Animated Engines, Atkinson

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.

10/1/00 Update:

A sharp-eyed, well informed visitor to this site has just discovered an important error in one of the above statements, and in my illustration. In Atkinson's original engine, the power stroke was made longer than the intake stroke. Apparently, by allowing the mixture to expand to a larger volume than was drawn into the cylinder, more energy was extracted giving greater fuel efficiency than the Otto-cycle (everything else being equal). Thank yous go to George, for pointing this out; and John for the additional information.

My illustration draws from a book about building a model of the Atkinson engine. I may have my measurements wrong, or perhaps even the model designer may not have taken the stroke lengths so carefully into account. After all, he didn't have much to work with: "...Jim was able to build his engine using one simple drawing and a photo..."²

In any case I still recommend the book, and still feel that I've correctly illustrated the fundamental operation of the Atkinson. I would love to hear any additional comments about this delightful engine.

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