Four stroke Atkinson-cycle engine

The Atkinson cycle may also refer to a four stroke piston engine in which the intake valve is held open longer than normal to allow a reverse flow of intake air into the intake manifold. This reduces the effective compression ratio and, when combined with an increased stroke and/or reduced combustion chamber volume, allows the expansion ratio to exceed the compression ratio while retaining a normal compression pressure. This is desirable for improved fuel economy because the compression ratio in a spark ignition engine is limited by the octane rating of the fuel used.

Atkinson-cycle engine

A high expansion ratio delivers a longer power stroke, allowing more expansion of the combustion gases and reducing the amount of heat wasted in the exhaust. This makes for a more efficient engine. The disadvantage of the four-stroke Atkinson-cycle engine versus the more common Otto-cycle engine is reduced power density.

Because a smaller portion of the intake stroke is 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 with this same type of intake valve motion but with forced induction (supercharging) are known as Miller cycle engines.

4strock - 4stroke atkinson cycle - 4stock valve clearance - 4strock otto cycle - 4strock valve train

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