Pretty sure it's just a way to get more expansion from the same air charge. It's a similar idea to the Atkinson cycle. You have dissimilar compression and expa

Pretty sure it's just a way to get more expansion from the same air charge. It's a similar idea to the Atkinson cycle. You have dissimilar compression and expansion strokes. In normal engines, there's a limit to compression ratio because if it's too high, it causes knocking. But a bigger expansion ratio lets you extract more energy out of the combusted gas, which leads to higher efficiency. The original Atkinson cycle idea was to use some complex linkage to get dissimilar compression and expansion strokes, but the way it's implemented in things like the Prius is to have a high compression engine, but mess with the intake valve timing such that you only use a small part of that compression during the intake phase so you effectively handicap your compression ratio to avoid knock, while still retaining the full stroke during the expansion phase.