2-stroke diesel is the (near) future?

On 16 May 2005 11:01:36 -0700, "Sport Pilot"
wrote:


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.


Ever seen a "super modified" pulling tractor???
I think, from the sound of one at full tilt, that 9000 RPM would be on
the low side - turboed, and sometimes also supercharged
(turbo-compounded) they scream like a wildcat in heat.

wrote:
On 16 May 2005 11:01:36 -0700, "Sport Pilot"
wrote:


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.


Ever seen a "super modified" pulling tractor???
I think, from the sound of one at full tilt, that 9000 RPM would be
on
the low side - turboed, and sometimes also supercharged
(turbo-compounded) they scream like a wildcat in heat.


Compare that maximum to the gasoline maximum. Formula 1 cars will
exceed 20,000 RPM.

"Steve" wrote

In practical terms, no one really
wants or needs a 9000 RPM diesel, though.

Oh, I don't know. NASCAR might be interested in going diesel ! g
--
Jim in NC

Steve wrote:

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.

Actually, detonation can and does occur in Diesels. With today's Diesel
fuels and injection methods the fuel does not burn INSTANTLY upon
injection. The droplets are too large. The droplets tend to evaporate
from their surface area. It IS possible for unburned gases evaporated
from the fuel to exist in the chamber and detonate in local regions. The
effect is nowhere as severe as the detonation that occurs in spark
engines at times. Interesting traces made with high frequency pressure
sensors indicate these local "peaks" in combustion pressure when this
occurs. It is the cause of the "tinkling" sound that Diesels
occasionally make.

Detonation is not a full or nothing situation. Local regions can
undergo detonation even while the majority of the charge undergoes
normal wavefront burning, in either Diesel or SI.

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.

Thought I had responded to this before, but cannot find it.

There IS a fundamental reason diesels do not turn as many revolutions
as a gas engine. Injecting the fuel throught most of the expansion
cycle prevents speed, but does give a constant push. You could shorten
the injection so that the end of the injection is closer to TDC, but
then it would be more of an otto cycle. If you put too much fuel at or
near TDC then you would have the same problem as an otto engine with
high compression and low octane fuel. Diesel fuel is not high octane.