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Exhaust systems must satisfy a wide range of requirements, including lowering emissions to comply with future fuel economy and emissions regulations. To help meet these requirements, new emissions control systems have been developed today. In addition, since air-fuel ratio (hereafter, A/F) control has a major impact on emissions, a new two-A/F sensor system with A/F sensors provided both upstream and downstream of the catalyst was developed, incorporating an A/F control capable of further lowering emissions with greater robustness. This development identified the hysteresis characteristics of the O2 sensor downstream of the catalyst as an important factor affecting emissions during conventional A/F control. Subsequently, reaction analysis was carried out using sensor reaction models and by evaluating sensors under real-world operating conditions.

New 2A/F systems different from usual A/F-O2 systems are being developed to cope with strict regulation of exhaust gas. In the 2A/F systems, 2A/F sensors are equipped in front and rear of a three-way catalyst. The A/F-O2 systems are ideas which use a rear O2 to detect exhaust gas leaked from three-way catalyst early and feed back. On the other hand, the 2A/F systems are ideas which use a rear A/F sensor to detect nearly stoichiometric gas discharged from the three-way catalyst accurately, and to prevent leakage of exhaust gas from the three-way catalyst. Therefore, accurate detection of nearly stoichiometric gas by the rear A/F sensor is the most importrant for the 2A/F systems. In general, the A/F sensors can be classified into two types, so called, one-cell type and two-cell type. Because the one-cell type A/F sensors don’t have hysteresis, they have potential for higher accuracy.