Here are a couple of posts from theSprinter list at Yahoo explaining diesel torque.
Oh, and one reason a diesel has so much torque is because the fuel burning cycle is
> much, much longer than on a gasser. Instead of a single combustion event when the
> plug fires in a gasser, the fuel in a diesel keeps burning as it's being injected -- the
> added torque is because the fuel keeps burning (and pushing the piston down)
> through more of the power stroke. The sound of a diesel is the multiple shock waves
> colliding in the combustion chamber as the fuel is injected. The reason newer diesels
> are quieter is because the current generation of electronic fuel injection is much
> better at controling fuel flow and a lot of fuel can be quickly injected thus reducing
> noise.
>
> And while we're at it -- some folks think the limited RPM range of a diesel is due to
> it's greater rotating mass. While that plays a part, the main reason is the slowness of
> diesel combustion -- if the engine is turning too fast you can't burn all the fuel. Many
> diesel engines in commercial trucking have 2 rpm ranges -- one while under power
> and the other while in "jake" braking mode. In the latter fuel burn rates aren't an issue
> (because fueling is turned off the the valve action changed to create an air
> compressor in the engine) and the engine is allowed to turn faster to provide more
> braking effect.
>
> -Mike
>
There are many factors that come in to play regarding torque output in a
diesel engine. As mentioned, the combustion phase is of longer duration
relative to to the cycle of the piston on the downstroke. Overall, the
mean effective pressure in the cylinder of a diesel engine is much
higher. In addition, diesel engines, in comparison to gasoline engines,
generally have a longer stroke. This is needed to accomplish higher
compression ratios. The longer the stroke, the more offset the
crankshaft pin is from the centerline of the crankshaft. This means the
connecting rod can exert more leverage to turn the crank as the piston
descends on the power stroke. Torque is directly related to the force
on the piston and the length of the moment arm, the crankpin offset
in this case. In addition, a gasoline engine develops its highest
cylinder pressure much closer to TDC, top dead center. At this point,
the moment arm is the shortest with respect to the connecting rod
and crank pin. In reality, a gasoline engine develops its highest
pressure some point after TDC, but is still at a much less optimal
point relative to the crank arm position to develop the most torque.
--
Ray Muno
http://www.aem.umn.edu/people/staff/muno
University of Minnesota e-mail:
muno@aem.umn.edu