Development of High Accuracy and Quick Light-off NOx Sensor 2018-01-0334
For the purpose of coping with the strengthening of NOx exhaust gas control and fuel consumption control, it is indispensable to improve the NOx purification capacity. In view of this, vehicle manufacturers are in the course of developing high performance SCR (Selective Catalytic Reduction) systems [1, 2]. For such SCR systems to be realized, high precision NOx sensors for carrying out urea injection quantity control and SCR degradation diagnosis are absolutely indispensable. Detection of NOx concentration by means of a NOx sensor is generally performed as follows: O2 is discharged by means of an O2 detection electrode; remaining NOx is decomposed by a NOx detection electrode; NOx concentration is then detected as electric current that flows when oxygen ions are conduct through solid electrolyte. In order to detect NOx of ppm-order, it is necessary to detect minute current of nA-order with high accuracy. Very small amounts of residual O2 and conducted electrons (having temperature characteristics) that flow in solid electrolyte constitute causal factors for accuracy errors. In order to cancel these causal factors for errors, a technology whereby electrodes for cancelling causal factors for errors are placed next to NOx detection electrodes (three-electrode structure), has been established. As a result, high accuracy has been achieved. For NOx detection electrodes, Rh is generally used as a noble metal active against NOx. However, Rh is liable to be oxidized. When sensors are to be started up, therefore, it takes time to resolve oxidation, with the result that the light-off time is delayed. To solve this problem, quick light-off has been realized by a special control process (whereby H2O is electrolyzed by utilizing water vapor in exhaust gas, leading to the generation of H2, with the result that the adsorbed oxygen is forcibly removed). Due to these technologies, a NOx sensor capable of detecting NOx with high accuracy at an early timing has been developed. A report is given hereby of this sensor.