A hydrogen powered car, by definition is one that uses hydrogen as it's on board fuel. The chemical energy of hydrogen is converted to mechanical energy by the power plants of these cars.

There are basically two types of hydrogen cars. One type is hydrogen fuel cell cars and the other is hydrogen on demand cars. The former type requires hydrogen to be pumped into the car. This means that there has to be hydrogen fuel stations which would also mean that there has to be a hydrogen fuel storage system and a distribution system both inside stations as well as on the car. Since hydrogen is a gas, it has to be stored under high pressure and gas under pressure always has a risk factor. Hydrogen on demand (HOD) cars are a totally new thing (although the concept has been there for ages). The difference of HOD cars from the former type is that there is neither pumping of hydrogen nor storage of hydrogen involved in these cars. This also means that there is no risk of 'gas under pressure'. Then where on earth does the hydrogen come from? The hydrogen is made inside the car as and when needed (remember the name of the car? hydrogen ON DEMAND!). Hydrogen is made using a simple principle. A simple electrolyser attached to the fuel system breaks down water into hydrogen and oxygen. Another major difference between HOD cars and fuel cell cars is that HOD cars do not run entirely on water while fuel cell cars do. In HOD cars, hydrogen is simply used to suppliment the energy produced by gasoline. A little bit of free extra energy to push down the piston!

There are also two types of hydrogen on demand cars. One is commercially made while the other is a small conversion you do at home for your existing petrol and/or diesel car/van/SUV/truck. Commercial hydrogen on demand cars are something totally new. The Scorpion is the first such car to be made and the first 190 cars will be released to the US market in Fall of 2008. It is expected to give 40mpg with hydrogen and 27mpg without hydrogen. Unfortunately it comes with a high price tag of around US$150,000 since it is a sports car. Hopefully somebody will take the initiative to produce regular HOD cars so that they will be affordable to the average person.

Converting your present gasoline or diesel car to run on hydrogen can be done at a very small price tag, as low as $150. However there are also more expensive systems in the market such as the Hydrorunner system for $3000-10,000, and other systems on Amazon.com for around US$1750.00. Because hydrogen generation in these HOD cars is done using water, they're also known as water fuel cars. You can read more on water fuel cars on waterfuel-water4gas.blogspot.com

As mentioned earlier, hydrogen on demand cars produce hydrogen by a process of electrolysis. Basically, electrolysis of water produces a burnable gas, which is a combination of H₂ and O₂. Depending on the type of electrolyser that you use (common ducted vs independently ducted), you can get either oxyhydrogen or Brown's gas. brownsgas.wordpress.com is a good resource to read about the science of Brown's gas. This site also talks of a similar gas trademarked MagneGas by a Nobel prize nominated scientist, a definitely recommended reading.