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2-stroke diesel is the (near) future?



 
 
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  #71  
Old May 16th 05, 06:43 PM
Sport Pilot
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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.

  #72  
Old May 16th 05, 06:49 PM
Sport Pilot
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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.

  #73  
Old May 16th 05, 06:52 PM
Sport Pilot
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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.

  #74  
Old May 16th 05, 07:01 PM
Sport Pilot
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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.

  #75  
Old May 16th 05, 07:04 PM
Sport Pilot
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Sorry about the two similar posts. I didn't think the first "took".

  #76  
Old May 16th 05, 09:03 PM
Steve
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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.

  #77  
Old May 16th 05, 09:05 PM
Steve
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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.

  #78  
Old May 16th 05, 09:08 PM
Steve
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Posts: n/a
Default

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...).

  #79  
Old May 16th 05, 09:30 PM
Sport Pilot
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Posts: n/a
Default


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

  #80  
Old May 16th 05, 10:45 PM
Steve
external usenet poster
 
Posts: n/a
Default

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.
 




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