Atkinson Cycle Engines

Hannu Jääskeläinen

Abstract: The term Atkinson cycle refers to engines using a crankshaft mechanism, invented by James Atkinson, that allows a higher expansion ratio than compression ratio. This approach offers potential efficiency gains without the need for complex intake pressure boosting systems to maintain power density.

Atkinson Cycle
Automotive Applications

Atkinson Cycle

In the Atkinson cycle, named after the inventor James Atkinson, the geometric compression ratio and geometric expansion ratio are not equal. Atkinson’s approach to achieve this was to use two rotating shafts connected with a trigon link in place of the single crankshaft used in conventional internal combustion engines, Figure 1 [1916]. This allows the piston to have a longer stroke during the expansion stroke than during the compression stroke. As discussed elsewhere, it is the expansion stroke that is most critical for determining engine efficiency. Atkinson’s approach allows the efficiency benefits of high expansion ratio while avoiding some of the downsides of a high compression ratio.

Figure 1. An embodiment of James Atkinson’s concept for an engine with different compression and expansion ratios

Early commercial versions based on Atkinson’s ideas were built by the British Engine Company in the 19^th century. A modern example was commercialized by Honda in 2011 for household cogeneration units, Video 1 [3975].

Video 1. Honda EXlink and conventional engine comparison

(Source: Honda)

As discussed under Miller Cycle Engines, the term Atkinson cycle is commonly used to describe engines with late intake valve closing that achieves an effect similar to Atkinson’s invention using only intake valve timing. For this discussion, the term Atkinson cycle will refer only to engines using Atkinson’s dual shaft/trigon link approach. The term mechanical Atkinson cycle can also be used to differentiate James Atkinson’s approach from those that use valve timing to achieve a similar effect [6760].

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