Editing Four-stroke engine (section)

=== Atkinson cycle ===
[[Image:Prius2004.JPG|thumb|230px|This 2004 [[Toyota Prius]] hybrid has an Atkinson-cycle engine as the petrol-electric hybrid engine]]
[[Image:T cycle AtkinsonMiller.png|thumb|170px|The Atkinson Gas Cycle]]
{{main|Atkinson cycle}}

The Atkinson-cycle engine is a type of single stroke internal combustion engine invented by [[James Atkinson (inventor)|James Atkinson]] in 1882. The Atkinson cycle is designed to provide efficiency at the expense of [[power density]], and is used in some modern hybrid electric applications.

The original Atkinson-cycle piston engine allowed the intake, compression, power, and exhaust strokes of the four-stroke cycle to occur in a single turn of the crankshaft and was designed to avoid infringing certain patents covering Otto-cycle engines.<ref name=pat367496>{{cite patent|country=US|number=367496|inventor=J. Atkinson|title=Gas Engine|gdate=1887-08-02}}</ref>

Due to the unique [[crankshaft]] design of the Atkinson, its expansion ratio can differ from its compression ratio and, with a power stroke longer than its compression stroke, the engine can achieve greater [[thermal efficiency]] than a traditional piston engine. While Atkinson's original design is no more than a historical curiosity, many modern engines use unconventional valve timing to produce the effect of a shorter compression stroke/longer power stroke, thus realizing the [[Fuel economy in automobiles|fuel economy]] improvements the Atkinson cycle can provide.<ref>{{cite web |url=http://www.autos.ca/auto-tech/auto-tech-atkinson-cycle-engines-and-hybrids/ |title=Auto Tech: Atkinson Cycle engines and Hybrids |work=Autos.ca |date=2010-07-14 |access-date=2013-02-23}}</ref>