With the direct injection method would there not be a certain range of boost
     > in which the impluse would occur without spark or glow?  It seems that in
     > aircraft applications supercharging for that arrangement is doable because
     > we only run a very narrow range of RPM

Mike this is probably a true statement, but unless the boost
 is provided by a turbocharger, recovering wasted exhaust
 energy, it is unlikely to improve the efficiency. And when
using a turbo the heating of the intake charge causes the need
for intercoolers or possible problems with detonation. Most
omnivorous or multi-fuel engines go one way or the other. They
either run spark all the time (which is no loss if you need it to
start anyway) or are designed as Diesels that will tolerate gas
for a short time to get you out of a fix. As Francois mentioned
there is no great improvement without a large investment in
complex and expensive injectors, and then to no great benefit.
The availability of Jet A almost anywhere is a big plus, as is the
lower cost of Jet A. One other down side that most people forget
about if not Diesel savy is the increased weight of the fuel. Diesel
is heavier per gallon so you must add the additional weight to
your efficency equations. True Diesels are usually efficient
enough to overcome the fuel weight differential but the mechanical
complexity of high pressure pumps and controls makes these
engines marginal for aircraft. Diesels have the perfect powerband
for aircraft and most make peak power below 3000 RPM, so if it
was easy to extract the power at the same weight we would all
be flying Diesels now!  Unfortunatly it is not that easy. Witness
the long developement times of the diesels being advertized.
Bill Jepson

Heck Continental gave up.

Paul Lamar
 
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