What is the Atkinson cycle and why is it used in hybrid vehicles?

The Atkinson cycle uses a longer expansion stroke than compression stroke, achieved through late intake valve closing that pushes some intake charge back out. This results in higher thermal efficiency (up to 40% vs 35% for Otto) but lower power density since effective displacement is reduced. It is ideal for hybrids because the electric motor compensates for reduced power, while the efficient engine operation maximizes fuel economy. Modern implementations use variable valve timing to switch between Otto and Atkinson cycles.