"Roy Smith" wrote in message
...
It occurs to me that compared to a piston engine, the turbine is 1) more
expensive, and 2) more reliable. But, why are those things true?
Looking at it another way, is there some inherent reason why piston
engines are cheaper to produce? Is there also some inherent reason why
they're less reliable?

I believe that there are at least two factors:

A turbine needs to be constructed out of more expensive materials, because
of higher temperatures involved in the operation of the engine, and it needs
to be constructed to higher tolerances, because it's very sensitive to
imbalances. These contribute to cost.

On the other hand, a turbine has no parts that reverse direction, while a
piston engine has many such parts. So the turbine suffers less stress, when
constructed correctly, than a piston engine does. It's also "simpler", in
the sense that the engine doesn't need as many moving parts to accomplish
the same thing. These contribute to reliability.

The above ignores higher maintenance costs, which are probably related to
several factors, including cost of parts, cost of training for a mechanic,
and stricter maintenance guidelines (meaning maintenance happens more often
and is more thorough).

If I were to give you the $/HP budget a turbine designer has to work
with, would you be able to design a piston engine that was as reliable
as a turbine?

Well, one problem is that the assertion that turbines are more reliable is,
in my opinion, unproved. A well-maintained piston engine can be VERY
reliable, while a poorly maintained turbine might not last very long at all.
It's hard to know for sure, because most turbines are operated in an
environment where there are strict maintenance standards. Those standards
applied to piston engines might well result equally reliable piston engines.

I think one interesting way to address your question is to look at what
causes engine failures. In piston engines, it's usually some secondary
component, such as fuel delivery or oil circulation. When it's a primary
component, often it's something that's either suffered from poor operation
techniques (valves and pistons, for example) or a manufacturing defect
(crankshafts).

Turbines do suffer from manufacturing defects (if I recall, there was an
uncontained failure in the 90's on some rear-engine jet -- 727, DC-9 or
something like that -- where the blade failure was due to some metallurgical
problem). But they have stricter maintenance regimes (which more often will
catch problems with secondary components), and perhaps more importantly,
they have stricter operating standards and instrumentation to monitor
operation (for example, overtemp operation is strictly monitored and limits
specified, and if those limits are exceeded, the engine is automatically up
for inspection and/or repair).

Which is a long way of saying that I think it's entirely possible that if
you spent as much on a piston engine as you might spend on a turbine, and
followed similar practices with respect to operation and maintenance, you
could achieve similar reliability rates.

Pete