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<< Lancair Builders' Mail List >>
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<< 310HP continuos and has a torque of 750NM at 2500 RPM !! It needs
about 40l (10gallons) per hour. <<
Really ? 10 gallons of diesel per hour ? About 60 lbs or so per hour?
10 gallons of diesel per hour at the 270 Hp you required of the 300+ Hp
diesel engine?
That is a Brake Specific of ~ 0.22 lbs fuel/hour/horsepower. >>
Actually, since this was quoted in liters we need to do our conversions from
there.
40 l is ~10.5 gal. US and this is ~71 lb. (diesel, not avgas). At 270 HP,
this would still be an unbelieveable .263 lb./HP/hr SFC, but looking at the
post he didn't say what power level that fuel flow was for.
More realistic SFC for modern diesels is in the mid to high .3's for normally
aspirated and low to mid .3's for turbocharged. Some stationary
heat-recycling type diesels can even achieve SFC in the high .2's (this was
actually done over 30 years ago). Modern diesels may be compression
ignition, glow plug / stratified combustion, or even spark assisted, but the
one thing they are NOT is a "diesel cycle" where fuel is burned at constant
pressure during the entire power stroke. Modern diesels are all true Otto
cycles: all the combustion takes place at or near TDC creating very high
peak pressures and allowing maximum extraction of energy from the charge
during the full power stroke.
But we are not burning diesel, we are burning Jet A; a cruddy mix of mostly
kerosene with some junk gasoline thrown in to increase the energy density and
make light off in turbine engines easier. Gasoline has 5 to 8% more energy
per lb. than kerosene, but kerosene is denser so a gallon of kerosene has 4
to 6% more energy than a gallon of gasoline (standard day, sea level
conditions). Oh, yeah, add deicing fluid (Jet A is hygroscopic and that
water freezes out at the worst times...) and algaecide (things just LOVE to
grow in that water); don't let it sit around forever, turbine operators
don't have problems with this because the burner cans pass the s--t easier
than piston engines and turbine guys tend to fly more hours per year.
Figure losing 3 to 6% of your diesel SFC if you run Jet A in the same engine.
Jet A also will not take the compression that diesel will take, so take that
down a notch and lose another few percent. My bet for success in this area
will be stratified combustion to maintain high compression ratios and (the
inverse) high expansion ratios. Efficiency of heat engines (Turbine, Otto,
Diesel, Sterling, Brayton, etc.) is simply peak temperature and pressure of
the working gas times the expansion ratio minus losses. Lots of the
"mysterious diesel efficiency" is really just the higher compression ratio at
work in the equation.
Back to the VW engine, we will probably find it operates at ~.36 lb./HP/hr.
for about 197 HP from 10.5 gph (40 l). Much better than we are used to, the
next questions are can it run at 270 continuous at FL250 and what does it
weigh? If the RPM is really 2500 at power and altitude, no reduction is
needed but a thrust and prop support housing must be added. Be sure to count
that in the total, all accessories weight (and then try to get an honest
weight on a Lyc or Con <LOL>) before comparing apples to apples.
IMHO, aviation turbo-piston-Otto-cycle Jet A (not "diesel") engines are
something that will come and are on their way. But a plug-in replacement for
the TSIO-550 and TIO-540 isn't here quite yet.
Eric
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