Abstract
The desire for improved fuel economy of gasoline combustion engines has turned the attention of the European and Japanese car makers as well as the associated suppliers towards the development of lean burn engines. Currently, special interest is paid to the most advanced and forward looking lean burn technology: the gasoline direct injection engine. Related to the introduction of this promising engine technology is the problem of efficiently reducing NOx emissions under lean exhaust conditions in order to meet the stringent EURO III/IV emission standards. The present paper demonstrates that NOx storage catalysts show superior performance compared to other currently evaluated catalysts such as the Ir-type or Pt-type catalyst.
The successful applicability of a NOx storage catalyst depends largely on the availability of an appropriate electronic control system. This system precisely determines the NOx loading of the storage catalyst at any time during operation, terminates the lean operating mode when the NOx uptake has dropped to an unacceptable level and releases well defined rich spikes in order to convert the adsorbed NOx into N2. Based upon semi-empirical mathematical formulas, a computer model was developed which generates precise data concerning the NOx filling level of the storage catalyst dependent on the most relevant parameters, i.e. gas inlet temperature and NOx mass flow. The model requires the incorporation of certain key parameters which have to be derived from a limited set of NOx storage measurements with the individual catalyst formulation. NOx adsorption catalysts loose efficiency by high temperature ageing as well as by sulfur poisoning. The present paper proposes the utilisation of a conventional two-point l-sensor for the detection of catalyst degradation. The laboratory test results show that sulfur-poisoning of NOx storage catalysts can most effectively be tackled by a desulfation strategy comprising high temperature treatment at reducing exhaust conditions. The thermal damages of the NOx storage catalyst can be counteracted by means of a suitable parameter adjustment of the NOx storage model.