An advanced catalytic exhaust after-treatment system addresses the problem of NOX emissions from heavy-duty diesel trucks, relying on real-time catalyst modelling. The system consists of de-NOX catalysts, a device for injection of a reducing agent (diesel fuel) upstream the catalysts, and computer programmes to control the injection of the reducing agent and to model the engine and catalysts in real time. Experiments with 5 different air-assisted injectors were performed to determine the effect of injector design on the distribution of the injected diesel in the exhaust gas stream. A two-injector set-up was investigated to determine whether system efficiency could be increased without increasing the amount of catalyst or the amount of reducing agent necessary for the desired outcome. The results were verified by performing European standard transient cycle tests as well as stationary tests. It is shown that the injector design and the injector position strongly influence the distribution of diesel fuel spray in the exhaust gas system. The use of a second injector could not be shown to increase NOX conversion substantially during stationary or transient conditions although the potential seems to exist. The NOX conversion during a standard European transient cycle reached 39% at a fuel penalty of 5%. The outlet hydrocarbon and CO levels were decreased substantially and the formation of N2O was very low, with levels close to the detection limit of the instrument. The choice of injection strategy parameters is shown to have a large impact on system performance. The exhaust gas system described in this paper is ready for use onboard heavy-duty trucks; it provides a means of decreasing the amounts of NOX emitted from mobile diesel powered sources.