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Author Topic: Can thermal mass of piston engine change dynamically for improved Carnot cycle?  (Read 2259 times)

Offline peppercorn

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My thinking on this started from evaluating ways to pack cooler, denser air into a cylinder.   Several 'cold-air' systems are already available to improve power, but not usually for efficiency, but it is possible to use higher compressor pressures to allow more heat rejection via a turbocompressor with a sandwiched intercooler, as well as other methods.

However, this is an aside. Eventually I asked myself, where actually does the charge air take on most of its heat prior to inlet valve shut-off?  I would hazard a guess that on a N/A or well intercooled turbo engine, the vast majority of heat absorbsion during the air flowing into the cylinder is conducted back from the cylinder walls and piston crown left over from the last 'burn'.

It should be noted that until the inlet valve shuts any heating of gas in the cylinder is effectively acting as an increased back pressure, restricting further incoming air. Clearly, the air is still experiencing a vacuum, due to the dropping piston, but it is less than there would be in a cold-walled cylinder.

So, what I've got as far as thinking is 'could some of the upper cylinder block 'mass' be moved away (like an expanding outer shell - only the width of small air gap) whilst the inlet stroke is taking place.  This would involve cylinder walls being as thin as possible (so that the majority of the heat from the last 'bang' is unavailable to be transferred back), but when the structural integrity is again needed during compression and combustion strokes the walls appear as if a solid mass of metal (ie. the big ask of the scheme!).

In addition, if the 'shell' is kept thermally removed from the cylinder walls for all, but the combustion stroke, a heat recuperation 'mode' could be holding the shell elements close to red-hot by passing exhaust gases through them - On mating with the cylinder walls again, the heat from them would add a small 'external-heat' boost to the expanding gases just before the start of combustion.

I might try and add a diagram if this is as clear as mud!!!
« Last Edit: 01/11/2010 16:21:35 by peppercorn »


 

Offline Geezer

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I wonder if the cooling effect of the next charge accounts for some waste heat recovery? I'm not sure about that though. I do know that four-stroke engines do rely on the intake charge to cool the combustion chambers.

The biggest problem might be luberifrification of the cylinder walls. The oil might get a bit too toasty to do anything useful.
 

Offline peppercorn

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I wonder if the cooling effect of the next charge accounts for some waste heat recovery? I'm not sure about that though. I do know that four-stroke engines do rely on the intake charge to cool the combustion chambers.

The biggest problem might be luberifrification of the cylinder walls. The oil might get a bit too toasty to do anything useful.

The cooling effect of the next incoming charge can only allow waste-heat-recovery once the intake valve is closed (as expansion prior to this point is reducing the ability to put in a larger mass of charge air).

In terms of four stroke's needing that cooling effect I would say there are other ways to ensure a 'safe' wall temp (sodium 'channel' in the walls? - like a sodium-filled exhaust valve).  The cooling is taking the load off the water jacket, but it is also adding expansion of gas before it can be utilised.

Good point about lower cylinder lube temp. though Geez! ...
I had initially pictured the design with the 'hot-shell' reciprocating in a mimic of the cylinders movement, but then thought an expanding shell could be a an easier engineer.  There would be an advantage if the shell was just be lowered from above (but much greater recip. mass) as the peak temps near the piston rings would be lower.
« Last Edit: 09/11/2010 10:57:08 by peppercorn »
 

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