Placement of converters unlimited

require a temperature of 400 °C (752 °F) to operate effectively. Therefore, they are placed as close to the engine as possible, or one or more smaller catalytic converters (known as “per-cats”) are placed immediately after the exhaust manifold.

A 2-way (or “oxidation”, sometimes called an “ox-cat”) catalytic converter has two simultaneous tasks:

1. Oxidation of carbon monoxide to carbon dioxide: 2 CO + O₂ → 2 CO₂
2. Oxidation of hydrocarbons (unburnt and partially burned fuel) to carbon dioxide and water: C_xH_2x+2 + [(3x+1)/2] O₂ → x CO₂ + (x+1) H₂O (a combustion reaction)

This type of converters unlimited is widely used on diesel engines to reduce hydrocarbon and carbon monoxide emissions. They were also used on gasoline engines in American- and Canadian-market automobiles until 1981. Because of their inability to control oxides of nitrogen, they were superseded by three-way converters.

Three-way  converters unlimited have the additional advantage of controlling the emission of nitric oxide (NO) and nitrogen dioxide (NO₂) (both together abbreviated with NO_x and not to be confused with nitrous oxide (N₂O)), which are precursors to acid rain and smog.^[19]

Since 1981, “three-way” (oxidation-reduction)  converters unlimited have been used in vehicle emission control systems in the United States and Canada; many other countries have also adopted stringent vehicle emission regulations that in effect require three-way converters on gasoline-powered vehicles. The reduction and oxidation catalysts are typically contained in a common housing; however, in some instances, they may be housed separately. A three-way  converters unlimited has three simultaneous tasks:^[19]

Reduction of nitrogen oxides to nitrogen (N₂)

• {\display style {\text{C}}+2{\text{NO}}_{2}\,\rightarrow \,{\text{CO}}_{2}+2{\text{NO}}}
• {\displaystyle {\text{CO}}+{\text{NO}}\,\rightarrow \,{\text{CO}}_{2}+{\frac {1}{2}}{\text{N}}_{2}}
• {\displaystyle 2{\text{CO}}+{\text{NO}}_{2}\,\rightarrow \,2{\text{CO}}_{2}+{\frac {1}{2}}{\text{N}}_{2}}
• {\displaystyle {\text{H}}_{2}+{\text{NO}}\,\rightarrow \,{\text{H}}_{2}{\text{O}}+{\frac {1}{2}}{\text{N}}_{2}}

Oxidation of carbon, hydrocarbons, and carbon monoxide to carbon dioxide

• {\displaystyle {\text{C}}+{\text{O}}_{2}\,\rightarrow \,{\text{CO}}_{2}}
• {\displaystyle {\text{CO}}+{\frac {1}{2}}{\text{O}}_{2}\,\rightarrow \,{\text{CO}}_{2}}
• {\displaystyle a\,{\text{C}}_{x}{\text{H}}_{y}+b\,{\text{O}}_{2}\,\rightarrow \,c\,{\text{CO}}_{2}+d\,{\text{H}}_{2}{\text{O}}\qquad a,\,b,\,c,\,d,\,x,\,y\in \mathbb {Z} }

These three reactions occur most efficiently when the converters unlimited receives exhaust from an engine running slightly above the stoichiometric point. For gasoline combustion, this ratio is between 14.6 and 14.8 parts air to one part fuel, by weight. The ratio for autogas (or liquefied petroleum gas LPG), natural gas, and ethanol fuels can vary significantly for each, notably so with oxygenated or alcohol based fuels, with e85 requiring approximately 34% more fuel, requiring modified fuel system tuning and components when using those fuels. In general, engines fitted with 3-way conrters are equipped with a computerized closed-loop feedback fuel injection system using one or more oxygen sensors, though early in the deployment of three-way converters, carburetors equipped with feedback mixture control were used.