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In the case of the Prius engine , the effective compression ratio is about 8:1, while the expansion ratio is about 13:1. As a result, it is 12% to 14% more efficient, in terms of power output per fuel consumed, than the non-Atkinson engine upon which it is based.
so, static compression is 13:1, while the engine only has to drive the load equivalent of 8:1 compression. 61.5% difference in compression ratios. from the way it is operated(pushing the air back through the intake rather than through the exhaust), pumping losses are also cut down as well. and the result is a ~13% improvement in BSFC. certainly something interesting to look into.....