Temperature-controlled CO, CO2 and NOx sensing in a diesel engine exhaust stream

Results are presented for in situ measurement of CO2, NOx and CO concentrations in exhaust emitted from an automotive diesel engine. These data were obtained with a series of temperature-controlled sensor probes and compared to exhaust concentrations measured simultaneously with an exhaust motor analyzer. The CO2 sensor is a Nernstian type with a Li2CO3|Au sensing electrode, a Li3PO4 electrolyte and a Li2TiO3 + TiO2|Au reference electrode. The total NOx sensor measures the difference between potential responses of two porous Pt electrodes on an ion-conducting stabilized cubic zirconia support. The potential difference results because one Pt electrode is covered with a catalyst to achieve a locally equilibrated NOx mixture, whereas the other electrode exposed to the sensing gas mixture, pre-equilibrated at a different temperature. The CO sensor is a resistive type and responds to selective sorption of reducing species. All sensor types have simple planar configurations but require accurate temperature control to deal with significant fluctuations in the engine exhaust stream. A constant sensor temperature of typically 400 °C is achieved with a ceramic heater strip and tight control using a direct digital PID algorithm. The NOx sensor requires a second temperature-controlled pre-conditioning filter. This filter is integrated in the probe and controlled with a thermo-coax heating wire and another PID implementation. Generally, the sensors responses in the engine agree with the laboratory tests, but calibration errors resulted due to lack of thermal homogeneity of the sensor. In addition, some drift is observed due to particulate contamination from the exhaust stream. The CO2 sensor appeared to be the most robust, though the probe signal may have been influenced by the evacuation effect at high gas velocities present in the car exhaust.