Minor improvement
There are some variations of engine cycles, such as the Atkinson cycle (right) which have a minor effect on efficiency.
The Scuderi engine for example calls their cycle a "split-cycle" engine. It is a minor variation of the usual 4 stroke cycle, in that the compression stroke is shorter than the expansion stroke. The Scuderi engine has a smaller compression cylinder than the expansion cylinder. This is otherwise known as an "Atkinson" cycle engine. Other atkinson cycle engines have been made, including Ford's version which is implemented more simply by adjusting the valve timing. Since a partial compression with an open valve is effectively no compression, compression does not begin till the valve closes, instead of at bottom dead center.
This yields the diagram at right, with differences from 4 stroke cycle highlighted.
It is easy to overestimate the benefit of this engine, because it ventures in to a region where all the work done inside the cylinder is not exported outside the piston (engine), but is instead offsetting atmospheric pressure (the pressure envelope). So while a larger portion of heat is indeed converted to work inside the cylinder, only about half the extra work makes it to the crankshaft.
In summary, this engine cycle makes a few percent difference in efficiency, which is still swamped by the 90% heat loss to water cooling at low speeds.
A Drawback or tradeoff for both these engines will be some power for improved fuel economy.
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Atksoin cycle plotted vs 4 stroke cycle
Note the green area is very small in comparison to the blue area. This is a minor change. The red area also represents lost work (work against the atmospheric pressure represented by the green line.). The red area also represents some unrecyclable heat, making the Atkinson cycle a poor choice for heat recycling engines.
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