Exhaust Gas Sensor with High Water Splash Resistant Layer for Lower Emission

Increasingly stringent regulations call for the reduction of emissions at engine startup to purify exhaust gas and reduce the amount of CO₂ emitted. Air-fuel ratio (A/F) sensors detect the composition of exhaust gas and provide feedback to control the fuel injection quantity in order to ensure the optimal functioning of the catalytic converter. Reducing the time needed to obtain feedback control and enabling the restriction-free installation of A/F sensors can help meet regulations. Conventional sensors do not activate feedback control immediately after engine startup as the combination of high temperatures and splashes of condensed water in the exhaust pipe can cause thermal shock to the sensor element. Moreover, sensors need to be installed near the engine to increase the catalyst reaction efficiency. This increases the possibility of water splash from the condensed water in the catalyst. Therefore, to achieve both a quick feedback control and restriction-free installation, the development of a high water splash resistance exhaust gas sensor is necessary. As a means to improve water splash resistance, we focused on the water repellent function of the Leidenfrost effect. We discovered that the water repellent function of the Leidenfrost effect appeared when the thermal conductivity of the water splash resistant layer is 1W/m·K or higher in the range of sensor control temperature. In addition, by adopting α-Al₂O₃ in the aggregate of water splash resistant layer and keeping the porosity within 30% to 40%, we were able to achieve both good sensor responsiveness and water repellent function, which normally contradict each other. Adopting a water splash resistant layer with water-repellent function significantly improved the water splash toughness, enabling both a quick feedback control immediately after engine startup and restriction-free installation.