[P-51]

darkpuppet,Aug 8 2005, 09:52 PM Wrote:

P-51,Aug 8 2005, 05: Wrote:Yup, I think it's a great idea, the Miller Cycle, but I think it must be used with a blower.  The idea of being able to run high boost at an effective CR of 8:1, but getting 10:1 work out of it is great.  The only problem is the wickid reversion you have going on in the intake manifold.
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pretty much all the reading I found on the miller cycle say it needs supercharging, and indirectly, positive displacement superchargers.

but then again, it sort of confuses me how they can get more work out of a lower effective compression ratio.. I mean, if you're letting air flow back out so the cylinder isn't wasting energy compressing it, what do you do with all the air and fuel that's headed back out?

I must do more reading on this... pretty neat concept.
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It's pretty easy to understand why it works, it's the how (what happens to the air/fuel that is pushed back out?) that is tricky.

Why, is simply because, in a standard Otto Cycle, the exhaust gas temps are as high as 1600 degrees. That means there is a LOT of extra energy left in the gas that is being waste. That energy could be used through further expansion doing work on the piston. But in an Otto cycle, you are limited because to get that huge stroke for mechanical expansion, leads to a really high CR. Too high for most gasolines, you get deto. Also, that large stroke also just means more displacement, you're sucking in more air to start with, so you're right back where you started.

Miller Cycle allows you to have an "expansion ratio" of say 15:1, to get more of the work out of the exhaust, but only 8 or 9:1 CR, allowing for regular octane. AND, the "effective" displacement is reduced by 9/15... um, that's not right. But the effective displacement is reduced by some amount. The math eludes me at the moment.