Selective catalytic reduction (SCR) has become widely used to reduce NOx emissions especially for medium and heavy duty CI engines, while lean NOx traps (LNT) are a typical choice for light duty engines. In the field of small engines, SCR is usually not applied, because an expensive additional AdBlue supply is necessary and the emission limits can be also achieved with less complex methods. Nevertheless, if the engine is used as range extender of a hybrid electric vehicle, in engine-only operation it must also fulfill the Diesel emission legislation. There SCR is a sensible and maybe in view of future legislation only choice. A key problem of this choice, is that for small engines, typically the available space for the SCR is limited and leads to a strongly reduced storage capability of the catalyst. Some of the features of a small system render a purely data based approach feasible as system dynamics are faster and phenomenon like storage are less pronounced. Thus in this work a black box modeling approach is presented and combined with an NMPC strategy for closed loop SCR control. The proposed control strategy is validated in simulation and experiments on a testbench.