Studies of a Heavy-Duty Diesel Aftertreatment System Based on the NOX Storage and Reduction Technology

The legislative limits for NOX emissions in the exhaust gas from heavy-duty diesel engines will be decreased in the European Union from today 3.5 g/kWh (Euro IV) to 2.0 g/kWh in 2008 (Euro V). In the USA and Japan a similar trend can be seen. Accordingly, the development of an exhaust aftertreatment system for heavy-duty trucks seems unavoidable. In this work, an 11 l diesel engine rig has been built up including an aftertreatment system which is based on the NOX storage and reduction approach. BaO-based NOX storage and reduction catalysts of in total 16.8 l were used and also oxidation catalysts of in total 8.4 l to oxidise NO to NO2 and pre-oxidise the injected hydrocarbons.

The NOX storage and reduction technology has already been applied in lean-burn gasoline vehicles and has shown NOX reduction degrees of 80 % and above. It is thus a promising alternative for the reduction of NOX in diesel truck exhausts. In the NOX storage and reduction concept, NOX is stored as Ba(NO3)2 under long lean periods (1-2 min) and released and reduced under short rich periods (0-10 s). It is the aim of this project to both optimise the aftertreatment system with respect to a high NOX reduction and a low fuel penalty and also create models of the catalysts which may be used to control the NOX reduction performance under transient conditions.

Stationary NOX reduction experiments have been performed on the engine rig. The maximal degree of NOX reduction was approximately 60 % under stationary operation. The degree of NOX reduction depends on a number of parameters as the catalyst temperature, the injection time, the cycle time or the dosed amount of hydrocarbons (diesel fuel). To keep the fuel penalty low, a bypass system has been installed, which bypassed approximately 90 % of the exhaust flow under the regeneration periods. The period when the flow through the catalyst is reduced, the bypass time, has turned out to be an important parameter to achieve an improved low temperature NOX reduction without increasing the fuel penalty. An optimisation of the NOX reduction performance has been done at different temperatures using Design of Experiments. The optimisation results served as the base of a temperature dependent dosing strategy which controlled the injections under transient conditions. In a European Transient Cycle (ETC), a 60 % NOX conversion was achieved with a fuel penalty of 6.6 %.

A first model for the NOX storage and reduction catalyst has been developed and tested under stationary conditions. A mean-field model of the oxidation catalyst placed upstream of the NOX storage and reduction catalyst has also been created and tested successfully in an ETC.