Aftertreatment Catalyst Design for the New DaimlerChrysler Supercharged 4-Cylinder Engine with Direct Gasoline Injection

The launching of direct injection gasoline engines is currently one of the major challenges for the automotive industry in the European Union. Besides its potential for a notable reduction of fuel consumption, the engine with direct gasoline injection also offers increased power during stoichiometric and stratified operation. These advantages will most probably lead to a significant market potential of the direct injection concept in the near future.

In order to meet the increasingly more stringent European emission levels (EURO IV), new strategies for the exhaust gas aftertreatment are required. The most promising technique developed in recent years, especially for NO_x conversion in lean exhaust gases, is the so-called NO_x storage catalyst. The NO_x storage technology achieves a high level of NO_x conversion by storing nitrogen oxides reversibly as nitrates during lean operating conditions, while the periodic regeneration of the NO_x storage components takes place under rich, i.e. under reducing conditions. In order to take full advantage of the fuel savings over the entire life-time of the vehicle, the most important task for the car and the catalyst manufacturers is the application and improvement of the thermal aging stability of this catalyst technology.

The present paper focuses on the application of a NO_x storage catalyst system on a new supercharged 4-cylinder engine with direct gasoline injection and also on the progress of NO_x storage catalysts' development. On the basis of model gas tests and detailed studies at the engine test bench different NO_x storage catalyst generations are compared. The key criteria to evaluate the different catalyst generations are on one hand the thermal aging stability and on the other hand the NO_x storage capacity as a function of temperature under dynamic engine operating. Both, the model gas tests as well as the tests at the engine bench show that the new NO_x storage technology possesses a notable improved stability with respect to thermal stress and also that the NO_x storage window has been extended in the higher temperature range. Due to an intelligent combination of active components the improvement of the NO_x conversion was achieved without any loss in the overall behavior of the catalyst. The temperature for a complete desulfation of the new catalyst generation is in the range of 600°C which is comparable to commercial NO_x storage catalysts. Under oxidizing conditions as well as during the NO_x regeneration phases the hydrocarbon oxidation activity remains at a high level.