The Prius' intake cam holds the valve open longer while the piston goes up on the compression stroke, pushing air back into the intake manifold. Its effective displacement and compression ratio are both reduced this way, but on the power stroke the exhaust valve stays closed while the expanding gas pushes the piston down nearly to the bottom of the stroke. The gas expands through a higher ratio than it's compressed, extracting more energy. Original Atkinson cycle engines did this through a complicated mechanical linkage that actually varied the stroke. The Prius has a mechanical compression ratio of 13:1, while the Yaris has 10.5:1. The "Atkinson Cycle" intake valve hold-open drops the effective compression ratio of the Prius to about the same as the Yaris.
13:1 is a HIGH compression ratio for a spark ignition engine! Some large Diesels run in that compression range. Might work with E85. Might require a different ECU program to make it work on that fuel.
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03-21-2009, 05:07 PM
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