Two Equals Four: The Atkinson Cycle Engine

The Atkinson cycle engine faded from view for a time, but variations of the Atkinson cycle have reappeared in modern hybrids.


| December/January 2015


In the mechanical world, the past seems to have a habit of catching up with current and future needs. The Atkinson cycle engine is a perfect case in point.

Invented by James Atkinson in England (first patented in 1886 and then refined through a second patent in 1887), the Atkinson cycle engine completed all four strokes of the Otto cycle engine in a single crankshaft revolution instead of the two required by the Otto. This was made possible by connecting the piston to the crankshaft through four connecting links instead of a single connecting rod.

In Atkinson’s design the crankshaft was not on the same axial line as the cylinder. The piston rod (C) connected to an intermediate T-shaped crankshaft rod (E). The “T” rod connected to a short rod rotating on a fixed center (H) and to the crankshaft. The final “link” was the throw on the crankshaft. In an Otto cycle, the piston covers the same distance for each phase of operation – intake, compression, combustion/expansion and exhaust. On the Atkinson, no two of the four links were the same length, so the piston traveled different distances during each of the engine’s four phases. Because of this link arrangement, in the Atkinson engine the piston traveled a longer distance in the combustion/expansion phase than it did in the compression phase, and it traveled a shorter distance in the intake phase than it did the exhaust phase. The ratio of the phases could be changed by varying the distance between the link centers.

With a longer combustion/expansion cycle than intake or compression cycles the engine promised great gains in efficiency. Unfortunately, the extra mechanical hardware required, with extra linkages, bushings, and of course weight, meant it cost more to manufacture than conventional Otto cycle engines. Further, the design did not lend itself to high-speed applications. Promising as it was, the Atkinson faded from view. Until recently.



Automotive manufacturers looking for new ways to gain efficiencies have rediscovered the Atkinson cycle and are applying a variation of the scheme to current production car engines, specifically hybrid gas/electric engines. Toyota employs a variation of the Atkinson cycle in the hybrid Prius, as does Lexus with its Hybrid Drive. While neither is a true Atkinson engine, both use variable valve timing to alter the duration of the intake and combustion/expansion phases to achieve a shorter intake and longer expansion for increased efficiency, just as with the original Atkinson so many years ago.

For an excellent animation of an Atkinson engine in motion, go to Animated Engines.














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