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amcpacer: The modern implementation of the Atkinson cycle is an *extended* duration intake. A much later intake valve closing than typical to reduce dynamic compression while achieving benefit of high expansion ratio. Like Christ explained the original utilized "dual cranks" for small to typical compression ratio with much higher expansion ratio. The cam solution achieves the same ends without the mechanical complexity of shifting crankshaft throw lengths.
The Miller cycle used forced induction with the extended IVC timing to achieve complete cylinder fill and cut down on pumping losses but was unable to dramatically increase the mechanical compression ratio as there was no decrease of dynamic compression. Slight improvement in thermal efficiency with displacement-specific power on parity or higher than the Otto cycle (Atkinson cycle is much lower)
MazdaMatt: wouldn't do that if I were you. Creating the vacuum on the downstroke from an incomplete fill would result in much higher pumping losses on the intake side. It's a lot easier to pull mixture into the cylinder and bleed a bit of it off than it is to pull the cylinder down against a pair of closed valves. You'd wind up with MUCH less power.
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