[Ecky]

There are conversion losses, yes. Some quick math:

-An alternator generally has something like 30-40% losses, so the greater the proportion of energy you transfer to the front wheels (powered by an alternator) the greater your losses.

-For every 10HP you add to the front wheels, you'll be adding 15HP of drag on the engine, and you'll be turning 5HP (3750 watts) of gasoline burned directly into heat.

-An extra 10HP (5 to each wheel) would therefore mean the alternator would need to dissipate 3750 watts of waste heat, and produce 7,500 watts of power, or approximately 500 amps at 15v. Your alternator is probably rated for anywhere between 60 and 110 amps, so to get 5HP to each of the front wheels, powered solely by alternators, you'd need 6-10 of the bolted to your engine.

-Alternators also are not reliable for constant loads, and will probably die quickly if used this way. Frankly, they're terrible, and have only been ignored for being as terrible as they are for as long as they have, because cars have historically had such low electrical demands and manufacturers would prefer a cheap and inefficient part that lasts "long enough".

~

You could, of course, bolt an altermotor onto the engine in place of the alternator, one that can double as a starter, produce power at high efficiency (probably 90% or greater), high voltage (meaning low amps needed, and no conversion losses) and can assist the engine during acceleration. But, you still don't want to be constantly taking power from the rear and adding it to the front, because it's still going to be lossy. At the very least, you have a 90% highly efficient altermotor producing power, and 90% efficient hub motors in the wheels, resulting in 20% of the energy produced being wasted in return for gaining full-time AWD.

Though, to be fair, adding a mechanical linkage in a traditional AWD system is going to lose 10-20% of your power and fuel economy too - this is why 4WD and AWD vehicles have lower gas mileage, all else being equal, and will even accelerate more slowly unless they're running into traction limits. These are to be avoided unless necessary.

My opinion is to abandon the idea of full-time AWD. Maybe apply power to all 4 wheels during initial acceleration and passing, and then once up to speed, let the hypothetical front hub motors just freewheel. When you slow down, use all 4 to generate power and fill your batteries.

Or, if you don't feel the need for AWD, a single altermotor on a belt to your engine will let you enjoy more efficient electricity production (saves gas and less loss = more power), greater low-end torque and acceleration on demand, and gas savings through regenerative braking. If you want to add full time electric assist, you're either going to have to charge the battery manually or accept a large fuel economy hit for transferring power to all 4 wheels all the time.

EDITED for clarity