What's the difference between a fuel cell vehicle and a hydrogen-on-demand vehicle?

By: Jamie Page Deaton

The Tiguan HyMotion is powered by a hydrogen fuel cell system. See more pictures of alternative fuel vehicles.
The Tiguan HyMotion is powered by a hydrogen fuel cell system. See more pictures of alternative fuel vehicles.

If you could run your car on a fuel that burned so cleanly that its

only emissions were water vapor, you'd do it in a heartbeat, right?


It turns out that you can. Automakers are at the cusp of technology that would allow us to use hydrogen instead of gasoline to fuel our cars. Hydrogen is readily available and has fewer environmental drawbacks than petroleum-based fuels like gasoline. Though the technology is in its infancy, there are two ways hydrogen-as-car-fuel will likely be developed -- hydrogen-on-demand for internal combustion engine applications and for fuel cell vehicles.

Alternative Fuel Vehicle Image Gallery

Because of the promise of these two clean energy technologies, there are a number of scams trying to capitalize on the demand. In this article, we're going to look at fuel cell vehicles, hydrogen-powered internal combustion vehicles and we'll even take a look at some recent advertisements that claim with a simple conversion kit, you can run your car on water.

All hydrogen technologies work on the same principal as gasoline to power cars. Fuels, like gasoline and hydrogen, contain chemical energy. That energy can be released by creating a chemical reaction. In the case of a gasoline or hydrogen-powered engine, that chemical reaction is combustion, or burning the fuel. In a fuel cell vehicle, the hydrogen fuel is combined with oxygen. While the combustion in a hydrogen- or gasoline-powered engine is converted into mechanical energy, in a fuel cell vehicle, the chemical energy from the hydrogen and oxygen is converted into electrical energy. Of course, this is a very simplified version of what's actually going on under the hood. If you're not quite sure about the differences between internal combustion engines and fuel cells, it might be a good idea for you to check out How Car Engines Work and How Fuel Cells Work.

Keep reading to find out if you really can power your car with water.


A Filipino inventor, Glen Castillio, collects hydrogen from his portable hydrogen reactor which is connected to a 12-volt battery in the car. His reactor separates enough hydrogen from water to run an automobile engine.
A Filipino inventor, Glen Castillio, collects hydrogen from his portable hydrogen reactor which is connected to a 12-volt battery in the car. His reactor separates enough hydrogen from water to run an automobile engine.

Hydrogen-on-demand can mean two different things. A hydrogen-on-demand system can provide hydrogen for use by a fuel cell or for an internal combustion engine.

A number of companies, and individual inventors, have claimed to have created aftermarket parts that allow consumers to convert their own vehicles to run on hydrogen-on-demand systems. The most common of these aftermarket systems use a hydrogen-on-demand system to create hydrogen as a gasoline additive. This process is also called hydrogen fuel enhancement or hydrogen injection. Most hydrogen-on-demand systems create the hydrogen by using water -- which is made up of a single oxygen atom sandwiched between two hydrogen atoms -- and converting it into HHO gas. HHO gas is a mix of oxygen and hydrogen gases. It's also called oxyhydrogen, or Brown's Gas, after the scientist who first created it.


In most aftermarket hydrogen-on-demand systems, the HHO gas is created by running an electrical charge through water and another chemical -- the type of chemical varies, depending on the system. The electrical charge is generated by the vehicle's battery. The process creates HHO gas, which is then fed into the engine through the intake manifold where it mixes with the gasoline and is burned in the combustion chamber. Supposedly, adding the HHO gas to the gasoline allows it to burn at a lower temperature, increasing efficiency and decreasing harmful emissions.

Most companies and inventors who make these aftermarket parts describe them as a way to power your car with water. In truth, it's not water that's powering the car -- it's the chemical the water is reacting with -- along with the gasoline or diesel fuel that you normally put into the fuel tank. In fact, powering a car with water would be nearly impossible. Chemical reactions release energy by altering chemical bonds. The chemical bond of water is so stable that to alter it requires more energy than is gained in the chemical reaction. So while these aftermarket systems may increase a gasoline engine's efficiency, the energy required to create the HHO gas is substantial enough to negate the benefits.

But these aftermarket systems aren't the only way to introduce hydrogen into a vehicle's power plant. Read the next page to find out how automakers are making good use of hydrogen.


Hydrogen-powered Cars

A BMW Hydrogen 7 vehicle arrives at the Santa Monica Pier in Santa Monica, Calif.
A BMW Hydrogen 7 vehicle arrives at the Santa Monica Pier in Santa Monica, Calif.
Vince Bucci/Getty Images for BMW

As we mentioned earlier, fuel cells and internal combustion engines can be fueled by hydrogen. The BMW Hydrogen 7, for example, has an internal combustion engine that can run on both gasoline and hydrogen. One very important distinction to make here is that the Hydrogen 7 runs on liquid hydrogen, which must be kept extremely cold. If it isn't, the hydrogen will turn into a gas, and as a gas, it will not be able to power the engine. The major modifications needed for running a car on hydrogen go to the fuel tank, not the engine. Keeping all that liquid hydrogen cold requires some serious technology.

Beyond that is the issue of engine tolerances. Car engines are built to run in specific conditions and with specific fuels. That's why putting a lower octane gasoline in your car's engine can cause a change in performance. The car may be able to run on lower octane fuel, but it's not ideal -- sort of like how you could survive on a diet of licorice, but not for long. Cars like the BMW Hydrogen 7 have engines that have been built to run on hydrogen, and sophisticated computer systems to monitor the engine and make sure it's operating optimally. Could someone convert their car's engine to run on hydrogen? Sure. After all, anything is possible -- not likely, but possible.


One more factor to consider is that even if you could power your car on hydrogen, where would you get it? Hydrogen fueling stations are few and far between. In the United States, Southern California has a just a few. It is possible to get a hydrogen refueling system for your home, but these systems tap into natural gas lines. The natural gas has to be converted into hydrogen fuel for a vehicle.

All things considered, it appears as though reliable hydrogen fuel technology is still a ways off. Hydrogen does promise a chance for a highly renewable resource to power vehicles and to decrease harmful emissions, yet there are some significant technological hurdles to jump before it becomes a mainstream fuel source. Automakers are in various developmental stages of fuel cell technology, and hydrogen-powered vehicles may be widespread soon; however, the much less credible "water-powered" cars will likely never come to the showroom floor.

For more information about hydrogen, fuel cells and other related topics, follow the links on the next page.

Lots More Information

Related HowStuffWorks Articles

More Great Links


  • Auto-facts. "Water4gas Scam: Reviewed." http://www.auto-facts.org/water4gas-scam.html
  • Motavalli, Jim. "Gentlemen, Start Your Fuel Cells! The Hydrogen Road Tour Takes Off." The New York Times. August 12, 2008. http://wheels.blogs.nytimes.com/2008/08/12/gentlemen-start-your-fuel-cells-the-hydrogen- road-tour-takes-off/?scp=2&sq=BMW%20Hydrogen%207&st=cse
  • Strom, Ron. "Cars run on water: Miracle or scam?" WorldNetDaily. May 20, 2006. http://www.worldnetdaily.com/index.php?fa=PAGE.printable&pageId=36226
  • Ulrich, Lawrence. "BMW Hydrogen 7: A Sedan Fueled by the Future." The New York Times. July 20, 2008. http://www.nytimes.com/2008/07/20/automobiles/autosreviews/20AUTO.html?_r=1&scp= 1&sq=BMW%20Hydrogen%207&st=cse&oref=slogin
  • Valdes-Dapena, Peter. "Coming soon: Hydrogen-powered BMW." CNNMoney. Sept. 12, 2006. http://money.cnn.com/2006/09/12/autos/bmw_hydrogen/index.htm?postversion=2006091212