This paragraph struck me as being less than informative about diesels. I
was expecting the false notion of  "GM's converted gas 350", but it
didn't come. Anyway, your comparison the diesels of a couple generations
ago is like assuming fuel injection for gas engines is not here
(mechanical or electronic.) The current Cummins diesel 5.9L has 305 Hp @
2900 rpm and 555 Ft. Lbs @ 1400 rpm. Compare this to the Dodge gas HEMI
5.7L 330 Hp @ 4800 rpm and 375 Ft. Lbs @ 4200 rpm. Aside from the small
weight differences, couldn't this current generation diesel turn a much
coarser prop and give you shorter take-offs (assuming the right prop for
each engine's required rpm) ?
-dave

"Them Diesels make more torque than gas engines". This is a true mistake
of perception,brought on by comparing big truck Diesels with car
engines. This myth has been at least Partially discredited by General
Motors unfortunate 350 inch "Olds" Diesel. The truth is, that for the
same displacement and manifold pressure, the Diesel will provide
significantly less torque (and horsepower) than the equivalent gasoline
engine. The reason for this is the fact that direct injection has
problems providing a uniform mixture ratio distribution, such that at
full throttle you run into the "smoke limit". This is typically at a
much leaner mixture than the near stochiometric operation at full
throttle in a gasoline engine. If you can't usefully burn the fuel, you
cannot make the power.

I agree! It was rather an inadequate paragraph.

It is a big weight difference. In the case of the Renault aircraft diesel
it is about a 25% weight increase. The reason for this is the robust nature
of the true diesel engine to withstand the 20:1 compression ratio.
This cuts another way in that it limits the RPM compared to a lower
compression gas engine due to higher inertial forces as a consequence of
the piston engines reciprocating motion. This results in a 350-450 HP over
the road tractor trailer truck engines to weigh in the neighborhood of 
2000 pounds. God knows what a 650 HP road train tractor diesel engine weighs? :)
BTW See the picture below I took at Mount Isa in the outback.

Most pickup diesels on the other hand are somewhat lighter duty engines.

As I recall there is another, probably more important problem, with  
diesels. It is high speed burning that also limits the RPM. I am sure that 
Vance can fill us in on this. 

OK given the RPM limits for a given HP at a lower RPM the torque MUST BE 
higher. It is mandatory. No two ways about it.  It is a law of physics and 
not just a good idea. HP = T X RPM X K  You make RPM lower and torque
MUST go up to keep HP the same. Unfortunately airplanes fly on HP,
HP to weight ratio and not torque alone.

Paul Lamar
 
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