NOx storage and reduction experiments have been performed with stationary operation of a heavy-duty diesel engine rig. An optimization of the NOx reduction performance has been done using experimental design. The adjustable parameters in this study were cycle time, injection time, injection rate and bypass time (period of reduced flow through catalysts). NOx was reduced by 50-60% (3.3-4.1 g/kWh) with a fuel penalty below 5%. It was shown that experimental design was efficient for optimizing the NOx reduction and this systematic approach enabled important conclusions to be drawn about the system performance. (c) 2004 Elsevier B.V. All rights reserved.