Catalytic converter type
Two-way
A 2-way (or "oxidation", sometimes called an "oxi-cat") catalytic converter has two simultaneous tasks:
Oxidation of carbon monoxide to carbon dioxide: 2 CO + O2 → 2 CO2
Oxidation of hydrocarbons (unburnt and partially burned fuel) to carbon dioxide and water: CxH2x+2 + [(3x+1)/2] O2 → x CO2 + (x+1) H2O (a combustionreaction)
This type of catalytic converter 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 Because of their inability to control oxides of nitrogen, they were superseded by three-way converters.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
Three-way catalytic converters have the additional advantage of controlling the emission of nitric oxide (NO) and nitrogen dioxide (NO2) (both together abbreviated with NOx and not to be confused with nitrous oxide (N2O)), which are precursors to acid rain and smog.
Since 1981, "three-way" (oxidation-reduction) catalytic converters 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 catalytic converter has three simultaneous tasks:Reduction of nitrogen oxides to nitrogen (N2) Oxidation of carbon, hydrocarbons, and carbon monoxide to carbon dioxide
These three reactions occur most efficiently when the catalytic converter 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 be significantly different 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 catalytic converters are equipped with a computerized closed-loop feedback fuel injection system using one or more oxygen sensors,[citation needed] though early in the deployment of three-way converters, carburetors equipped with feedback mixture control were used.
Three-way converters are effective when the engine is operated within a narrow band of air-fuel ratios near the stoichiometric point, such that the exhaust gas composition oscillates between rich (excess fuel) and lean (excess oxygen). Conversion efficiency falls very rapidly when the engine is operated outside of this band. Under lean engine operation, the exhaust contains excess oxygen, and the reduction of NOx is not favored. Under rich conditions, the excess fuel consumes all of the available oxygen prior to the catalyst, leaving only oxygen stored in the catalyst available for the oxidation function.
Closed-loop engine control systems are necessary for effective operation of three-way catalytic converters because of the continuous balancing required for effective NOx reduction and HC oxidation. The control system must prevent the NOx reduction catalyst from becoming fully oxidized, yet replenish the oxygen storage material so that its function as an oxidation catalyst is maintained.
Three-way catalytic converters can store oxygen from the exhaust gas stream, usually when the air–fuel ratio goes lean.[20] When sufficient oxygen is not available from the exhaust stream, the stored oxygen is released and consumed (see cerium(IV) oxide). A lack of sufficient oxygen occurs either when oxygen derived from NOx reduction is unavailable or when certain maneuvers such as hard acceleration enrich the mixture beyond the ability of the converter to supply oxygen.
