Hi Gang,
With the recent discussion about the Innodyne turbine and
diesel aircraft engines in general there has been some misinformation about the
inherent advantage of Jet A over 100LL with regards to energy content. Here is
an excellent link to a Chevron
web site that covers everything you might want to know about various
aviation fuels. In particular, the energy density of jet fuel, on a BTU/Gal
basis does show about an 11.8% advantage over Avgas. On the other hand, if you
look at the energy density on a BTU/Lb basis, Avgas has the advantage by about
1%.
So where does the supposed 30% energy advantage of diesel engines
over Avgas engines come from? Do diesel engines really enjoy a 30% efficiency
advantage over aircraft engines? In terms of BSFC, they are starting from a 1%
disadvantage solely on the basis of the energy content per pound of fuel.
Granted, diesel engines operate at a much higher compression ratio (the
Thielert Centurion 4.0L engine is 18.5:1) than our Avgas engines, but is that
difference sufficient to extract more than 30% efficiency. I don’t think
so.
Thielert claims a 0.36 BSFC for their 1.7L turbocharged
diesel engine with a compression ratio of 18:1. The GAMI folks report that our turbocharged
Avgas engines typically achieve a BSFC in the range of 0.41-0.43 when operated
LOP. This works out to a 16% efficiency advantage per pound of fuel for the
turbo-diesel engine.
Taking it a step further, when we fuel up our diesel powered
airplane, our fuel tanks will hold the same number of gallons of fuel whether
it is Jet A or 100LL, but the Jet A will weigh 17% more than a load of 100LL.
Ahh, this is how we get the 30% advantage over our 10LL engines. When we
combine the 16% greater load of BTUs per wing with the 16% better efficiency of
a turbo-diesel engine, we do, in fact end up with an aircraft that has a 30%
greater potential energy conversion than our current rides. Now the question
is, can we really go 30% farther between fill ups?
Wow, that 350HP, 4.0L certificated (in Europe)
turbo-diesel engine is starting to sound pretty good. Too bad it weighs 600
pounds, although Thielert claims the installed weight is comparable to a TCM
engine. Time will tell…
Best Regards,
Mike
http://www.chevron.com/products/prodserv/fuels/bulletin/aviationfuel/2_at_fuel_perf.shtm
http://www.centurion-engines.com/c40/c40_data.htm