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Im not an aircraft expert and I just realized that there are two different terms - turbofan and turboprop. I always had them combined in my head as a term for a large jet engine (like you would see on a 747). Apparently this is not the case though. So what is the difference between them? And are there are types of "turbo" engines?

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fooot's answer is excellent. Large modern commercial airliners use turbofan engines; you still see turboprops in use in smaller regional passenger planes. Most modern military aircraft are turbofan-driven, though types that prioritize endurance over speed will use turboprops. –  Russell Borogove May 18 at 22:06
    
Really good description of how turbine engines work. It might be worth mentioning that early turbine engines used centrifugal rather than axial compressors? –  ccl2341 Jul 2 at 4:20

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Both engines use a turbine for power. This is where the "turbo" part of the name comes from. In a turbine engine, air is compressed and then fuel is ignited in this compressed air. The energy produced by the ignition turns the turbine. The turbine is then able to drive both the compressor at the front of the engine and also some useful load. In airplanes, it produces thrust.

The first jet engine was a turbojet. This is a simple turbine engine that produces all of its thrust from the exhaust from the turbine section. However, because all of the air is passing through the whole turbine, all of it must burn fuel. This means it is inefficient, and the solution is the turbofan.

Jet engine schematic


In a turbofan, the turbine primarily drives a fan at the front of the engine. Most engines drive the fan directly from the turbine. There are usually at least two separate shafts to allow the fan to spin slower than the inner core of the engine. The fan is surrounded by a cowl which guides the air to and from the fan. Part of the air enters the turbine section of the engine, and the rest is bypassed around the engine. In high-bypass engines, most of the air only goes through the fan and bypasses the rest of the engine and providing most of the thrust. Turbofan engine schematic


In a turboprop, the turbine primarily drives a propeller at the front of the engine. There is no cowl around the prop. Some air enters the turbine, the rest does not. The propeller is geared to allow it to spin slower than the turbine.

Turboprop engine schematic


Turboprops are more efficient at lower speeds since the prop can move much more air with a smaller turbine than the fan on a turbofan engine. The cowl around the turbofan's large fan allows it to perform better than an open propeller at high speeds.

At supersonic speeds, turbojets have more of a performance benefit. They develop all of their thrust from the high velocity turbine exhaust, while turbofans supplement that with the lower velocity air from the fan. Since the air from the fan is also not compressed nearly as much as the core turbine flow, it is also harder to prevent the flow from going supersonic and causing losses.

The Concorde used turbojets because it was designed to cruise for long periods at supersonic speeds. Modern fighter jet engines are turbofans, which provide a compromise between efficiency and speed.

There are other benefits and drawbacks between turbojets, turbofans, and turboprops, but I think they are beyond the scope of this question.

Work has been done on creating a "propfan" engine, in an attempt to get the efficiency of a turboprop and the speed of a turbofan. They have yet to come up with a viable design.

Prop fan engine schematic

Elsewhere in aviation, turbine engines are used in helicopters (as a turboshaft engine driving the rotors instead of a propeller), and in most jet aircraft and large turboprop aircraft (as an auxiliary power source, the APU).

Turbines also find use outside of aviation in power plants (to generate electricity), and even vehicles (like the Abrams tank).

Turbocharged piston engines use a turbine much differently from the examples above. Instead of being the primary power source, the turbine only assists the piston engine. A turbocharger uses a turbine to compress air sent to the engine intake. The increased compression helps the engine generate more power. The turbine of a turbocharger is driven by engine exhaust gasses, and a supercharger is similar but is directly powered by the engine. See the Wikipedia page for more detail.

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Great answer. Now, are turbojets better than turbofans for high altitude supersonic flight? I see most 1960s fighter jets and the Concorde used turbojet engines... –  shortstheory May 19 at 2:54
    
@fooot Great, informative answer. You may want to mention turbocharged piston engines. That was a confusion for me when I first began researching the different types of airplane engines. –  Garrison Neely May 19 at 19:21
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Why is the exhaust air ducted down in the turbo prop example? Wouldn't it make sense to have it shoot straight through? –  RoboKaren Jul 5 at 16:27
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@RoboKaren: Don't think of it as ducted down. Rather, think of it as being fed to an exhaust. Turboprops, like piston engines, are normally installed in front of the plane. Thus you need to deflect the exhaust somewhere to prevent it from blasting the cockpit and passenger compartment. If the aircraft doesn't have this restirction (twin engine mounted on the wings for example) then it would indeed make sense to just vent the jet straight through. Indeed some real-world designs do exactly this. –  slebetman Jul 7 at 3:52
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@RoboKaren: The PT-6A (a common turboprop engine) is "backwards." Air enters at the back, works its way forwards and exits out exhausts on the the sides. The main turbine is on the front, tied to a gearbox, and then the propeller. The nature of turbojets and turbofans is that the exhaust gasses have significant velocity. With a turboprop, there's relatively little velocity in the exhaust. Most of the energy in the exhaust has been captured by the main turbine and turned into rotational power. –  Meower68 Jul 22 at 21:55

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