Hayashita, G., Ohtani, M., Aoki, K., and Okazaki, S., "New Exhaust Emission Control System with Two A/F Sensors," SAE Technical Paper 2017-01-0917, 2017, doi:10.4271/2017-01-0917.
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. The results of this analysis found that O2 sensor hysteresis is generated based on certain output principles, and that the adoption of a limit current type A/F sensor would be an effective countermeasure. Finally, control logic was proposed using the high stoichiometric detection performance of A/F sensors to prevent saturation of the oxygen storage capacity. As a result, the developed emissions control systems achieves excellent low emissions performance, including a 45% decrease in NOx.This control method was applied to a 2.5 liter in-line 4-cylinder engine and it was verified to result in lower exhaust emissions than conventional methods. Toyota plans to deploy the new A/F control system as a new common concept in all of its Toyota New Global Architectures (hereafter, TNGA) engine series.