Naked Science Forum
Non Life Sciences => Technology => Topic started by: peppercorn on 06/09/2010 14:44:09
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Instead of using a butterfly valve or adjustable valve timing could the air charge be controlled to match the fuel ratio (in stoichiometric or lean-burn config.) with cold air moderating the charge density.
I picture a on-off type electromech. valve (replacing the intake butterflies) in addition to the cold air system, that would be closed some way through the induction stroke during partial load conditions - keeping the air mass ratio correct, but with far lower pumping losses. This would avoid the complexity of in-cylinder fuel-injectors and VV-timing.
The cold air could be controlled by bypassing a portion of the incoming air through a turboexpander with a intercooler in the middle.
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Crickets [::)]
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Huh! Still crickets [::)]
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Geezer, the analogy I'm going to go for is tumble weed...
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C'mon Geez - you know 'bout this stuff; where's your tupence worth?
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I tried to understand it - 'onest guvner, but the Geezobrain sort of iced up.
Maybe a piccy would help?
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I tried to understand it - 'onest guvner, but the Geezobrain sort of iced up. Maybe a piccy would help?
I'll get back to y'all on that later....
time to get the crayons out, me thinks [:D]
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Okay, it was kind of a coming together of two ideas:
I had been reading:
http://www.wdlpower.com/downloads/PTNSS_2005_TCS_Paper.pdf
Where a turboexpander downstream from a supercharger (turbo or mechanical) can improve efficiency by noticeably lowering the induction air temperature (admittedly a the cost of slightly higher pumping losses).
The principle is that a t-expander in conjunction with an intercooler can allow far higher heat rejection than a standard turbo. Thus, the exhaust pressure is now adding overall efficiency as opposed to more power alone - as with a simple turbo'd engine.
The second thing that occurred to me is that the regulation of charge that is so critical in a S.I. engine can be adjusted by controlling the instantaneous density of the charge air rather than throttling the flow.
A potential way to do this would seem to control the air's temperature (independent of ambient temp.) to 'pack-in' the charge in full power mode and thin out (by less cooling) in part load conditions.
As you can tell these are incomplete ideas, but I had hoped some of you engineering types could help me see if this is anything but a dead-end?
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I'm not sure I really understood the question but; if you change the amount of air you let into an engine, don't you mess up the compression ratio?
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I'm not sure I really understood the question but; if you change the amount of air you let into an engine, don't you mess up the compression ratio?
I'm not sure I understand it yet either.
The throttle on any spark ignition (SI) engine controls the amount of air that you let into an engine. The compression ratio is simply the maximum volume of the cylinder divided by the minimum volume of the cylinder (at least it used to be until they started messing around with the valve timing so that the expansion ratio can be greater than the effective compression ratio.)
The carburettor (or some fancy electronic equivalent) supplies the right amount of fuel to obtain the desired combustion products from the mass of air that was inhaled.
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Where a turboexpander downstream from a supercharger (turbo or mechanical) can improve efficiency by noticeably lowering the induction air temperature (admittedly a the cost of slightly higher pumping losses).
The principle is that a t-expander in conjunction with an intercooler can allow far higher heat rejection than a standard turbo. Thus, the exhaust pressure is now adding overall efficiency as opposed to more power alone - as with a simple turbo'd engine.
I looked at the paper (well, I sort of skimmed it!) and I can see how you can get a greater air mass per cycle and therefore more power for a certain displacement, but it was not too clear to me why the efficiency improves.
The second thing that occurred to me is that the regulation of charge that is so critical in a S.I. engine can be adjusted by controlling the instantaneous density of the charge air rather than throttling the flow.
A potential way to do this would seem to control the air's temperature (independent of ambient temp.) to 'pack-in' the charge in full power mode and thin out (by less cooling) in part load conditions.
"can be adjusted by controlling the instantaneous density of the charge air rather than throttling the flow"
But isn't that what a throttle does? The air after the throttle is less dense than the atmospheric air.
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I'm not sure I really understood the question but; if you change the amount of air you let into an engine, don't you mess up the compression ratio?
I'm not sure I'd use the term 'mess up'. The quoted CR on a SI engine, being a ratio is independent of the charge density.
Although for a direct injection Diesel, the volume and charge pressure prior to fuel injection should be more or less constant, but with SI this is adjusted (either by throttle butterfly or dynamic valve timing). Traditional supercharging adds pressure, but at the cost of elevated charge temps.
I looked at the paper (well, I sort of skimmed it!) and I can see how you can get a greater air mass per cycle and therefore more power for a certain displacement, but it was not too clear to me why the efficiency improves.
The efficiency improves because you are trading some of the initial pressure gain from the standard turbocharger for a lower temperature charge (with lower pressure) after the turboexpander.
"can be adjusted by controlling the instantaneous density of the charge air rather than throttling the flow" But isn't that what a throttle does? The air after the throttle is less dense than the atmospheric air.
I think if such a thing as an ideal throttle butterfly existed it would reduce the air flow to the engine without restricting the pressure (impossible I know, but).
Even with new dynamic valve timing systems their must be a pumping loss cost at low loads, as shutting the intake valve earlier has its own (admittedly far smaller) energy cost as, sealed it will be a gas spring for the fraction of a second before TDC.
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Just as an aside:
By far the simplest way to add overall cycle efficiency to a piston engine, especially if around a fifth of the horsepower is required in direct electrical load, would be to have a non-supercharging (or very 'low-blow') exhaust turbine that directly couples to a high-speed electric generator.
This is a pretty good efficiency deal on a modern Diesel especially one needed for constant mechanical (+ 20% electrical) load.
But back in the real world (particularly the automotive one) there is a need to make engines as dynamic as possible - both by changing the modes that engine can operate in (lean-burn, charge augmentation, etc) and by hybridisation (evening out power demand on the prime-mover).
Sorry if this sounds like it turning into a lecture - I'm just trying lay out how I see the problems of efficiency.
Going back to the turbo-generator principle if you accept that maybe storing all the recovered exhaust energy in chemical batteries is not always a completely suitable situation, I would argue that this:
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(from the article)
could be a simple way to recuperate the power. It has a compressor coupled with a motor and a expander. An intercooler is mounted downstream from the compressor - rejecting lots of heat.
Cooling the charge has many advantages, especially with gaseous-fuel engines that don't suffer from poor vaporisation at low temps.