h a l f b a k e r yWhat was the question again?
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Modelled is a 2-stroke with a perforated cylinder. Filtered intake air is continuously fed through the perforations at the calculated post-combustion pressure (to keep air and any particulates from flowing backwards immediately following combustion).
So, to follow the cycle: combustion occurs and
the piston moves down. Some air is still entering the cylinder (though not much against the cylinder pressure) which cools the cylinder walls and helps burn off any HC's hanging around. Piston hits BDC and the exhaust valve opens, exhaust rushes out and, relieved of the pressure, the air comes in faster. Partways through the upstroke the exhaust valve is closed.
Perforation size and placement determined by airflow requirements. Pressure determined dynamically by calculating maximum combustion chamber pressure. Cylinder wall and piston cooling done by the intake air.
(not saying it would work *well* mind you)
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Is any of that influenced by the droplet size or surface tension of the fuel? If the perforations are larger than the droplets, i would expect the vapour to go into them and either explode, which would widen the capillary, or fail to ignite entirely. Surface tension would also seem to me to be important because of capillary action. I have a feeling this could become a major problem unless you make the holes very small and have relatively strong surface tension or don't use a liquid fuel. Maybe you could use custard powder. |
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This has been Baked in a variety of 2-stroke designs since about 1930. |
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You're not going to be able to pump a significant
volume of air against the combustion pressure and
still produce a net power output. It'll take too
much energy away from the engine. |
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Thermo:
Work = integral pressure d Volume |
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What is the real purpose of adding air during
combustion? Yes, a leaner mixture is generally
more efficient, and you'll reduce hydrocarbon
emissions. (You'll also increase NOx emissions in
the process, but we'll worry about that later.) |
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A stratified charge system would probably be a
better way to accomplish what you have in mind.
The idea is to richly burn the fuel off in one corner
of the chamber and let it spread lean once the fire
is lit. To accomplish this with a two-stroke, you're
going to need a direct injection system. (The
volume of fuel is insignificant versus the volume of
air, and that's why pumping against some of the
compression pressure can still be efficient in this
case.) |
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Basically it's cooling, lubricating, extending the power stroke and improving the burn efficiency all in one :) |
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Except at maximum combustion pressure, the air is always flowing in. This takes care of lubrication for the piston and cooling for the cylinder. |
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A rich mixture is what burns initially, but then as the piston moves downward the pressure lessens, more air flows in and the heretofore uncombusted fuel combusts with the new air. |
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The exhaust valve isn't opened until BDC thus allowing the maximum power stroke. After BDC the exhaust exhausts helped along by the intake air, then the exhaust valve is closed. |
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The piston is now on it's way up but air is still flowing in: this provides a full charge for the next combustion. |
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Perforations would be the smallest that would accomodate filling the cylinder with the right volume of air in the time allotted. |
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//Baked...//
you mean sleeve-valve technology which this isn't even remotely related to ? |
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So lets say you have a supply of air pressurized to 750 psi and you plumb it to these slits: |
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First why not plumb it to a port in the cylinder wall? Slits are going to cause massive friction. |
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Since you are going to need a pump of roughly similar output to produce this pressure how is it going to be efficient? |
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//why not plumb it to a port in the cylinder wall?//
well for one thing then it wouldn't be ingress via carefully sized and placed perforations. |
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As envisioned, the upstroke is the exhaust stroke (as well as the intake stroke). The exhaust port opens, the exhaust does what it's supposed to and air is still being ingressed. If the exhaust port closes halfway up the stroke, we can still get a complete cylinder's worth of air in because the air is being pumped in continuously at whatever pressure has been calculated. |
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We could use a device to pump a soundwave timed to hit all the perforations at initial combustion meaning a less-constant overall ingress pressure would be needed, but that would just be complicated. |
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For, say, a 250cc engine, you need a pump to supply 250cc of air at say 750 PSI (50 Bar) each cycle. |
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At 6,000 RPM this means 25 litres per second. |
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25 litres per second at 50 Bar is 125,000 watts! |
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What do you expect the output of your engine to be? |
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are you saying that a one-cylinder diesel one*quarter* litre @ 6K rpm (35atm pre-combustion pressure) uses 80KW on the compression stroke ? |
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This idea is like a stone thrown in the glass house of reason. |
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Somewhere in this system there is the need for a supercharger that can produce produce as high a pressure as combustion does. Since producing and maintaining the static 1.5 bar pressure used in detroit super-turbo diesel two strokes consumes a 20% surplus of idle output we can see that a 30 bar static intake manifold pressure would be simply impossible. It would also require the paradoxical situation of COMBUSTION NOT INCREASING THE CHAMBER PRESSURE OF THE CYLINDER HEAD. Which as we all know would prevent the engine from producing any power at all. |
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The air-pressure metering is based on combustion pressure for a given fuel firing amount. |
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But the perforations' size limits the speed that the air can fill the cylinder so you wouldn't have the occasion where the internal cylinder pressure is already at combustion level before combustion takes place. |
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