Optimal Operation of a Combined NOx/Oxygen Zirconia Sensor Under Lean Burn Conditions

This article describes the development and tests of a combined NO_x/oxygen Zirconia sensor. To achieve accurate NO_x and oxygen measurements under lean-burn conditions, preliminary model scale experiments with a premixed methane-air burner are carried out with current production oxygen sensors as well as with our own oxygen probe. It is found from these tests that accurate oxygen sensing requires sensor operating temperatures much lower than usually set, about 600°C. Moreover, this temperature is a good compromise for NO_x sensitivity and time response of the sensor. Accordingly, a new NO_x/O₂ sensor has been designed and prototyped. Held in a spark plug type housing, it simultaneously monitors the NO_x and oxygen content of the exhaust gas. Over the wide 0₂ range of interest, model scale tests of the sensor with burnt gases have shown promising agreement with laboratory analysers.