You can get 1000 watts or so from a car alternator and that is your energy budget. That amount of output from the alternator may get you about 0.1% of the energy that goes into the engine. You have converted energy in gasoline to engine rotation (20% efficient), to alternator rotation, to electrical energy (70% efficient) , to create hydrolysis of water (50% efficient) to putting the O2 and H2 into the engine again for rotation of the engine shaft (20% efficient), The total loses are (.2 x. .7 x .5 x .2 = 3.5% efficient). Quite a trip with lots of loses for very small results.