guys, fuel cell technology do exists, and people world over have already implemented it in cars, though that water heating from silencer is a different thing.

one guy in maharashtra has also done similar water heating modification. it is different technology. in that water is boiled and converted into steam either by silencer heat or engine heat, and then fed into combustion chamber, steam when heated expands further thereby giving more power, but in this menthod, engine still runs on petrol, just that its mileage is increased.

whereas in fuel cell technology, it is converted to electrical energy and drives a motor.