Difference Between Turboprop, Turbojet and Turbofan Engines!
By converting the shaft power of the turboprop into thrust and the fuel consumption per power into fuel consumption per unit thrust, a comparison between turbojet, turboprop and turbofan can be made.
Assuming that the engines are equivalent as to compression ratio and temperature and that the engines are installed in equal-sized aircraft best suited to the particular type of the engine being used.
Turbojet – Characteristics and Uses:
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1. Low thrust at low forward speed.
2. Relatively high TFSC at low altitudes and air speeds. This disadvantage decreases as altitude and air speed increases.
3. Long take off road required.
4. Small frontal area results in ground clearance problems.
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5. Lightest specific weight.
6. Ability to take advantage of high ram-pressure ratio.
Turbo-Prop—Characteristics and Uses:
1. High propulsive efficiency at low speeds, which falls off rapidly as airspeed increases. This results in shorter rolls. This engine is able to develop very high thrust at low air speeds because the propeller can accelerate large quantities of air at zero forward velocity of the airplane.
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2. More complicated and heavier than turbojet.
3. Lowest TFSC.
4. Large frontal area of propeller and engine combination necessitates longer landing gears for low-wing airplanes but does not necessarily increase parasitic drag.
5. Efficient reverse thrust possible.
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These characteristics show that turboprop engines are superior for lifting heavy loads off short and medium runways. They are limited in speeds to 800 km/hr approximately. Since propeller efficiency falls off rapidly with increase in speeds and altitude.
Turbo-Fan—Characteristics and Uses:
1. Increased thrust at forward speeds similar to a turbo prop results in a relatively short take off. But unlike the turbo prop the turbo fan thrust is not penalised with increasing air speeds upto approximately. Mach-1 with current fan designs.
2. Weight falls between turbojet and turbo prop.
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3. Ground clearances are less than turbo prop, but not as good as turbojet.
4. TSFC and specific weight fall between turbo prop and turbojet.
5. Considerable noise reduction over the turbojet. This results in the reduction of acoustic fatigue in surrounding aircraft parts and it is less objectionable to the people on ground.
6. Superior to turbojet ‘hot-day’ performance.
From the above characteristics it is learnt that the turbofan engine is suitable for long range, relatively high speed flight and has a definite place in the prolific gas turbine family.
Comparative suitability for (left to right) turboshaft, low bypass and turbojet to fly at 10 km altitude in various speeds. Horizontal axis—speed, m/s. Vertical axis carries only logical meaning.
Efficiency as a function of speed of different Jet types. Although efficiency plummets with speed, greater distances are covered, it turns out that efficiency per unit distance (per km or mile) is roughly independent of speed for Jet engines as a group; however airframes become inefficient at supersonic speeds.
The motion impulse of the engine is equal to the air mass, multiplied by the speed that the engine emits this mass-
I = mc
where m is the air mass per second and c is the exhaust speed. In other words, the plane will fly faster if the engine emits the air mass with a higher speed or if it emits more air per second with the same speed. However, when the plane flies with certain velocity v, the air moves towards it, creating the opposing ram drag at the air intake = mv
Most types of jet engine have an air intake, which provides the bulk of the gas exiting the exhaust. Conventional rocket motors, however, do not have an air intake, the oxidizer and fuel both being carried within the airframe. Therefore, rocket motors do not have ram drag; the gross thrust of the nozzle is the net thrust of the engine.
Consequently, the thrust characteristics of a rocket motor are completely different from that of an air breathing jet engine.
The air breathing engine is only useful if the velocity of the gas from the engine, c, is greater than the airplane velocity, v. The net engine thrust is the same as if the gas were emitted with the velocity c – v. So the pushing moment is actually equal to-
S = m(c – v)
The turboprop has a wide rotating fan that takes and accelerates the large mass of air but only till the limited speed of any propeller driven airplane. When the plane speed exceeds this limit, propellers no longer provide any thrust (c – v < 0).
The turbojets and other similar engines accelerate much smaller mass of the air and burned fuel, but they emit it at the much higher speeds possible with a de Laval nozzle. This is why they are suitable for supersonic and higher speeds.
From the other side, the energy efficiency is higher when the engine pushes as large as possible mass of air at the speed, comparable to the airplane velocity. The exact formula, given in the literature, is-
The low bypass turbofans have the mixed exhaust of the two air flows, running at different speeds (c1 and c2). The pushing moment of such engine is-
S = m1 (c1 – v) + m2 (c2 – v)
where m1 and m2 are the air masses, being blown from the both exhausts. Such engines are effective at lower speeds, than the pure jets, but at higher speeds than the turboshafts and propellers in general. For instance, at the 10 km attitude, turboshafts are most effective at about 0.4 Mach, low bypass turbofans become more effective at about 0.75 Mach and true jets become more effective as mixed exhaust engines when the speed approaches 1 Mach—the speed of sound.
Rocket engines are best suited for high speeds and altitudes. At any given throttle, the thrust and efficiency of a rocket motor improves slightly with increasing altitude (because the back-pressure falls thus increasing net thrust at the nozzle exit plane), whereas with a turbojet (or turbofan) the falling density of the air entering the intake (and the hot gases leaving the nozzle) causes the net thrust to decrease with increasing altitude.