The Prius uses an Atkinson cycle engine. This used to mean that all four strokes of the cylinder occurred with one revolution of the crankshaft. Is that still what it means, or does it have to do with compression and valves?
It is all done with valve timing adjustments now. The more complex crankshaft mechanism became unnecessary when the patents on the normal Otto cycle engine expired long long ago.
That wasn't ever the key point of the Atkinson cycle. The essence of Atkinson cycle is to have the compression ratio be smaller than the expansion ratio (rather than both equal, as in the Otto cycle). In the earliest Atkinson-cycle designs, that was done with an elaborate mechanical linkage between the pistons and crankshaft, as you can see in the animated picture on the "Atkinson cycle" Wikipedia page. The way that particular animation shows it, yeah, that design would have all four strokes in one crank rev ... but look at the complexity of that thing, it's got three more bearings per piston than a conventional Otto engine. The Toyota Atkinson engines accomplish the same thing (compression ratio lower than expansion ratio) by using the same mechanical setup as an Otto engine, and adjusting the valve timing. -Chap
The original Atkinson cycle engine was also designed to avoid infringing upon Otto's patent, But since that is now longer expired and licensing fees can no longer be demanded, the patent-avoidance feature of the complex mechanical linkage is no longer necessary. The goal of differing compress and expansion ratios can be achieved more simply by blending their two competing but expired patents.
