This from one of many WEB sites talking about a "new" molecular species: HHO

"Denny Klein just patented his process of converting H2O to HHO, producing a gas that combines the atomic power of hydrogen with the chemical stability of water. "it turns right back to water. In fact, you can see the h20 running off the sheet metal." Klein originally designed his water-burning engine for cutting metal. He thought his invention could replace acetylene in welding factories. Then one day as he drove to his laboratory in Clearwater, he thought of another way to burn his HHO gas. "On a 100 mile trip, we use about four ounces of water." Klein says his prototype 1994 Ford Escort can travel exclusively on water, though he currently has it rigged to run as a water and gasoline hybrid."
Read the above carefully - "Convert water to HHO, then take HHO back to water." So you fuel a car by converting water to water.

Energy is a state function. What is the energy change for the overall reaction water -> XX - > water? Zero.

From another WEB Site:

"HHO acts as an oxidant through air inflow to make gasoline burn more efficiently."
If the oxidation state of oxygen in HHO is -2, then the electrons are happy. In H2O, the oxidation numbers are +1 for H and -2 for oxygen. Remember that oxygen likes to be -2; it won't give up electrons without energy input. It also can't take on more electrons (they'd end up in the n=3 shell - not good). Similarly H doesn't mind being H+; it won't take on electrons easily. It also won't give up any electrons (it doesn't have any!). So the idea that you can "burn" water is chemical nonsense.
But there's more:
"H2O is the chemical formula for a water molecule, where a single oxygen atom is bonded with two hydrogen atoms. HHO is a popular way of writing OxyHydrogen - a mixture of hydrogen and oxygen gases, usually in a 2:1 ratio. Thus, HHO = Hydrogen-Hydrogen-Oxygen. Since the oxygen and hydrogen are in mixed gas mode and not bonded, it is not H2O (water). Bonding hydrogen and oxygen requires an ignition and chemical reaction. When the Oxyhydrogen gas is lighted, oxygen and hydrogen atoms bonds and releases energy, the end result of the chemical reaction is water vapor."
OK, so now we have a little more reality. If HHO is simply a mixture of H2 and O2, then of course we can use that as a fuel. O2 is an oxidant, H2 in this case is the reductant. They react in a redox reaction. Or O2 can react with gasoline (just as does in your regular car).

But this isn't worth investing in, as it takes energy to generate H2 and O2 from water. Therein lies the trouble. How do we create that? From a battery they say. Why not just use the battery to drive the car directly?

"Hey guys listen up... I have built my own HHO generator that produces HHO gas using only 3 amps of power. I've installed one on my car and ups my mileage by 30%... my next project is to set up my home generator to run on this gas."
He'll use electricity to make HHO, then run that in his home generator to make electricity? Consuming water in the first step and producing water in the second. Does anyone see a problem?

Bringing back an earlier class quote:

The Law of conservation of energy tells us we can't get something for nothing, but we refuse to believe it. - Isaac Asimov
Finally, there is a danger issue here. If HHO is really a mixture of H2 and O2 gases, I'll want to stay VERY far away from this guy's house. The Hindenburg was pure H2 and burned "relatively" slowly, only because it took time for enough atmospheric O2 to mix in with the H2. A mixture of the two (in proper stoichiometry, no less) will release all of the enthalpy of the reaction at once, with all of the gas expansion at once. A VERY big explosion (lots of heat and lots of PV (gas expansion) work, in a very short time). You'll hear it for miles.

Hydrogen as a fuel is really just a battery. We still need to charge the battery. That means fossil fuels, solar, hydroelectric, nuclear, etc.