StorminMatt |
10-26-2007 12:39 AM |
Quote:
Originally Posted by DaX
(Post 78379)
For general aviation anyhow, gas turbine engines in the form of turbo-props are cool, but use WAY more fuel than their av-gas burning counterparts.
A 320 ci engine burning av-gas can cruise at around 7 gph (maybe better even). Most gas turbine engines are eating something like 20+ gph.
And as far as I understand, most gas-turbine engines use Jet-A fuel - which is interchangeable with diesel.
If Lycoming made a diesel engine for GA, people could fill up with diesel or Jet-A, both of which are typically cheaper than 100LL.
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Then again, there are a couple of considerations here as far as efficiency. A 320ci engine like that found in a Cessna 172 produces a meager 160HP. But even a low-powered turboprop produces at least a few hundred HP - rarely under 500HP. These engines also tend to power larger and heavier planes. So given all these factors, fuel consumption is going to be higher. But it is SPECIFIC fuel consumption that matters. In other words, how many gallons of fuel per hour are you burning per hundred HP?
There are other factors as well. One MAJOR factor is the age and design of the engine. Early gas turbines were HORRIBLE when it came to fuel consumption. A typical early turborop/jet had an eficiency of around 15% (or so). So a plane using such an engine would tend to be a fuel hog. And since turbines last a long time, there are certainly ALOT of these older, less efficient ones running around out there. But even newer turbines can be guzzlers simply because a manufacturer might continue to build an older design. These older turbine designs tend to be cheaper to produce, since (1) there is no need to engineer a new engine, and (2) they use less exotic materials and less precise tolerances. They may guzzle. But at least they are reliable and proven. So the design continues to be produced. And these cheaper turbines tend to be used on lower powered turbines used on smaller and cheaper planes.
Speaking of low power turbines, another factor that reduces efficiency is the use of radial flow turbines/compressors vs axial flow. Radial flow engines are inherently MUCH less efficient than axial flow engines. And, as with older designs, radial flow engines are cheaper to produce. So they tend to find their place in ALOT of general aviation applications.
On the other hand, larger turbines used in airliners, like the GE CF6-80 series, are VERY efficient - around 40%. While it is hard to say how well the efficiency would scale down to a smaller size, it is certainly true that MUCH higher efficiency is possible even on smaller engines with design improvements. On the other hand, it would increase cost, which is probably why such improvements are generally not made.
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