I can see this if it's integrated into the design of the vehicle and it's electronic control system.
OTOH, I drive an '89 Volvo which has elec. ignition and fuel injection. The car starts on 12V but when running it sees more like 13-14V. So I'd be very wary of disconnecting the alternator electrically, and forcing the car to operate on only 12V.
Plus the battery's reserve life is only something like 30-60 minutes; when you allow the needed time to recharge the battery for the next start that doesn't leave much time to run without the alt.
Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.
One way to replace the alternator would be to use NiMH batteries. You can get large form-factor ones at 1.2V each, and put enought together to give 13.2V or 14.4V. You could have a switch on the dash which either:
(1) Runs the alternator together with the normal car battery
(2) OR disconnectes the alternator and runs off the NiMHs.
The NiMHs could be charged at home and would give a period of time without using the alternator, especially on short journeys. The challenge would be to get the most out of the batteries by reducing electrical load e.g. higher efficiency fuel pump, LED lighting inside the car + maybe tail lights, HID headlights at night, etc
__________________ Team GasMisers5 - #1 for first three rounds of the original GS Fuel Economy Challenge
Miles displaced by e-bike since 1 Jan 2008: 62.6 (0 kWh used)