While the prius does not have a "true" atkinson cycle engine, it does have the attribute of having the intake valve open for a little bit of time when the compression stroke begins.
More information can be found about this subject here:
http://www.hybrid-vehicle.org/hybrid-vehicle-ice.html
So my idea would be to adjust the cam gearing (retard it?) so that the intake valve is open for just a little bit of time as the compression stroke begins, therefore acting as a atkinson cycle engine and theoretically reducing pumping losses.
While people speculate that increasing cam timing would increase fuel economy because of the increase in torque, many have found that it only increases low end torque but does not help fuel economy and that in practice, retarding the cam timing is what yields better fuel economy.
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