Atkinson Engine

The Atkinson engine is essentially an Otto-cycle engine with a different means of linking the piston to the crankshaft. It was originally designed to compete with the Otto engine, but without infringing on any of Otto's patents.
The clever arrangement of levers allows the Atkinson to cycle the piston through all four strokes in only one revolution of the main crankshaft, and allows the strokes to be different lengths -- the intake and exhaust strokes are longer than the compression and power strokes (In this illustration... see below).

This also obviates the need for a separate cam shaft. The intake (if used), exhaust, and ignition cams are located on the main crank shaft. My illustration shows only an exhaust cam.

Everything I know about the Atkinson engine came out of Building the Atkinson cycle Engine. This illustration draws heavily from that excellent book.