Combustion Emissions

Most autos run on gas or diesel fuels, using an internal combustion engine. Gas and diesel fuels are mixtures of hydrocarbons (meaning they contain hydrogen and carbon atoms). If this hydrocarbon fuel were mixed with the perfect amount of oxygen and completely burned, the only by-products (pollution) would be carbon dioxide and water. If this were true, then nitrogen in the air would remain unaffected.

Unfortunately, we don't live in a perfect world. Typical engine combustion (as opposed to ideal engine combustion) looks more like this:

Fuel (gas or diesel) + Air = leftover hydrocarbons + nitrogen oxides + carbon monoxide + carbon dioxide + water

The hydrocarbons aren't completely consumed in the process, so they come out the tailpipe along with nitrogen oxides and carbon monoxide, and the expected carbon dioxide and water.

All engines that run on hydrocarbon fuels (gas, diesel) undergo the combustion process. All engines that undergo combustion emit pollutants. These include both "on-road" and "off-road" vehicles. However, not all vehicles emit the same amount of pollution - some pollute more or less than others. Check out EPA's guide to vehicle emissions for more information.