Don Stauffer wrote:

Steve wrote:

Sport Pilot wrote:


Diesel fuel is not conducive to high speed running. Nor is a long
injection period through much of the expansion phase. Yes you can
boost the turbocharger and the other things, but an Otto cycle engine
will respond with even higher speeds. Parts failure from speed is not
a problem with diesel engines, the rotating parts have to be bigger
than an otto engine because of the higher compression, yet the otto
engine will turn higer RPM's with smaller parts.



All of the above is true in the common practice of diesel design, but
none of it is necessarily true. There is nothing FUNDAMENTAL that
limits a diesel to low-RPM designs only. One can build a screaming
high-RPM diesel with light-weight rotating parts, but one would have
to ask "why?" Gasoline engines are made high-revving in order to
increase power output from a small package, but diesels can develop a
lot more low-RPM torque through high boost because they don't detonate
when "lugged," so there's no NEED to make them scream. If you need
more power, don't spin them faster, just boost them harder. High RPM
is an aggravation, not an advantage (no matter what Honda VTEC drivers
think...).

I would assume that use of hydrogen as a fuel would allow very high rpm
in a Diesel.

But no one has come up with a completely satisfactory way to do direct
injection with a gaseous fuel, which is the same problem diesels that
run on natural gas face.

NG diesels are interesting beasts. They actually mix the gas with the
intake air and compress it like a spark-ignition engine would, setting
up the possibility of detonation, but since NG is has a very high
relative "octane" rating, it doesn't ignite until a *tiny* shot of
diesel is injected to initiate combustion. They do have to operate at
somewhat lower compression than a straight diesel, but its still in the
neighborhood of 14:1 or 15:1 which is much higher than you can achieve
with gasoline, at least on any fuel short of leaded aviation racing fuel
that is brewed up in ridiculously small (and expensive) quantities for
the Reno air racers.