Stan Blanton wrote:

Will the BSFC of a turbo engine be higher than that of a normally aspirated one(at the same HP)? For example at 250 HP would a 13B turbo or a 20B NA have lower fuel consumption?

Thanks,

Stan Blanton

A good question Stan. I don't know. We don't have any data so far. Here 
is what I suspect. If a variable pitch prop is used with a turbo engine 
the same HP can be had at lower RPM where in theory the engine friction 
is lower. That means the prop pitch can be increased to provide the 
same cruising speed.

In general here are my feelings on rotary BSFC.

We think, after hundreds of hours of testing, the BSFC of a NA 13B
in real world cross country flights  is .47 running 100 degrees lean
of peak. It happily runs on just about any auto fuel or 100 LL as long as 
it does not eat your fuel tank. 

The 13B BSFC was done in actual flight test over and over again knowing the drag 
polar on the RV4. It was rather conservative because the drag of the test 
RV4 was probably higher than a stock RV4 due to rather poor design of the 
cooling system and an external muffler. Side by side flights of a 13B and
an O-320 or O-360 RV4s were also made.

Tracy routinely makes OSH from Florida 1100 miles in seven flying hours 
with one fuel stop on the stock 32 gallon RV4 fuel tanks.

Piston BSFC numbers below .38 claims are rather doubtful as that is diesel
territory. If that were the case why build a diesel? It is easy
to make fantastic BSFC claims because it is nearly impossible for the 
average user to verify the numbers. Also you won't see these numbers
with low compression ratio AC engines so expensive 100 LL fuel is mandatory.

0.38 is also the number claimed for the Continental liquid
cooled engine used in the Rutan Voyager and turbo compound radials. Also 
all those numbers are based on a perfect lean of peak operating point which 
is difficult to maintain manually. Screw up by adding too much power and you 
could burn a $250 Lycoming exhaust valve. So far there is no real effective 
electronic fuel injection system for air cooled aircraft engines that 
I know about. Its a shame because that would add greatly to the real world
life of aircraft engines. Most car engine computers routinely control the 
mixture to close tolerances for smog reasons. However close mixture tolerances
are not necessary with the 13B as there are no exhaust valves to burn.

At this point it make little difference in operating cost or range anyway.
It is just a numbers game. .45 to .40 is a 12% reduction in fuel burn.
First you must pay twice as much for 100LL fuel. If you fly 50 hours a year
and burn 10 gallons an hour your fuel cost for 100 LL will be around
$1000. 12% more is $120. You could lose a lot more than that if you
burned a piston engine exhaust valve. If you ignore a leaking exhaust 
valve the valve cooling will be seriously degraded with the possibility 
of a dropped valve head and catastrophic piston engine failure. 

People will gladly pay a lot more for the engine and tolerate a BSFC 
of .65 for a turbo prop engine just for the extra reliability.


In any event the RX8 engine soon to be announced is incrementally improved
in lower fuel consumption.

Paul Lamar
 
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