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

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.

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.

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.

I could have sworn the Rabbit Diesel had a way different engine,
but I could be wrong.


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.

Not pickups AFAIK; station wagons and maybe sedans. This was Roger
Smith at his finest.

A friend bought one with a dead@55000 mile engine. It was an stock
gas block; no where NEAR beefy enough. The blowby was so bad, the
engine soiled itself at every seal; he'd get 250 miles to the quart;
all leakage. At least it didn't rust!

It had a one-of-kind starter and flywheel. The distributor was
replaced with a vacuum pump to drive the HVAC door flaps. It had
dual batteries, designed wrong. The brakes were run off the PS pump,
so when the engine stalled, stop NOW.

He put in a gas 350 and drove it for 10 years more.

--
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

In rec.aviation.owning David Lesher wrote:

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.

I could have sworn the Rabbit Diesel had a way different engine,
but I could be wrong.


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.

Not pickups AFAIK; station wagons and maybe sedans. This was Roger
Smith at his finest.

A friend bought one with a dead@55000 mile engine. It was an stock
gas block; no where NEAR beefy enough. The blowby was so bad, the
engine soiled itself at every seal; he'd get 250 miles to the quart;
all leakage. At least it didn't rust!

It had a one-of-kind starter and flywheel. The distributor was
replaced with a vacuum pump to drive the HVAC door flaps. It had
dual batteries, designed wrong. The brakes were run off the PS pump,
so when the engine stalled, stop NOW.

He put in a gas 350 and drove it for 10 years more.

--
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

There were pickups.

A friend bought one new and had the engine blow at about 40k miles.

Thanks to California smog laws, he found his choices were replace it
with another new diesel (big bucks) or get an old gas engine and convert
it to propane and try to recover some of the investment. This was way
before 50k warranties.

--
Jim Pennino

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wrote:



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

It is a problem. It is solved by heating the fuel.

writes:



There were pickups.

A friend bought one new and had the engine blow at about 40k miles.

Thanks to California smog laws, he found his choices were replace it
with another new diesel (big bucks) or get an old gas engine and convert
it to propane and try to recover some of the investment. This was way
before 50k warranties.


Err, my friend bought the car from an office neighbor, after it had
been stolen and partially stripped. [Wheels, radio... He actually
drove it home on 4 borrowed space-saver spares...]

About a month before, the injector pump croaked. Amazingly, when it
arrived at the dealership on a towtruck, the speedometer said 49,986
miles. I disclaim any knowledge as to how that could have been.

They bitched, but put in a new pump under warranty. [By that time,
I suspected they rather all the OlsmoDiesels went to Burning Man or
similar.] I can't recall what was [not] covered, but that pump
was.

The Diesels that last, the Mercedes 240D, for example, are group-up
designs. And they do weigh more than gas blocks.

Any talk about Jet-A jelling sounds....bogus.. There's an ongoing
issue with DC-9's and 'cold-soak' of the fuel; causing icing
on short turn arounds. You'd think they'd suffer from any jelling
but...
--
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

"Newps" wrote in message
...


wrote:



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

It is a problem. It is solved by heating the fuel.

.... and since hydraulic fluid needs cooling they locate the hyd.cooling
coils inside the fuel tank(s). That helps solve two problems.

Excerpted from other posts.......

Any talk about Jet-A jelling sounds....bogus..

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

It is a problem. It is solved by heating the fuel.


It is a problem on long flights at high altitudes and high
latitudes. The fuel filters on the Boeings that I flew
were heated to prevent the screens from "waxing" over.
The fuel itself was not heated. The filters were heated
with hot engine bleed air and heated for one minute every
thirty minutes when the fuel temperature dropped below zero
degrees celsius.

At PanAm, we had three procedures for dealing with extremely
low temperatures across the North Atlantic.

1. Re-route to a more southernly (warmer) route.
2. Reduce altitude to a warmer OAT.
3. Increase speed for a greater friction effect on the tanks.
At around M.80, the Ram Air Temperature is about thirty
degrees higher than the True Air Temperature.

All of these required extra fuel of course and we depended
on the Dispatcher providing a good Temp Aloft forecast.

Bob Moore
ATP B-707 B-727
PanAm (retired)

Aha,

But much of the cost is due to changing components other than the engine.

If you had a glass cockpit, and fuel system that were compatible to start
with, then all you would need to change was the engine, mount, prop, sending
units, and software.

That would seem to be less than what the europeans are giong through to put
the Theilert in a skyhawk.




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.

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