Hydrogen or H2O systems may be a waste. On 10/4/2008 I had a Mileage Monster Single Cell HHO System installed for a total cost of $679.95 including the $250 installation charge. My first fillup today showed a MPG of 23.8 MPG, down from my average for this vehicle of 26.3 MPG. We will see what the future holds. To date it looks like I may have wasted $679.95. If you simply must try it for yourself check it out at www.iuseH2O.com.
These things are a complete rip-off. Basically, this is how they work:
You have a container of water. The system uses electricity generated by your alternator to electrolyze the water into Oxygen and Hydrogen, which they call HHO gas. The hydrogen and oxygen gases then are introduced into your intake and sucked into the engine where they combust, releasing energy and adding water vapor to your exhaust. Sounds great, right? Well, the part people forget about is that your alternator puts drag on the engine to generate electricity, thus consuming more fuel. The other important thing to remember is that energy is conserved. So you start with water in it's base energy state, you add energy to it via electricity (which you used gas to generate), resulting in a gas that has chemical potential energy, you combust it thereby releasing that chemical potential energy, and about 15-20% (depending on your engine's efficiency) of that potential energy is recaptured by the engine and used for moving the vehicle. You are left with hot water vapor that actually has more energy than you started with since you wasted about 80% of the combustion energy as heat. Anyway, the point is that this will reduce your gas mileage since you're only reclaiming about 20% of the energy you used to make the "HHO" gas.
Sorry that's kind of a convoluted explanation, but the short version is: you start with water, you end with water that has either the same or more energy than the water you began with. You cannot make energy from nothing, even if they claim there is a magic catalyst involved. Good luck getting your money back - the way scams usually work is they don't honor their guarantee.
Sorry, just read www.iuseH20.com, which claims that their system works by improving your combustion efficiency (not by actually extracting energy from the water vapor, as many others of these scammers claim). This would theoretically work, except that modern engines already have extremely high combustion efficiency if they are working properly. I don't think this would have any effect, and you would still lose energy on the electrolysis side of things.
Electrolysis works poorly at present because of the inefficiency of one of the electrodes. A recent development at MIT (a cobalt electrode that works efficiently at neutral pH) can change this. What WOULD really change things is if efficient electrolysis were also powered by solar panels incorporated into the body of the car (or the bed of a pickup) so electrolysis could be going on all day when the vehicle is in the sun - then you would be able to drive off the solar energy without hauling around a half ton of batteries. This would require a holding tank for the gases produced, but might be the shortest route to reducing petroleum use.