Catalytic Converter: Description and Operation

The catalytic converter consists of an aluminum carrier coated with a catalyst (active material) such as platinum or palladium which is enclosed in a heat resistant steel shell. The catalyst material uses residual oxygen in the exhaust gasses, and on most models secondary air from the AIS or AS systems, to convert HC, CO and in some cases NOx into less harmful substances. The secondary air from the injection systems is used to increase catalyst temperatures, allowing for more efficient operation. Since large amounts of unburned air/fuel mixtures can raise catalytic converter temperatures to dangerous levels, high temperature (overheating) warning lamps are included on many models, and a provision to stop secondary air flow is included on most models with the AIS system. Stopping secondary air flow removes oxygen from the exhaust stream, reducing oxidation and lowering converter temperature.
Three basic catalytic converters are used. The Oxidation Catalyst (OC) reduces HC and CO through catalytic oxidation, converting them into water and carbon dioxide. The Three Way Catalyst (TWC) reduces oxides of nitrogen (NOx) as well as HC and CO, producing nitrogen, water and carbon dioxide. The third type converter used is a two-stage, two-celled converter called Three Way and Sleeper Catalyst (TWC-OC). Exhaust gasses enter the first stage of this converter, and sufficient catalytic oxidation of the pollutants occurs to measurably raise the temperature of the exhaust gasses. The remaining gasses are transferred to the second cell where they are mixed with secondary air from the injection system and further reduced to nitrogen, water and carbon dioxide. The TWC-OC catalyst is the most effective at reducing the three major pollutants contained in engine exhaust gasses.