Doug and Patty wrote:

What would be required to run on jet fuel or diesel ? Could a guy start it
on gas and switch over - or is there no advantage to running jet fuel?
Thanks, Doug

I have heard of that being done. 

Wankel Gmbh Germany makes a rotary engine with
direct high pressure fuel injection and a spark plug. You can 
probably see one run at OSH in their booth.  That particular
engine does not get much better fuel burn than the gasoline version
because the compression ration is about the same as the gasoline version.

In order to show a fuel burn advantage a diesel engine must run 
a much higher compression ratio. In that case the entire structure
of the engine must be beefed up with more material. For this
reason a heavy duty truck diesel generating only 250 to 300 HP weighs
about 2000 pounds. 

I have always maintained that a truck diesel in a low wing
loading aircraft would make a good banner tow aircraft.
Just replace the AC engine and hopper in a duster aircraft
with a truck diesel.

The Renault diesel aircraft engine is artificially subsidized in 
Europe because of fuel taxes. Diesel fuel, unlike the US, is half the 
price of gasoline in Europe strictly because of taxes. The real cost
of diesel fuel is about the same as gasoline. The Renault will not need
a high compression ratio to succeed in Europe. All it need do is burn 
diesel fuel.

Back in the thirties through 1944 the Germans built quite a lot of
two cycle true diesel aircraft engines with high compression ratios. 
They found that the extra weight of the true diesel engine reduced 
the payload of the aircraft making the better fuel burn not 
worth it for all normal use. Only in extremely long range, 
slow aircraft with low wing loading did the Germans 
use diesel engines. Later, after the war, the turbo compound
aircraft engine matched the fuel burn of the diesel without
the weight penalty and consequently set many un-refueled  
distance records.

Packard in the US also built diesel aircraft engines in 1928
and it was as light as a gasoline engine at the time. They did 
this by using only one valve  that served for both intake and exhaust. 
Consequently no exhaust manifold was possible. This 
meant the exhaust was dumped straight out into the air stream
and "the smell drove drove users out of their minds" :-) according 
to Herchel Smith author of A History of Aircraft Piston Engines.

BTW I can highly recommend this book published by Sunflower University
Press in Manhattan Kansas.
  
Paul Lamar
 
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