Ive been looking at doing this for the trike Im building now and here is what Ive come up with:
Use a 10hp chineese diesel derated to 8 hp and have it run in its "sweetspot" of fuel efficiency (about half the fuel used if a motorcycle engine of about the same hp output) (the diesel would have been only 25% better except the BSFC jumps up in the "sweet spot"
Anyways, the etek style motors have always been 90% plus efficiency and they just (finally)! released a motor I beleive has enough umph to actually drive my trike to highway speeds the mars 1003
So here is what im thinking;
Gen) Mars 709 (known as etek rt) running at 96v and 55 to 60a constant
Drive) Mars 1003 (known as etek-s)
either 96v x 32 ah of 8ah headways or 96v of 100ah of prismatics (Possibly both)
For around town and shorter trips, just the batts driving through the controller.
For extended highway driving gen "passing through" the batts and on to the drive motor.
For charging at roadside cafe while having a cup of coffee, just run the motor im thinking. (I would LOVE to have a chat with someone smarter then me to see what im missing in the way of a charge controller? im thinking that the amp output from the gen wouldn't fry the batts, it would just need a hvc cutoff on the high side right?
So, if my math is right, assuming 91% on both motors by way of efficiency:
8hp diesel into gen = 7.28
7.28 going into batts and controller @95% 6.91hp
going into drive motor @91% = 6.28
5% for loss in the chain itself leaves 5.96 hp for moving the car (for my trike this should be fine)
so with 180 grams per hp per hour thats a whole lotta MPG
Using the 15% example for mechanical losses in the transmission (and the fact that you cant engineer a "sweet spot" for the gen because you would have to cycle through the gears and change throttle position 8hp in a motorcycle engine would show up as 6.8 hp at the rear wheel
at 354 grams per hp per hour (a standard 250cc motorcycle engine) the math starts to look better
assuming the car needs 5kwh to travel 70mph for one hour (6.67 "hp hour")
the diesel in example a is going to burn .46 gallons of fuel
the MC engine (non hybrid, just driving a rear wheel) will burn .91 gallons of gas.
Is my math right here, I did it fast and made a lot of assumptions, but it looks like BSFC is the key to this being viable (That and no voodoo hiding in a charge controller) lol.