- Some hydrogen cars like Honda's FCX Clarity are powered by fuel cells, devices that use fuel to generate electrical current. Other hydrogen cars like BMW's Hydrogen 7 use conventional combustion engines that burn hydrogen in a cylinder to drive a piston that turns the crankshaft.
- Both fuel cells and combustion engines combine hydrogen with oxygen to create water or H2O. Water vapor is the only exhaust from hydrogen fuel cell cars.
- Hydrogen combustion engines primarily emit water vapor as well but can also produce small amounts of nitrogen oxides due to high cylinder temperatures. The amount of nitrogen oxide in the exhaust from these cars, however, is far less than the amount from a comparable gasoline engine.
- Hydrogen cars have the potential to dramatically decrease pollution, since hydrogen car exhaust is essentially water vapor. Unlike fossil fuels, burning hydrogen doesn't release carbon dioxide and could thus help reduce CO2 emissions.
- Whether hydrogen is a truly green and sustainable fuel will depend not only on how it's used but on how it's produced as well. Using fossil fuels to produce hydrogen, for example, would emit pollution and potentially negate some of the benefits; consequently, most researchers in the field are seeking new methods to produce hydrogen fuel using solar energy.
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