Diesel aircraft engines and are the light jets pushing out the twins?

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?

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?

Roy Smith wrote:

Looking at it another way, is there some inherent reason why piston
engines are cheaper to produce?

The materials are cheaper and the tolerances (especially balancing) much looser.

Is there also some inherent reason why
they're less reliable?

The internal pressures are higher and the moving parts are constantly and rapidly
reversing direction.

George Patterson
If a man gets into a fight 3,000 miles away from home, he *had* to have
been looking for it.

"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

Peter Duniho wrote:

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.

Perhaps a study of the durability of engines used for things like APUs, rather than
aircraft powerplants would be informative. Such engines, both piston and turbine, are
likely to be only moderately well maintained.

George Patterson
If a man gets into a fight 3,000 miles away from home, he *had* to have
been looking for it.

If you really wanted to know...

You could likely compare figures derived from different models of armored
fighting vehicles. Also, military aircraft used to have a mix a long time
ago.


"Peter Duniho" wrote in message
...
"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

"Peter Duniho" wrote in message
...
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).

Sioux City DC10.

Paul

"Paul Sengupta" wrote in message
...
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).

Sioux City DC10.

Not actually the accident I'm thinking of. But yes, that's another example
of blade failure (did they eventually determine it was a manufacturing
defect, or a maintenance problem?).

The accident to which I was referring only involved one or two fatalities,
of a passenger or of passengers sitting right next to the engine.

Pete

I believe you are referring to a Delta MD-80/88 that
was taking off from Pensacola. I think there were
two killed and a couple of injuries.

Mike Pvt/IFT N44979 PA28-181 at RYY

Peter Duniho wrote:
"Paul Sengupta" wrote in message
...

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).

Sioux City DC10.


Not actually the accident I'm thinking of. But yes, that's another example
of blade failure (did they eventually determine it was a manufacturing
defect, or a maintenance problem?).

The accident to which I was referring only involved one or two fatalities,
of a passenger or of passengers sitting right next to the engine.

Pete

Mike H writes:

I believe you are referring to a Delta MD-80/88 that
was taking off from Pensacola. I think there were
two killed and a couple of injuries.


http://www.ntsb.gov/ntsb/brief.asp?e...08X06203&key=1

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A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

"Peter Duniho" wrote in message
...
"Paul Sengupta" wrote in message
...
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).

Sioux City DC10.

Not actually the accident I'm thinking of. But yes, that's another
example
of blade failure (did they eventually determine it was a manufacturing
defect, or a maintenance problem?).

They say it was a manufacturing defect about the size of a grain of
sand.

http://www.ntsb.gov/speeches/former/hall/jh970912.htm

"Metallurgical examination of the titanium fan hub revealed that a fatigue
crack originated from an inclusion near the surface of the hub's bore. The
inclusion had been formed during the titanium vacuum-melting process at the
time of manufacture about 2 decades earlier, which developed an internal
cavity during final machining and/or shot peening. At the time of
manufacture, the fan hub had been ultrasonic and macroetch inspected."

The accident to which I was referring only involved one or two fatalities,
of a passenger or of passengers sitting right next to the engine.

Yes, I know the one you're talking about.

It's mentioned on the page referenced above:
"We will soon conclude our investigation on that Delta Air Lines MD-88
engine failure I mentioned earlier. Metallurgical examination of the
fracture surface of that fan hub revealed that a fatigue crack had
originated from a machining defect in a tie rod hole. Further, the fan hub
had been fluorescent particle inspected only seven months before the
failure, when the crack was estimated to be approximately ½-inch long."

Also http://www.ntsb.gov/pressrel/1998/980113d.htm

Paul