Editing Atkinson cycle (section)

==Modern Atkinson-cycle engines==
[[File:Atkinson-cycle engine.jpg|thumb|A small engine with Atkinson-style linkages between the piston and flywheel. Modern Atkinson-cycle engines do away with this complex energy path.]]

In the late 20th century, the term "Atkinson cycle" began to be used to describe a modified [[Otto-cycle]] engine—in which the intake valve is held open longer than normal, allowing a reverse flow of intake air into the intake manifold. This "simulated" Atkinson cycle is most notably used in the [[Toyota NZ engine#1NZ-FXE|Toyota 1NZ-FXE]] engine from the early [[Prius]] and the [[Toyota Dynamic Force engine]]s. 

The effective compression ratio is reduced—for the time the air is escaping the cylinder freely rather than being compressed—but the ''expansion'' ratio is unchanged (i.e., the compression ratio is smaller than the expansion ratio). The goal of the modern Atkinson cycle is to make the pressure in the combustion chamber at the end of the power stroke equal to atmospheric pressure. When this occurs, all available energy has been obtained from the combustion process. For any given portion of air, the greater expansion ratio converts more energy from heat to useful mechanical energy—meaning the engine is more efficient.

The disadvantage of the four-stroke Atkinson-cycle engine versus the more common Otto-cycle engine is reduced power density. Due to a smaller portion of the compression stroke being devoted to compressing the intake air, an Atkinson-cycle engine does not take in as much air as would a similarly designed and sized Otto-cycle engine. Four-stroke engines of this type that use the same type of intake valve motion but also utilize [[forced induction]] to make up for the loss of power density are known as [[Miller cycle|Miller-cycle]] engines.

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