Incomplete combustion can produce a variety of different byproducts, depending on the fuel burned. In many cases, these substances can be toxic. For instance, incomplete burning of hydrocarbons produces carbon monoxide, a poisonous gas. Soot, hydrogen and nitrogen oxides are other common byproducts of incomplete combustion.
Complete combustion of a hydrocarbon fuel results in very few byproducts, primarily carbon dioxide and water along with a few oxides. If there is not enough oxygen for complete combustion, or a heat sink of some sort interferes with the combustion process, the fuel does not burn completely. This partial oxidation of hydrocarbons results in molecules that are not completely broken down, releasing potentially toxic substances into the atmosphere.
One common method of reducing these potentially toxic byproducts involves either burning the exhaust products in an afterburner or using a catalyst to alter the composition of the exhaust. The catalytic converter on an automobile is the second type of device, using precious metals to oxidize exhaust gases. Modern catalytic converters can oxidize carbon monoxide into carbon dioxide, convert unburned fuel particles into carbon dioxide and water, and reduce nitrogen oxide into nitrogen and oxygen. These devices can significantly reduce, but not totally eliminate, the byproducts of incomplete combustion.