Diesel engine NOx and PM emissions are characterized by a combination of many system inputs. Since more than one combination of inputs leads to the same output and the high number of degrees of freedom, it is an ill-posed problem to determine the optimal input combination. In this paper the introduction of a two dimensional coordinate system is addressed, with the target of a separate and independent control of NOx and PM. Finally, a validation on testbench data is presented. Copyright © 2007 International Federation of Automatic Control All Rights Reserved.