Detection of water droplets on exhaust gas sensors

In order to avoid water droplet-borne stress on planar ceramic exhaust gas sensor elements, it is suggested to apply interdigital electrodes (IDE) on the sensor elements. As soon as water droplets impinge the sensor element, the capacitance of the IDE increases. Not before the water is evaporated, the capacitance reaches its initial value, indicating that the danger of droplet-borne stress is over. During engine cold start, this signal can be used to initiate sensor heating period. Initial experiments that demonstrate the feasibility of this concept are described in this study. The influence of droplet sizes on the capacitance of interdigital electrodes is investigated and a quadratic correlation between water droplet diameter and capacitance is found out and is explained qualitatively by a physics-based model. Transient experiments investigating the behavior during droplet evaporation as well as real engine cold start experiments conclude the paper. The engine cold start tests prove that the start point and the end point of water condensation on a ceramic exhaust gas sensor element can be detected.