The Atkinson engine is essentially an Otto four stroke engine with a different means of linking the piston to the crankshaft.
The clever arrangement of levers allows the Atkinson engine 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 design eliminates 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.
Since this page was first published, I've learned a great deal about the Atkinson engine. Visitors to this site originally clued me in, which prompted me to do a bit more reading.
This illustration closely follows the dimensions of a model engine, described in the excellent book: Building the Atkinson Cycle Engine. In this design, the intake and exhaust strokes appear to be longer than the compression and power strokes. It's not clear whether this was intentional; I suspect that the model designer was more interested in the linkage than the thermal cycle.
In a true Atkinson cycle, the power and exhaust strokes are longer than the intake and compression strokes.8 By starting with a small initial charge, and allowing it to expand to a larger volume than it originally occupied, a greater degree of fuel efficiency is realized.
Atkinson designed more than one engine to capitalize on this important property. I hope to have better animations of them all some day.
For more on the Atkinson and all internal combustion engines, I highly recommend Lyle Cummins' Internal Fire.
A number of visitors have arrived at this page after reading somewhere that Toyota's popular hybrid car, the Prius, uses an Atkinson cycle engine. I do not know where this claim originated, but I doubt that the Prius engine uses the linkage illustrated above.
It is possible to create the same effect as the Atkinson cycle by changing the valve timing on an otherwise ordinary Otto four stroke engine. I really should illustrate this, but in the meantime I hope the following explanation will suffice:
A cam is employed on both intake and exhaust valves (unlike my four stroke illustration). The intake valve cam is designed to hold the intake valve open for more than a single stroke:
The intake stroke begins as usual, initially drawing a full cylinder of fuel-air mixture.
When the piston begins its upward travel, the intake valve remains open. The piston pumps some of the fresh fuel mixture back out the intake port. The net effect is exactly the same as if the intake stroke were shortened.
The intake valve closes after the piston has moved some predetermined portion of this stroke. Compression does not actually begin until this point, effectively shortening the compression stroke to match the shortened intake stroke.
The power and exhaust strokes remain as in the four stroke, employing almost the entire length of the piston travel.