An On-Board Diagnosis Method for Three-Way Catalyst Deterioration

To meet On Board Diagnostic System Phase-II (OBD-II) regulations, a new diagnosis method is presented which can identify slight deterioration of three-way catalysts in a spark ignition engine. A correlation method is adopted for accurate diagnosis. The O2-feedback control signals, which keep the air-ratio stoichiometric, are used as search signals for the diagnosis. In this process, fluctuations of the air-fuel ratio are measured using oxygen sensors installed in the front and after of the catalyst and cross-correlation functions are calculated to indicate the catalyst efficiency. For precise diagnoses, the frequency of O 2-feedback control is modified without affecting emissions. For reliable diagnosis, the obtained indices are corrected by estimating catalyst temperature, space velocity, and engine speed. Steady state engine operation and FTP tests show that the catalyst deteriorated by heat damage can be discriminated from new catalyst material.