Thank you so much for reminding me of something my father use to say "Ask a stupid question and I'll give you a stupid answer!"
I saw this process 50 years ago in chemistry class. It just takes a DC current. Collect the O2 over one electrode and H over the other. Pipe the two into a bottle, hold a lighted match over the bottle and BOOOOM you have it. But it's not a practical solution for a car. It can only be used where space and weight are a premiem, such as a space ship. Just remember there is no such thing as a perpetual motion machine.