The Myth of the Water-Fueled Car

Given current gasoline prices, it would be a relief if someone really could invent a water-fueled car. But it just ain’t gonna happen — not until the laws of physics change, anyway.

Unlike most of our articles, this Mythbusters episode focuses on a single myth: that of the fabulous water-fueled car. Despite recent breathless announcements, this is just an urban myth that pops up repeatedly, especially in those times (oh, like right now) when petroleum fuels are expensive or very scarce, or both.

Sadly, cars that use water as fuel will forever remain nothing but fantasy — and not just because the big oil companies are suppressing the concept. It’s more a matter of someone having to figure out a way to rewrite the physical laws of the universe before it can work. In the following sections, we’ll tell you why this is so.

Water, water everywhere, and not a drop to burn

The biggest strike against the hypothetical water-fueled car is simply that, as writer Robert Heinlein once put it, there ain’t no such thing as a free lunch. You can’t extract energy from water without putting more energy into than you could get back later. This is because, in effect, water is already burned.

Think about it. Combustion — burning — occurs when something combines with oxygen to release energy and create a new waste product. Water is composed of two atoms of hydrogen bonded with an atom of oxygen. It’s always formed when hydrogen burns in the presence of oxygen (though it can be formed other ways).

The upshot? Water is simply the waste product of hydrogen burning. It’s an interesting waste product, and life as we know it would be impossible without it — but it’s not possible to burn it as fuel, because it’s already been burned. Hence, you’re never going to burn water in a water-fueled car.

But what about fuel cells?

What about them indeed? There are cars under development that use fuel cells that combine hydrogen in a controlled manner with oxygen, to generate the energy necessary to move a car. The only waste product? Water vapor (which, by the way, is a greenhouse gas).

It may even be possible, sometime in the future, to buy a so-called “water-fueled” car that directly uses electrolysis to split water into its component hydrogen and oxygen before burning it to provide energy. However, it’s not truly burning water, and will never produce as much energy as it uses. That’s not possible.

Even if it converts some of the car’s mechanical energy into battery power and uses that the energy to split more water, eventually you’ll reach the point of diminishing returns (due to friction and other factors), and the battery will run down. You’ll never recapture all the energy you’ve put into it in the first place.

It’s all a (tail)pipe dream

A car that uses ordinary tap water for fuel without using up more energy than it produces is simply impossible, at least as we currently understand physics. That would make it a perpetual motion device, and those never, ever work. Ultimately, sadly, that means we’ll just have to give up our dreams of a water-fueled car.