Sport Pilot 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...).



Actually diesels don't really deliver that much torque at similar
speeds. Ok they do but only because the valving is timed for the
slower speeds. Many gasoline tractor engines will diliver similar
torque, but with a higher fuel consumption due to the lower compression
ratio.

It depends on what you're comparing to. A lot of tractors use
normally-aspirated indirect-injection diesels, which are no comparison
to a turbocharged diesel. Or to a good normally-asipirated gasoline
engine, except in terms of longevity. Gasoline engines are hard to beat
for the *width* of their torque curve, but modern turbo-diesels do a
good job against them, and do so with better efficiency.

Diesel fuel burns slower and will knock when the injection
timing is shortened to allow higher speeds.


Higher speed requires more than just rapid injection, it also demands
higher boost pressure, which causes the fuel to burn faster without any
knock-like phenomenon. All serious diesels are turbocharged for that
reason (as well as others), and normally-aspirated diesels are limited
to very small power outputs and relatively low efficiency. You can only
get so much combustion chamber pressure through the compression ratio of
the engine- the rest has to come from forced induction. As someone
pointed out earlier, the VW TDI develops power up to 4500 RPM, which is
comparable to many gasoline engines, but it is able to do so only
because it is a turbo-diesel. As I said before, there's no reason that
building a 7000-RPM diesel isn't possible, but there's no REASON to
build one.