[pfdietz]

Prius uses an Atkinson cycle engine, doesn't it? Inherently more efficient than a conventional engine, albeit at the cost of lower power. You can get that effect with variable valve timing in some power ranges, at the cost of more complexity.

[kube-system]

Pretty much all mainstream hybrids run Atkinson cycle (technically not the original Atkinson design, but an otto-cycle engine that keeps the intake valve open longer, to produce the same effect)