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The old Outlander PHEV, which we still get, seemed to just use the same engine as the ICE model. Slapping a hybrid system onto a car without changes to the engine will help some, but the real reason hybrids like the Prius get great fuel economy is because the engine is also addressed. Mitsubishi may not have had an Atkinson engine available initially, but they probably could have used a smaller engine without much performance difference from the ICE model.

 

And hybrid system can let any engine spend as much time in those areas as possible, regardless of what "cycle"(these are all Otto engines at their heart) the engine is. Those Otto equipped PHEVs don't have poor fuel economy because of the Otto engine, it is because the manufacture didn't leverage the hybrid system by down sizing the engine. The secret of the Prius' efficiency is that the engine is sized so its output matches up with the majority of driving needs. Atkinsonization is just another way of downsizing; the engine's effective displacement is smaller. There is an efficiency advantage to the extended expansion stroke, but a hybrid with a smaller engine would still be more efficient than not changing the engine from the ICE model.

We are now in the era where engines don't have a Otto to Atkinson distinction. Valve timing to allow power and efficient operation of an engine has been around for decades. Consumers are now getting engines with a wider range of such timing. Some can switch between Otto and Atkinson.

 

The practical reason they started with Otto was because of budget, be it time, money, or both. The Outlander's engine had some small changes to improve efficiency over the one used in the ICE model. Same with the Volt. The 1.4 mill in the Cruze here had a turbo to give it acceptable power.

Well, the Passat and A class(no B) here only have a 2L, so a big step down for us, if we had the choice. Going by what PHEVs the European brands do offer here, the models are probably more focused on performance than getting the best efficiency and EV range.

I remember the Volt concept also used a turbo, so the power output was likely close to the 1.4L they did use. I'm sure the engineers tested such before the decision was make on the 4 cylinder.

I acknowledged an Atkinson/Miller would have an efficiency advantage over the Otto, even one sized so its efficient output matched most of the car's needs. That Otto would be more efficient in a hybrid than using one sized to run the ICE model.

Using a smaller 3 cylinder engine in Corolla hybrid would be more efficient than using the same 1.8L is in the ICE model, but less efficient than the 1.8 Atkinson.

When the engine is working under heavier loads, the driver is more than likely calling for more power. For its efficiency, the Atkinson cycle gives up power output. So staying in Atkinson mode while accelerating or climbing isn't the best idea when Otto mode is available for extra power. It isn't because the system technically couldn't stay in Atkinson during higher loads. Hybrids overcome the Atkinson cycle's lower power with the electric motor.

Hyundai CVVD: Full HP and Torque Performance – Fully Atkinsonized When Power Not Demanded | CleanMPG