Limiting-current type air–fuel ratio sensor using porous zirconia layer without inner gas chambers: proposal for a quick-startup sensor

Design of a quick-startup air–fuel ratio (A/F) sensor for automobiles is examined. The sensor detects oxygen and/or incomplete-combustion gas concentration in exhaust to find the A/F of engine operations. We propose a compact structure sensing part which is directly stacked on a heater, where porous and dense zirconia layers are combined without inner gas chambers. We report, in this paper, the basic function of each zirconia layer for further studies of the proposed sensor: (a) porous zirconia layer stacked on a heater showed clear limiting-currents of oxygen pump in several A/F conditions. The limiting-currents were obtained near the pumping voltage of 0.5 V. The currents were 1–3 mA in air depending on the gas transparency of the porous zirconia samples; (b) one of the platinum electrodes attached to the dense zirconia layer was designed to act as a built-in standard of oxygen concentration. Here an additional current source was utilized to keep oxygen ion current. We confirmed that the electrode maintain substantially constant amount of oxygen in a wide A/F range of 10.5–30.