Atkinson differential engine

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 density. A modern variation of this approach is used in some modern automobile engines. While originally seen exclusively in hybrid electric applications such as the Toyota Prius, some nonhybrid automobiles now feature engines that can run in the Atkinson cycle as a part-time operating regimen, giving good economy while running in Atkinson cycle, and conventional power density when running as a normal-cycle engine. Mazda Skyactiv models offering this capability include the Mazda 3 and MX-5.

Atkinson produced three different designs that had a short compression stroke and a longer expansion stroke. The first Atkinson-cycle engine, the differential engine, used opposed pistons. The second and most well-known design, was the cycle engine, which used an over-center arm to create four piston strokes in one crankshaft revolution. The reciprocating engine had the intake, compression, power, and exhaust strokes of the four-stroke cycle in a single turn of the crankshaft, and was designed to avoid infringing certain patents covering Otto-cycle engines. Atkinson's third and final engine, the utilite engine, operated much like any two-stroke engine. The common thread throughout Atkinson's designs is that the engines have an expansion stroke that is longer than the compression stroke, and by this method the engine achieves greater thermal efficiency than a traditional piston engine. Atkinson's engines were produced by the British Gas Engine Company and also licensed to other overseas manufacturers. Many modern engines now use unconventional valve timing to produce the effect of a shorter compression stroke/longer power stroke, Miller applied this technique to the four-stroke engine, so it is sometimes referred as the Atkinson/Miller cycle, US patent 2817322 dated Dec 24, 1957. In 1888, Charon filed a French patent and displayed an engine at the Paris Exhibition in 1889. The Charon gas engine (four- stroke) used a similar cycle to Miller, but without a supercharger. It is referred to as the "Charon cycle". Modern engine designers are realizing the potential fuel-efficiency improvements the Atkinson-type cycle can provide.

...
Wikipedia