From: StarAerospace@aol.com
Message-ID: <198.d2fc2d.296d538b@aol.com>
Date: Wed, 9 Jan 2002 03:04:27 EST
Subject: On to diesels...
To: lancair.list@olsusa.com
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Reply-To: lancair.list@olsusa.com

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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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