Both CO and hydrocarbons are converted in the oxidation catalyst to carbon dioxide and water vapor which are non-toxic gases. The conversion of CO and HC in the catalyst requires oxygen, as shown in the reaction equations. Usually there is not enough oxygen in the exhaust gases of LPG engines to burn all of the pollutants. Oxidation catalyst systems frequently require that extra air, called secondary air, be introduced into the exhaust system in front of the catalyst. Catalyst activity increases with temperature.
Higher temperatures are necessary for hydrocarbon conversion.
LPG exhaust contains short carbon chain hydrocarbons which are more difficult to convert in the catalyst than those found in diesel or gasoline exhaust. As illustrated in the graph, the shorter the carbon chain the higher the conversion temperature. In the three-way catalyst, reactions between CO, HC, and NO x result in the simultaneous removal of all three major exhaust pollutants. 2 Concentrations of pollutants in the exhaust gas depend on the fuel mixture composition. At lean fuel mixtures the exhaust gases contain little carbon monoxide or hydrocarbons but high concentrations of NO x .