h a l f b a k e r yQuis custodiet the custard?
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In automotive engines, when reducing the amount of fuel-air mixture allowed into the engine on each stroke, it would be useful to compress that mixture into a smaller volume. Additionally, it would be useful to minimize the volume of gas remaining in the cylinder between the end of the exhaust stroke
and the beginning of the next intake stroke.
It would seem like a conceptually-simple means of achieving both of these objectives would be to add some pre-compressed springs to the linkage between the connecting rod and the piston, so that the position of the piston at TDC would vary with pressure; at reduced-power operation, the piston could sit higher than at full-power operation, thus providng better compression.
If the spring were frictionless, and if the pressure during the exhaust stroke fell below that required to compress the spring, then the spring itself shouldn't cause any energy loss. During the compression stroke, some energy would be spent compressing the spring, but this energy would be recovered in the following power stroke. The spring should not significantly compress during the exhaust or intake strokes, so there would be no loss there.
There are two potential difficulties I can see with this approach: (1) the springs would have to be constructed quite well to avoid failure due to metal fatigue; I don't know if today's metalurgical technology is adequate; (2) if the spring mechanism froze up in the extended state, that could cause extremely excessive cylinder pressures. To avoid the latter problem, I'd engineer a failsafe breakable link in the cylinder or piston such that if overpressure did occur, that volume (but not displacement) of the cylinder would be increased. This would adversely affect engine performance, but would avoid sending bits of metal flying all over the place.
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Piston Springs. That's just north of Alice Springs, right? |
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I'm not sure how to type the sounds that I imagine will occur when this thing flies apart. |
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The troubles here are numerous. [-] |
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As noted, some clever engineering would probably be required to avoid catastrophic failures from metal fatigue. On the other hand, the same might be said about automotive engines in general (though decades worth of engineering have improved things a lot). |
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It would seem like the stress a conventional engine puts on the bearings would get concentrated on the springs instead; the springs would have to be engineered well to withstand the stress, but other parts of the engine would be stressed less, especially by things like lugging. |
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Is there any reason to expect that any probable failures would be any more spectacular than a seized bearing in a conventional engine? |
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I can not envisage any advantages to increasing the
compression chamber volume when under heavy load.
When I soup up an engine I always try to reduce this volume to a minimun (between the material strenght limits), stopping short of the valves hitting the piston head. Why, sometimes you machine special pistons to do that. That aside, we could try connecting rods of very strong elastomeric compounds, or make them telescopic and fit springs (or oil pressure) inside. Nah, the rubber band engine is already baked. On the other hand, it will reduce strain at the crankshaft.
Supercat, you are right, I once had a connecting rod fly through the oil pan because of just that. Very satisfying and messy. |
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finflazo: Don't engines tend to knock and do other such bad things when compression ratios get too high? I thought there were limits as to how much you could get away with compressing gas in the cylinder; I know that the pistons and valves have to be designed properly to reach those limits, but I believe it's possible to design a cylinder and piston such that the compression ratio would be unacceptably high at WOT. Am I mistaken? |
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Braubeaton: I'm not quite clear to what centrifigul force you are referring, since the springs would be reciprocating in entirely linear motion. As for resonant frequencies, I would expect engine controllers to account for that sort of problem and avoid them. |
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supercat : Very, very true. To increase indefinitely the compression ratio will end by totalling your engine.
That's why I said between limits. What I don't see is why changing the volume when under load will help.
That will lower power output, defeating the purpose
of hopening the throttle. Or perhaps what you want is a self-limiting engine? . |
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And that upper limit would be set by knock, not material strength. The first sentence of the posting says that compression ratio would be increased at part-throttle. At full load it would remain the same as other engines. |
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first bun. Something like this would be useful in changing the Atkinson ratio, in such an engine, while keeping a constant compression ratio. |
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