Catalytic Converter: Description and Operation

PURPOSE
The three way catalytic converter simultaneously removes up to 90% of all three major pollutants, (HC, CO, and oxides of nitrogen). A complete catalytic reaction depends on the fuel mixture staying within a very narrow range (14.7:1 +/- 1%), which can only be achieved with a properly functioning oxygen sensor system.

CONSTRUCTION

Typical Three Way Catalytic Converter:

The catalytic converter consists of a metal housing, and a ceramic grid substrate coated with small amounts of platinum and rhodium (the catalysts). Most converters contain only a few grams of the catalysts.

Two catalytic coverters are used on the V6 engine. This provides for a slower flow of exhaust thorough the catalytic converters and provides more time for complete catalytic reaction.

OPERATION
The effective conversion of pollutants begins at an operating temperature of about 250°C (480°F). The ideal operating temperature for maximum conversion and long service life is 400°C - 800°C (750°F - 1500°F). Engine malfunctions, for example misfires, can increase converter temperature to greater than 1400°C (2500°F). High temperatures like this can destroy the converter by melting of the substrate material. This renders the catalysts ineffective and can result in an exhaust restriction.

The use of leaded fuel must be avoided. The lead compounds will deposit in the pores and on the surfaces of the active material, reducing or eliminating exposure of the catalysts to the exhaust gasses. Excessive engine oil residues can also poison the catalyst.

DIAGNOSIS
The proper function of the catalytic converter is continuously evaluated by the On-Board Diagnostic II (OBD II) system. The OBD II system uses an oxygen sensor in the outlet of each catalytic converter to determine if the converter is properly functioning. The signals from the Oxygen sensors up stream of the catalytic converters are compared to the signals from the Oxygen sensors down stream of the catalytic converters and the OBD II system then determines the level the converters are functioning at.