Steve wrote:
> Luke Scharf wrote:
>
> > Steve wrote:
> >
> >> But there are more and more small diesels that run just as fast as

> >> gasoline engines. There's no fundamental limit thats any different

> >> than a gasoline engine, really, but up until now there's not been
a
> >> demand for high-RPM diesels.
> >
> >
> > When I was reading about the Volkswagen TDI engine, I vaguely
remember
> > coming across someone who said that the redline of that engine was
set
> > by the speed which which the burning fuel expanded.
> >
> > Sounds rather fundamental to me - but, then again, I'm a computer
guy.
> >
> > -Luke
>
> In almost ALL real-world engines, the actual limit is set by the
point
> at which some mechanical component would fail. The engine's torque
*may*
> drop off well before the mechanical failure point if it can't ingest
> enough fuel or air at high speed. In the case of a diesel, you can
> pretty much increase the burn rate to as high as the mechanical parts

> can tolerate by increasing turbocharger boost (and injection rate to
> match). Since detonation isn't possible (no fuel exists in the
cylinder
> until combustion is supposed to begin anyway) the only limits to
boost
> pressure are mechanical in nature. In practical terms, no one really
> wants or needs a 9000 RPM diesel, though.


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.