Believe it or not, the answer is a resounding yes. There are even conversion kits to help you. And some Web sites offer do-it-yourself guides that explain how to set up a hydrogen fuel cell for your car from common items you can find at any hardware store.
Tap water as fuel?
Can plain old tap water power your car? It's possible, but it may not be worth it.
Whether you'd want to convert your car to run on water is another matter. The process of turning water into fuel is based in science, but whether or not it will improve your gas mileage (as these sites claim) is debatable.
Most conversion kits are based on a concept spread by Dr. Yul Brown, who extolled the virtues of extracting energy from plain old water. Dr. Brown used the process of electrolysis to separate water into its components -- two hydrogen molecules and one oxygen molecule. This mixture, called HHO (also known as Brown's gas), can be burned and the resulting energy used to power your car [source: Yul Brown.org]. This fuel requires very little water and is meant to supplement the gas you use in your car [source: Run Your Car With Water.com].
Can plain old tap water power your car? It's possible, but it may not be worth it.
So what's the problem? Like most other alternative fuel sources, HHO conversion kits for use onboard a car have a negative net energy ratio [source: du Plessis]. This means that the amount of energy you get out of the conversion is actually less than the amount you put in. Think about it like this: If it takes one gallon of gasoline to convert water into HHO, your energy output will equal one-half gallon of gasoline. You've just used one gallon of gas to produce the energy of one-half gallon. Speaking strictly in terms of energy, you would've been better off simply using that one gallon of gas to fuel your car. You would have gotten the benefit of the whole gallon, rather than just half of it.
There's another side effect of using hydrogen as fuel that auto researchers are still grappling with. Studies of hydrogen (which is a viable fuel) show it can cause brittleness and breakage in metal car engine parts [source: CEC].
But skepticism toward the water conversion kits available online and a little brittleness aren't stopping researchers from figuring out how to overcome the negative net energy ratio of exploiting hydrogen aboard vehicles. Some car companies have already created electric cars that run on hydrogen you pump into a gas tank at the service station.
What if a process could be developed to separate hydrogen from water without using more energy? That would be the Holy Grail. One group from Purdue University is hot on the trail. They've come up with a process to split hydrogen from water that requires little or no energy input. Adding water to a composite metal (alloy) made of aluminum and gallium separates water into its component molecules. The composite attracts oxygen, but not hydrogen, leaving the former element free and separated.
The Purdue researchers are still developing the technology, and it may be awhile before you find it under your hood. If you can't wait to run your car on water, you could always order a conversion kit in the meantime.