'Convert' Conventional engine into 'Atkinson' by..
 

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:https://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.

In discussing this a while ago somewhere else, I think we came to the conclusions that it would just about work, but ideally you'd want to start with a high compression motor, because you want to capture extra gas expansion not just lose 10% of your 10% in pumping loss at lower RPMs, also we thought it would be unresponsive and annoying to drive, and would have the usual peril of with mods of freaking out the ECU enough to make it go into limp mode or open loop.

You mean like miller cycle without the forced induction right?

I have read GM is using the Atkinson cycle on the LE5 2.4 ecotec by holding the intake valve open during part of the compression stroke they are reducing part throttle fuel consumption. Also they are opening the throttle fully at the same time to reduce pumping losses (Quite ingenious) the result is effectively a lean burn cruise at WOT. My 2.2 ecotec is not VVT so no Atkinson cycle :(

https://en.wikipedia.org/wiki/Atkinson_cycle

Crazy, that cycle used to refer to the differing engine design, not just keeping the valve open.

Many Dodge (2.2L) and Honda owners have attributed better FE to increased (SOHC) cam timing. I personally have advanced mine by 9 degrees. I might retard it back to normal again in the future to see if I lose the gains I believe I've experienced....could be placebo.

Maybe the key is to increase the amount of time the valves are open, so in effect you're advancing the timing of the valves when they open and retarding the time in which the valves close. Also are you saying they're getting better overall fuel economy? Because I cannot imagine they're getting better highway MPG since you'd be burning more fuel while cruising, but for city driving I can see it being a possibility since you'd theoretically be able to shift earlier.

That's a good question....since the highest rpm my engine sees is while on the pulse of p/g or when accelerating at the beginning of a hill...might approach 2500rpm on the "worst" occations. I definitely think that optimal gearing would have my engine at about 2000rpm @ 60mph versus 50mph...this would take advantage of the additional low-end torque I have now.

Looking for this? www.keveney.com/Atkinson.html