2-stroke diesel is the (near) future?

Don Stauffer wrote:


I wouldn't say the ENTIRE expansion phase. That would not be that great
for efficiency- sort of like a steam engine running with no cutoff. And
if one injects fuel into the cylinder at the instant just before BDC,
that fuel would be pretty much wasted. I was under the impression that
the cycle would look something like the steam (Rankine) cycle in that
the way Diesel envisoned it it would be either constant enthalpy or
constant pressure of part of the expansion phase, adiabatic during the
rest. The point where it changed would correspond to what would be the
throttle opening on a steam or SI engine. At full "throttle" the
changeover would be late in stroke, light load early in stroke.

True, although diesel engine manufacturers of late have discovered that
they can do things like reduce the injection flow rate instead of just
reducing injection duration. That has the nice side-effect of REALLY
quieting diesel "rattle" at light throttle openings, as most clearly
evidenced by the current Cummins diesel in the Dodge Ram. The flow
rate/duration combination can be played with to do really neat things to
the torque curve also. The stumbling block that prevented this for many,
many years was the camshaft-driven fuel injection pump, and its limited
ability to vary injection timing and volume. Once the move was made to
computer control and use of electronically controlled engine
oil-pressure activated unit injectors, a lot of possibilities opened up.

Sport Pilot wrote:


As long as I get 90% or so right, I really don't give a damn.


Good luck on getting your next landing "90% right."

:P

Steve wrote:
wingsnaprop wrote:

Guess why?
*Duh* Pollution laws, and no need for the power to weight
requirements of Aviation! Duh... whats that got to do with this
topic?
Other than to show that 2 stroke Compression ignition engines are a
proven concept?


As already stated, 2-stroke diesels really don't have a
power-to-weight
advantage over 4-strokes. They still have to have a camshaft and
exhaust valves (they aren't like weed whacker engines, you know), so
they don't save that weight. Plus they have to have a blower for
scavenge air. The only area where they save weight is in that the
connecting rod and crank can be lighter, and that only helps offset
the
added weight of the blower.


I said something similar, but I don't know that a desiel has to have
the valve, as the old locomotive two strokes. Could it not be ported,
as the two stroke spark engines? The fuel is already oily so if the
bearings are sealed ball bearings, you may not have to add oil to the
fuel.

Not saying this is preferable, just possible.

Steve wrote:
Sport Pilot wrote:


As long as I get 90% or so right, I really don't give a damn.


Good luck on getting your next landing "90% right."

:P


I do, a good landing gear system is essential. Nobody makes a perfect
landing everytime.

Steve wrote:
Morgans wrote:

"Steve" wrote


As already stated, 2-stroke diesels really don't have a
power-to-weight
advantage over 4-strokes. They still have to have a camshaft and
exhaust valves (they aren't like weed whacker engines, you know),
so
they don't save that weight. Plus they have to have a blower for
scavenge air. The only area where they save weight is in that the
connecting rod and crank can be lighter, and that only helps offset
the
added weight of the blower.


How about the fact that they have power pulses in each revolution?
They
could possibly have half the displacement, and still get the same
power, (or
close to it) with less weight than the double displacement 4 cycle.
Yes,
the blower weight is added, but it is nice to make good power, way
up there.


The blower also takes away a significant chunk of crankshaft power.
The
blower has to do the same net work as those "non power" strokes in a
4-cycle diesel because its doing the same job- expelling burnt
mixture
and bringing in fresh air. You can't get something for nothing.

This is all old-hat. 2-stroke diesels have been in widespread use
since
Winton developed the basic foundation for what became both the EMD
and
Detroit Diesel 2-stroke engine architecture back in the 1920s.
2-strokes
became very simple to service and reliable, but they rarely won on
either fuel efficiency or total power output per unit weight. That's
why
you find 2-strokes in locomotives and ships where weight doesn't
matter
(or is a benefit), but they all but disappeared from on-road
applications by the end of the 1970s and DID completely disappear by
the
turn of the century.

I would have agreed at the start of this thread, but the two stroke
desiel does not have to be the same as the old locomotive desiels. The
blower is not needed if the crankcase is used to pump fuel/air mixture.

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.

Sorry about the two similar posts. I didn't think the first "took".

Sport Pilot wrote:

Steve wrote:

wingsnaprop wrote:


Guess why?
*Duh* Pollution laws, and no need for the power to weight
requirements of Aviation! Duh... whats that got to do with this

topic?

Other than to show that 2 stroke Compression ignition engines are a
proven concept?


As already stated, 2-stroke diesels really don't have a

power-to-weight

advantage over 4-strokes. They still have to have a camshaft and
exhaust valves (they aren't like weed whacker engines, you know), so
they don't save that weight. Plus they have to have a blower for
scavenge air. The only area where they save weight is in that the
connecting rod and crank can be lighter, and that only helps offset

the

added weight of the blower.



I said something similar, but I don't know that a desiel has to have
the valve, as the old locomotive two strokes. Could it not be ported,
as the two stroke spark engines?

In order to scavenge the cylinders properly, the inlet ports need to be
at the bottom of the cylinder and exhaust valves have to be located at
the top. The only viable alternative is the opposed piston engine (ala
Fairbanks-Morse) in which one piston uncovers an inlet port array and
the other uncovers the exhaust ports. But then you have the weight of an
additional CRANKSHAFT, without any increase in output power!

The fuel is already oily so if the
bearings are sealed ball bearings, you may not have to add oil to the
fuel.

I think you're confusing a weed-whacker/outboard motor type 2-stroke
with a 2-stroke diesel. A 2-stroke diesel has a closed crankcase just
like a 4-stroke.

Sport Pilot wrote:

Steve wrote:

Morgans wrote:


"Steve" wrote



As already stated, 2-stroke diesels really don't have a

power-to-weight

advantage over 4-strokes. They still have to have a camshaft and
exhaust valves (they aren't like weed whacker engines, you know),

so

they don't save that weight. Plus they have to have a blower for
scavenge air. The only area where they save weight is in that the
connecting rod and crank can be lighter, and that only helps offset

the

added weight of the blower.


How about the fact that they have power pulses in each revolution?

They

could possibly have half the displacement, and still get the same

power, (or

close to it) with less weight than the double displacement 4 cycle.

Yes,

the blower weight is added, but it is nice to make good power, way

up there.


The blower also takes away a significant chunk of crankshaft power.

The

blower has to do the same net work as those "non power" strokes in a
4-cycle diesel because its doing the same job- expelling burnt

mixture

and bringing in fresh air. You can't get something for nothing.

This is all old-hat. 2-stroke diesels have been in widespread use

since

Winton developed the basic foundation for what became both the EMD

and

Detroit Diesel 2-stroke engine architecture back in the 1920s.

2-strokes

became very simple to service and reliable, but they rarely won on
either fuel efficiency or total power output per unit weight. That's

why

you find 2-strokes in locomotives and ships where weight doesn't

matter

(or is a benefit), but they all but disappeared from on-road
applications by the end of the 1970s and DID completely disappear by

the

turn of the century.


I would have agreed at the start of this thread, but the two stroke
desiel does not have to be the same as the old locomotive desiels. The
blower is not needed if the crankcase is used to pump fuel/air mixture.


You're describing a weed-whacker engine, not a 2-stroke Diesel. Good for
cheap manufacture and relatively light total weight, but at the expense
of a very narrow power band, terrible efficiency, terrible emissions,
and except at the peak of the power band, terrible power/weight ratio in
spite of being lightweight.

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...).