Atkinson Patent Differential Compression Gas Engine circa 1886 built by D Forster, seen at a recent exhibition.

The Atkinson-cycle engine is a type of internal combustion engine invented by James Atkinson in 1882. He 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 design was the cycle engine, which used an over-centre 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.

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.