Automobiles have come equipped with catalytic converters since the mid-1970s. If you’ve been working on cars old or modern, catalysts or “cats” are quite familiar. These devices, installed in the exhaust stream of internal combustion engines, are designed to convert harmful vehicle tailpipe emissions to safer substances.
Depending on engine configuration, a manufacturer may install one or more catalysts in the exhaust system. Typically, a four-cylinder engine is equipped with one catalyst. Six- and eight-cylinder models nearly always are equipped with 2 or more catalysts.
The bulk of gasoline engine tailpipe emissions and their effects are as follows:
Carbon monoxide or CO, an odorless gas which is toxic to air-breathers.
Hydrocarbons or HC, a mixture of unburned organic molecules causing odors and toxicity.
Nitrous oxides or NxO , odorless gases which, combined with hydrocarbons in the presence of sunlight, produce “smog”.
Catalysts installed in the 1970s and early 1980s were called two-way or oxidation catalytic converters. The function of this kind of device was to interrupt the flow of some of the exhaust gases and convert them. CO was oxidized to CO2 (carbon dioxide), and HC was oxidized primarily to water vapor and CO2. [Bear in mind that CO2 is a significant factor in global climate change, though it is not toxic per se.]
In the mid-1980s, emission control regulations required that automobile manufacturers equip vehicles with three-way or oxidation-reduction catalysts (also referred to as TWC). These perform the same older functions of oxidization of carbon monoxide and hydrocarbons. But they have an additional function: Reduction of nitrous oxides to nitrogen (N2) and oxygen (O2).