Simply think of the hybrid system as having a SUPERCHARGER that increases the HP/torque output of the otherwise rather small and highly efficient engine, a SuperCharger that DOESN"T BURN FOSSIL FUEL and often runs of FREE FUEL, kinetic energy that would otherwise be lost via brake heating.

That SUPERCHARGING effect allows you to run "normally", cruising along at a relative constant speed, with an inordinantly small and highly efficient engine.

The Atkinson Cycle has two distinct advantages. Because the intake valve remains open for the early portion of the compression cycle the compression pumping losses are quite significantly reduced. Look at the "push" torque available at the piston head in relation to the angular position of the crankshaft, it reaches its peak value when the "crank" position is 90 degree to the piston rod and is at its minimum both a BDC & TDC. If it doesn't have to do any "work" for the first ~30 degrees after leaving BDC....??

The second advantage is that the power cycle, fuel burn period, is effectively elongated. The explosive power of the fuel is more completely exhausted versus the standard "otto" engine.

Remember that "free", waste energy, the free, otherwise wasted energy that is used to run a turbocharger..?? Well, with an Atkinson or Miller cycle engine that fuel is no longer "wasted". Which is why you will NEVER see a turbocharged Atkinson cycle engine.

And as many RDX owners are currently discovering there are other shortcomings of turbocharging an engine.

But that's another story.