Controlling NOx emission in exhaust gases passing through three-way catalyst followed by lambda sensor

摘要

An additional sensor is employed to measure NOx in the exhaust gases. The relationship between NOx concentration and lambda value in the exhaust gases is such that, for lambda greater than 1, NOx concentration in the exhaust, and sensor output, rise steeply. Cross sensitivity towards NH3 is exhibited, so that for lambda less than 1 the sensor (24) output rises similarly. The relationship between NOx concentration and lambda value, is used in conjunction with the output signal and an internal signal of the sensor (24) to correct the value of lambda o (optimal value for catalytic action). An Independent claim is included for corresponding equipment to carry out the method. Preferred features: The NOx sensor is used in controlling an NOx reduction or -storage catalyst (22) in the exhaust duct. The mixture is controlled such that the output signal of the NOx sensor (24) shows a predetermined NOx concentration corresponding with lambda o. The sign of the internal signal is evaluated to allocate the output signal to a region corresponding to lambda less than 1 and a region corresponding to lambda greater than 1. The internal signal is a pumping current of an oxygen measurement cell in the sensor (24), which adjusts the oxygen concentration in the measurement cell to a value corresponding to lambda = 1. A salient preferred feature of the equipment is that lambda o lies between 0.99 and 1.