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

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In rec.aviation.owning Mike Rapoport wrote:

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The question remains, at what HP level, based on the physics of the
engines,
does the crossover from piston to turbine occur?

As additional criteria, assume specific fuel consumption is the most
important parameter and that the A/C spends the majority of its time in
flight not doing touch and goes.


I think that you can look at the market to see where the crossover
occurs.
THere are currently no production piston aircraft engines over 450hp and
there are no aircraft turbines under 400hp.

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.

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.

Mike
MU-2

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.

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

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

--
Jim Pennino

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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

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

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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.

Large mobile electric generators are another common ground-based
application. Don't forget that weight and size are also relatively
unimportant in these applications, which makes a lot of engineering
problems much easier.

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.

Airplanes are designed around engines. Want to know what a
piston-powered Caravan looks like? It's called a Cessna 402.

The 'van is a pretty idiosyncratic plane- basically a flying box
truck. Great for hauling a heavy load a short distance into a small
strip. Sure, there's a bunch of rich boys out there flying them
around, too, but I suspect economics do not factor into their decision
in any way. The guys putting these things on amphibious floats with
executive interiors could probably afford to operate them even if they
only ran on vintage Champagne. A mainstream pilot can get a hell of a
lot more utility out of a SR-22 or 206 for probably 1/3rd or less of
the costs.

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

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

Considering that all the aviation diesels are being built to run on
jet-A, I'd say it's going to stay right where it is.

The only compromise we have to make with the diesels is to give up a
little useful load, otherwise they are equal or better on all counts.
Why isn't that enough for everybody to be excited about?

Best,
-cwk.

"C Kingsbury" wrote in message
om...
wrote in message
...
In rec.aviation.owning Mike Rapoport
wrote:

wrote in message
...

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.

Large mobile electric generators are another common ground-based
application. Don't forget that weight and size are also relatively
unimportant in these applications, which makes a lot of engineering
problems much easier.

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.

Airplanes are designed around engines. Want to know what a
piston-powered Caravan looks like? It's called a Cessna 402.

The 'van is a pretty idiosyncratic plane- basically a flying box
truck. Great for hauling a heavy load a short distance into a small
strip. Sure, there's a bunch of rich boys out there flying them
around, too, but I suspect economics do not factor into their decision
in any way. The guys putting these things on amphibious floats with
executive interiors could probably afford to operate them even if they
only ran on vintage Champagne. A mainstream pilot can get a hell of a
lot more utility out of a SR-22 or 206 for probably 1/3rd or less of
the costs.

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

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

Considering that all the aviation diesels are being built to run on
jet-A, I'd say it's going to stay right where it is.

The only compromise we have to make with the diesels is to give up a
little useful load, otherwise they are equal or better on all counts.
Why isn't that enough for everybody to be excited about?

Best,
-cwk.

Why do we have to give up useful load? On most flights of any duration, the
savings in fuel required will more than make up for the increase in engine
weight (if any)

Mike
MU-2