The Atkinson cycle engine is a type of Internal-combustion engine invented by James Atkinson in 1882. The Atkinson cycle is designed to provide efficiency at the expense of power.
The Atkinson cycle allows the intake, compression, power, and exhaust strokes of the Four-stroke cycle to occur in a single turn of the crankshaft. Owing to the linkage, the expansion ratio is greater than the compression ratio, leading to greater efficiency than with engines using the alternative Otto cycle.
The Atkinson cycle may also refer to a four stroke engine in which the intake valve is held open longer than normal to allow a reverse flow 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 good fuel economy because the compression ratio in a spark ignition engine is limited by the octane rating of the fuel used, while a high expansion ratio delivers a longer power stroke and reduces the heat wasted in the exhaust. This makes for a more efficient engine. Four stroke engines of this type with forced induction (supercharging) are known as Miller cycle engines.
The Toyota Prius (front-wheel-drive) and Ford Escape hybrid electric (front- and four-wheel drive) vehicles use the Atkinson cycle engine.
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