Air-Fuel Ratio Imbalance Diagnostic of Spark-Ignited Engines based on Time-varying Harmonic Signal Estimation

This paper presents air-fuel ratio (AFR) imbalance diagnostic of a spark-ignited (SI) engine. AFR imbalance occurs when AFR in one or more cylinders is different from other cylinders due to cylinder-specific malfunctions. It leads to the failure of accurate AFR control which is the main emission control mechanism of an SI engine. Thus, the legislative regulation has been imposed by California Air Resource Board (CARB) to detect significant AFR imbalance such that tailpipe emissions exceed the applicable emissions thresholds. To this end, the time-varying harmonic signal estimation-based AFR imbalance diagnostics are developed. Two different filters, including the time-varying notch filter (TV NF) and adaptive Kalman filter (AKF), are applied to exhaust gas oxygen (EGO) sensor outputs, such as a heated exhaust gas oxygen (HEGO) and universal exhaust gas oxygen (UEGO) sensors, to estimate the imbalance signal. The diagnostic metric depending the filter used is monitored to diagnose AFR imbalance real-time. The effectiveness of the developed diagnostics are illustrated with the turbo-charged SI natural gas (NG) engine.