object in rotation. Two objects on the same axis, with equal angular speed and moment of inertia, spinning in opposite directions, cancel the angular momentum. Source: Lagrangianos.

Formula of angular momentum vector \vec{L}.

\vec{L}=I\cdot \vec{\omega}

Where I is the inertia momentum and \vec{\omega} is the angular speed.

Another jet engines

Exist also the RAMJET and SCRAMJET, but these need dedicated posts, because the operation is different.

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About Pedro Ney Stroski

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