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

A diesel doesn't cost any more that a piston engine. A STC'd conversion
costs more but, in a new airplane the cost should be the same.

Mike
MU-2


wrote in message
...
In rec.aviation.owning Mike Rapoport wrote:

wrote in message
...
In rec.aviation.owning Mike Rapoport
wrote:
The Caravan has a 940hp engine flat rated to 675hp. Turbines are
typically
flat rated so that the engine can make rated power to reasonable
altitudes
and temperatures without having to design the gearbox for the full
thermodynamic horsepower. To keep the comparison with piston engines
apples
to apples you need to use thermodynamic ratings.

http://www.pwc.ca/en/3_0/3_0http://w.../3_0_2_1_2.asp

OK, that explains that.

To put some numbers on things, the engines in my MU-2 have a specific
fuel
consumption of .55lb/hp/hr and a piston engine is about .45 and
diesels
can
be under .40. Huge (ship) diesels can be under .30. Compare your
model
aircraft engines with the TFE731-60 used on the Falcon 900EX which
uses
.405lb/lb thrust/hr

Aha, numbers!

So if one assumes the motivation to switch from a piston to a turbine
is
the price of gas is roughly twice Jet-A, the crossover point would be a
turbine that did about .8 (to allow for the weight difference in the
fuels).

Any idea how small (in appropriate terms of hp) current technology can
make
a turbine with that consumption?


--
Jim Pennino

That would be the economic crossover point if the engines cost the same.
Of
course a plane that needed twice the fuel (in lbs) to achieve the same
performance wouldn't have much useful load or range.

Dropping a diesel in an airplane costs a bunch. The justification is the
cost is recovered in lowered fuel costs.

Your second point is certainly valid though and a minor problem with the
diesels according to the AVweb article on them.


--
Jim Pennino

Remove -spam-sux to reply.

wrote in message
...
In rec.aviation.owning Mike Rapoport wrote:

wrote in message
...

snip

There's lots of ground turbines under 400hp so we know there's a market
there; i.e. they must be practical and competive with pistons or they
wouldn't sell.


A lot of them are used to power natural gas compressors way out in the
middle of nowhere and reliability is much more important than fuel
efficiency and you have a large suitable fuel supply availible.

True.

So put it this way, if it were the turbine makers instead of the diesel
makers that jumped on this bandwagon, what would be their smallest
engine?

Given the high initial cost of turbines and the hgiher fuel comsumption,
I
doubt that turbines would be competitive with gasoline engines given
current
price differentials between the two fuels. The beauty of a diesel
aircraft
engine is that it should cost the same as a gas engine, has fewer parts,
uses less fuel and lasts longer. The turbine engine is more reliable but
costs more and uses more fuel. The lower the hp the less competitive the
turbine gets against the diesel.

Your first sentence overlooks the fact that turbines are currently
competitive at the Caravan level, but I pretty much agree with the
rest.


I don's see that I've overlooked something relative to the Caravan. The
Caravan has a 940hp engine. There is currently no suitable piston engine to
power such a large, single engine airplane. It couldn't be anything other
than a turbine.


OK, let's say I buy into about 400hp as the "up to now" crossover point.

Even 400hp is not an economic crossover. It just represents the limit of
what is practical in small aircraft turbine engines. The 400hp Allison
turbine is really a helicopter engine anyway. The smallest practical
application seems to be the around the Meridian/Caravan/TBM 700 size range
and these engines are all around 1000hp. The engineers designing airplanes
are not totally stupid, if it made sense to install 400hp turbines they
would do so.

Given the current fuel cost differential, where would you expect that
point
to move to assuming the engines were available?

It depends on how powerful diesels get for aircraft. Under several thousand
horsepower the diesel will always be cheaper and more fuel efficient than
anything else. There probably isn't an economic crossover point for
gasoline engines either unless the fuel price spread is artificially raised
even higher than it is now. You have to remember that the HSI and overhaul
costs on turbines is much greater than the cost of overhaul on a piston
engine. Given that the small turbine is going to consume a lot more fuel
and cost more to build and maintain it will never be cheaper.

Turbines will be used in applications where cost is a secondary
consideration to high power and high reliability. The gas turbine is a
mature 60yr old technology, huge improvements in cost or efficiency are
somewhat unlikely.

For a really efficient turbine see http://www.turbokart.com/about_ge90.htm


Jim Pennino

Remove -spam-sux to reply.

"Mike Rapoport" wrote:
A diesel doesn't cost any more that a piston engine. A STC'd conversion
costs more but, in a new airplane the cost should be the same.

Right. Some people seem to think that diesels are somehow magic. The
basic construction of a diesel and a gasoline engine are almost
identical. The only differences I can think of a

1) Higher compression ratio. This could be done with a longer-throw
crankshaft, a taller piston, a lower head, or some combination of all
three.

2) A fancier (higher-pressure) injector pump.

3) No spark plugs. Which means no ignition system (be it electronic or
magnetos).

4) Possibly the addition of some kind of starting assist such as glow
plugs.

The biggest problem I can see with a diesel is cold-weather operation.
I used to have a diesel car (1980's era VW Rabbit). It was a bitch to
start in really cold weather. If the glow plugs were in good shape, you
were fine down to about 20 F. Once you got down below about 10 F, you
probably weren't going to get it started without a preheat.

Somewhere down around 15 F, normal diesel fuel starts to gel. These are
temperatures commonly experienced aloft even at the altitudes spam cans
fly at in the winter in temperate climates. It would be real bad news
to get the engine going, only to have the fuel gel up in the tanks when
you reached cruising altitude. But, I suppose the Jet-A folks have
figured out the right additives to solve that problem.

In rec.aviation.owning Roy Smith wrote:
"Mike Rapoport" wrote:
A diesel doesn't cost any more that a piston engine. A STC'd conversion
costs more but, in a new airplane the cost should be the same.

Right. Some people seem to think that diesels are somehow magic. The
basic construction of a diesel and a gasoline engine are almost
identical. The only differences I can think of a

1) Higher compression ratio. This could be done with a longer-throw
crankshaft, a taller piston, a lower head, or some combination of all
three.

2) A fancier (higher-pressure) injector pump.

3) No spark plugs. Which means no ignition system (be it electronic or
magnetos).

4) Possibly the addition of some kind of starting assist such as glow
plugs.

The biggest problem I can see with a diesel is cold-weather operation.
I used to have a diesel car (1980's era VW Rabbit). It was a bitch to
start in really cold weather. If the glow plugs were in good shape, you
were fine down to about 20 F. Once you got down below about 10 F, you
probably weren't going to get it started without a preheat.

Somewhere down around 15 F, normal diesel fuel starts to gel. These are
temperatures commonly experienced aloft even at the altitudes spam cans
fly at in the winter in temperate climates. It would be real bad news
to get the engine going, only to have the fuel gel up in the tanks when
you reached cruising altitude. But, I suppose the Jet-A folks have
figured out the right additives to solve that problem.

Because of the higher compression ratio, a diesel has to be built
"beefier" than a gas engine to last as the automakers found out when
they tried a direct conversion on their gas engines in the 80's.

All the aircraft diesels have a constant speed prop and FADEC.

If gelling of Jet-A were a problem, airliners would be falling out of
the sky on a regular basis.

--
Jim Pennino

Remove -spam-sux to reply.

wrote:
Because of the higher compression ratio, a diesel has to be built
"beefier" than a gas engine to last as the automakers found out when
they tried a direct conversion on their gas engines in the 80's.

Well, the Rabbit I had was built with exactly the same block, pistons,
crank, etc, as the gas version. The basicly just slapped a shallower
head on the thing to increase the compression ratio.

We drove the car into the ground at about 160k miles. We replaced
pretty much all of the accessories (starter, water pump, alternator,
radiator, etc) at least once, and the clutch wore out at about 110k, and
the body was more rust than steel, and the electrical system was a mess,
but the core engine was just fine.

The only thing that ever happened to the engine core was a blown head
gasket, but that was really my fault. We had chronic overheating
problems due to a leak in the cooling system that we didn't fix for a
while. Eventually, the gasket said, "OK, if you want to keep abusing me
like that, I'm outta here".

In rec.aviation.owning Roy Smith wrote:
wrote:
Because of the higher compression ratio, a diesel has to be built
"beefier" than a gas engine to last as the automakers found out when
they tried a direct conversion on their gas engines in the 80's.

Well, the Rabbit I had was built with exactly the same block, pistons,
crank, etc, as the gas version. The basicly just slapped a shallower
head on the thing to increase the compression ratio.

We drove the car into the ground at about 160k miles. We replaced
pretty much all of the accessories (starter, water pump, alternator,
radiator, etc) at least once, and the clutch wore out at about 110k, and
the body was more rust than steel, and the electrical system was a mess,
but the core engine was just fine.

The only thing that ever happened to the engine core was a blown head
gasket, but that was really my fault. We had chronic overheating
problems due to a leak in the cooling system that we didn't fix for a
while. Eventually, the gasket said, "OK, if you want to keep abusing me
like that, I'm outta here".

The debacle I'm talking about was Chevey's (?) attempt to power pickups
with a gas engine converted to diesel by basically the same method.

You can get away with this if the basic engine is strong to start with
and you're not trying to pull too many horses out of it.

--
Jim Pennino

Remove -spam-sux to reply.

"Mike Rapoport" wrote in message
nk.net...
I don's see that I've overlooked something relative to the Caravan. The
Caravan has a 940hp engine. There is currently no suitable piston engine
to
power such a large, single engine airplane. It couldn't be anything other
than a turbine.

As you yourself pointed out, that 940hp engine is derated to 675hp. You
don't need a 940hp piston engine to provide the equivalent power, and a
675hp piston engine is not out of the question (for example, the Orenda V8
turbine replacement engines are in that ballpark, if I recall correctly).

Of course, the Orenda design is a good example of the general philosophy
that piston engines are more efficient, and cheaper to own and operate.
After all, why would anyone replace a turbine with a piston engine, if the
piston engine weren't cheaper? So I'm not saying this somehow disproves
your point...I'm just saying that you need to make sure you compare apples
to apples (and claiming that you need a 1000hp piston engine to do the same
thing a 1000hp turbine does is not comparing apples to apples).

[...] The engineers designing airplanes
are not totally stupid, if it made sense to install 400hp turbines they
would do so.

I agree the engineers are not totally stupid. I disagree that just because
it hasn't made sense so far, that it will not make sense in the future. It
really just depends on what factors influenced the original decision.

Am I saying that I think it will make sense in the future? No...I don't
know enough about the technology to be able to answer that question myself.
But so far, the people who do know about the technology haven't provided any
information that would suggest to me that the future will be completely void
of lower-power turbines.

[...]
horsepower the diesel will always be cheaper and more fuel efficient than
anything else. There probably isn't an economic crossover point for
gasoline engines either unless the fuel price spread is artificially
raised
even higher than it is now. You have to remember that the HSI and
overhaul
costs on turbines is much greater than the cost of overhaul on a piston
engine. [...]

Would a HSI cost the same on a smaller turbine? Does a HSI cost the same
for the PT-6 as it costs for whatever gargantuan engines the 777 uses?

Inspections and overhauls for piston engines generally scale up with engine
size, so it seems to me you need to compare apples to apples by comparing
the cost of a HSI and/or overhaul with the cost of an inspection on a
similarly powered piston engine.

Turbines will be used in applications where cost is a secondary
consideration to high power and high reliability. The gas turbine is a
mature 60yr old technology, huge improvements in cost or efficiency are
somewhat unlikely.

The same thing could theoretically be said about piston engine technology.
I suppose, in fact, that's one of the most compelling arguments in favor of
your claim: all of the engine technologies are relatively mature, so it's
reasonably safe to compare cost/benefit ratios at this time and assume that
they will remain similar in the future.

But can we be *sure* of that? You might think you can, but I'm not going to
claim that I can.

Pete

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