It doesn't make sense if you think about it like that but the fact is most internal combustion engines run very inefficiently. The right fuel/air mix can improve engine efficiency dramatically. Its also quite possible to combine liquid and gaseous fuels in and engine such as CNG and petrol or hydrogen and petrol.
There is obviously a lot of hype in these websites but perhaps the basic principle of adding certain gases to the combustions cylinder isn't that mad. Of course the energy really comes from the petrol, via the alternator to the battery. The battery splits the water into the gases. I've got no problem with it so far. The question is does adding the gas to the combustion mix really make the engine that much more efficient that it can account for the energy expended on the electrolysis?
