Atkinson Cycle

Most modern full hybrids use an Atkinson-cycle petrol engine. The electric motor makes up for the lower low-speed torque of the engine. Modern engines achieve the effect through valve timing rather than the original special crank mechanism.

Closing the intake valve late pushes some of the charge back out, so the effective compression stroke is shorter than the expansion stroke and more energy is extracted.

Atkinson engines feel less punchy on their own, which is why they pair well with electric assistance. Variable valve timing faults can affect how the engine switches between modes.