I think this idea has merit, but I'd change a couple of things. Since 80 percent of a car's braking is done with the front wheels, I'd keep the brakes there--maybe you could move them inboard to the half-shafts like a formula car. Or leave the front alone and put the hubmotors in the back.

Next thing is hubmotors are unsprung weight, so they have a disadvantage in ride and handling. Maybe not a big deal.

Also, I have yet to see a hubmotor that would do more than 2000 watts, where to move a car you'll probably need ten times that. Still, there's no reason that can't be made.

I guess you're not talking about using an actual controller to split power between the two engines--just use electric until it's gone and then go to gas, then plug in the next day? The controller is often the most complicated part.

Hrench, thanks for commenting. I considered putting the motors on the back wheels, but I chose the front wheels specifically because they do the most braking. Regen braking is one of the main points of the concept. Still, the back wheels are an option. The brake disc and caliper on most cars are pretty heavy, so replacing them with a motor might not increase unsprung weight all that much, though it will probably increase it at least some.

It occurred to me after I wrote this that the system could be used with an ultracapacitor and no battery to provide regen braking on an otherwise regular ICE powered car. I just now saw your flywheel concept and this would be coincidentally similar in purpose, though not in design. Theoretically, it could be designed so that batteries could be installed later to provide plug-in capability.

The batteries could effectively be optional and removable by the owner at any time. So for a long trip, you might remove the batteries to free up cargo space, then after you return from the trip, you could put them back in for commuting.

I know the electric-only power would be low. This would only be used for low-speed city driving. I envisioned the driver using the engine on the highway and switching to electric on surface streets.

Yes, I was wanting to keep the batteries and gas engine completely separate. So the controller simply controls battery power to the motors when the ICE is off. I suppose using regen braking with the ICE may complicate things, but maybe not to much. I must admit, I'm not an expert on controllers.

BTW, I envisioned that the motor would bolt on exactly like the disc and caliper it was replacing. The rotor (of the motor) would bolt on exactly where and how the disc bolted on. In many cars, the disc is part of the hub, so the motor rotor would have to have an identical hub built into it. The outside of the motor would have an extension that is shaped exactly like the caliper attachment area and would bolt on to the car exactly like the caliper did. Thus, the motor would use the exact same mounting points as the parts it replaces.

Obviously, the motor would only fit on one specific make of car. However, many car makers use the same exact brake system on several different models that are about the same size, so the motor could fit multiple models.

Dont forget about the speed junkies. We use alot of back breaking for drifting through the corners, and doing other tricks. Plus, you can control the car better by being able to choose between front and back breaks. But i do like your idea, though it sounds like its gonna cost me a good chunk of change.