Modern petroleum based fueled vehicles are fitted with three-way catalytic converters. 3-way converters convert hydrocarbons and carbon monoxide to water and carbon dioxide, as well as convert the oxides of nitrogen, nitric oxide and nitrogen dioxide, back into harmless nitrogen and oxygen molecules.
Older catalytic converters converted hydrocarbons and carbon monoxide into water and carbon dioxide, but were not able to convert the oxides of nitrogen.
The term 'three-way' is in relation to the three regulated emissions the converter is designed to reduce: carbon monoxide, hydrocarbons or volatile organic compounds, known as VOCs, produced from evaporated unburned fuel, and nitrogen oxides.
The converter uses two different types of catalysts to reduce the pollutants: a reduction catalyst and an oxidation catalyst.
The exhaust gases first pass over the reduction catalyst in the converter. The platinum and rhodium coating helps to reduce the oxides of nitrogen, together known as ‘NOX’ emissions.
When a nitric oxide or nitrogen dioxide molecule comes into contact with the coating, it strips the nitrogen atom out of the molecule and retains it. This frees up the one or two oxygen atoms in the molecule which combine in pairs to form molecules of oxygen.
The nitrogen atoms bond with other nitrogen atoms that are retained in the catalyst and form molecules of nitrogen. So two molecules of nitric oxide become one molecule each of nitrogen and oxygen, or two molecules of nitrogen dioxide become one molecule of nitrogen and two molecules of oxygen.
The exhaust gases then flow over the oxidation catalyst in the converter. This has the effect of reducing any unburned hydrocarbons and carbon monoxide by oxidizing them over the platinum and palladium coating. This aids the reaction of the carbon monoxide and hydrocarbons with any remaining oxygen in the exhaust gas.
Each carbon monoxide molecule combines with an oxygen molecule to make one less harmful carbon dioxide molecule. Because of strict emission requirements, vehicles with a 3-way catalytic converter have a feedback system, called looping.
The electronic control unit, or ECU, monitors the air-fuel ratio by using an exhaust gas oxygen, or EGO, sensor, also known as a lambda sensor. This sensor tells the engine computer how much oxygen is in the exhaust and uses this information via the ECU to control the fuel injection system.
The ECU can increase or decrease the amount of oxygen in the exhaust by adjusting the air-to-fuel ratio. The system ensures that the engine runs at close to the stoichiometric point in normal driving conditions. It also ensures that there is always sufficient oxygen in the exhaust system to allow the oxidization catalyst to deal with unburned hydrocarbons and carbon monoxide.
