CoO Sensors for Measurement and Control of Exhaust from Lean-Burn Engines

An experimental exhaust sensor using CoO ceramic is being developed for measuring and controlling air-to-fuel ratio (A/F) of internal combustion engines operating under lean-burn conditions. The operation of the sensor is based on the fact that at elevated temperatures the electrical resistance of CoO depends on the oxygen partial pressure (P_O2). At controlled temperatures, the combination of good P_O2 sensititivty, high reproducibility of the electrical properties and small temperature coefficient of resistance of CoO materials make this sensor suitable for the lean A/F range (15-20) where exhaust P_O2 depends only weakly on A/F ratio. At low P_O2 (A/F<13.5) and high temperatures, CoO reduces to Co; therefore, prolonged exposure of the sensor to these conditions must be avoided. The material properties and the sensor characteristics are discussed and results of tests on car-mountable CoO sensors are presented. Durability and large-scale manufacturability of these sensors have not yet been established.