Further improvements of emission control will require reliable estimation of emissions in real time. While many progresses are being done in terms of physical sensors, there is a wide agreement that virtual sensors and more in general real time emission models will play a central role in the next steps. While there is a deep understanding of the physics of the regulated pollutants, most general emission models tend to be too complex and poorly parametrized to be used on-line, while most data based models tend to be either insufficiently precise or of limited scope. To avoid this problem, this paper proposes a combined approach in which static maps are identified numerically, but the effect of dominant factors, like cylinder-head temperature and air path dynamics, is included on the basis of physical assumptions. Differently from most models developed for sensors, this approach is based on pure engine control unit (ECU) data, i.e. can be used for the computation of optimal control laws. As the paper shows, this strategy is able to provide not only real time estimation of NOx as a function of the ECU outputs, but also of particulate matter (PM). Copyright © 2007 International Federation of Automatic Control All Rights Reserved.