You know you have one under your car, but if you're lucky, you never have to think about it.
It's the catalytic converter, a component of your car's exhaust and emission-control system usually sited between the engine and the muffler.
It's a metal can containing a ceramic or metal honeycomb mesh coated with catalysts--often including small amounts of precious metals like platinum, rhodium, and palladium.
Passing exhaust gases over these substances produces chemical reactions that convert pollutants into less harmful substances, which can be either removed from the exhaust or safely emitted into the atmosphere.
Both carbon monoxide (CO) and unburned hydrocarbons (HC) are combined to produce carbon dioxide (CO2) and water (H2O), both of which can be emitted out the tailpipe.
The nitrous oxides (NOx), on the other hand, have their oxygen removed and are converted into nitrogen.
The first catalytic converters were introduced on U.S. cars for the 1975 model year, in response to new regulations limiting the maximum permissible exhaust emissions of new cars.
By 1981, all new U.S. passenger vehicles were fitted with catalytic converters, and throughout the next 25 years, catalytic converters spread into most of the world's production of new vehicles.
They are also fitted to other small engines, including those of motorcycles, chainsaws, and lawn mowers, as well as wood stoves.
The first "two-way" converters in 1975 controlled carbon monoxide and hydrocarbons; more recent "three-way" converters also process nitrous oxides.
A converter can remove up to 98 percent of the 15 tons of the three substances each year.