Setting up engines to meet emissions limits often involves extensive steady-state calibration activities combined with ad-hoc strategies to compensate for transient operation. As engines become more complex and acceptable emissions levels ever lower, this task is becoming increasingly time consuming and expensive.The inclusion of models in the engine control units offers a way to reduce some of this calibration effort. Model-based control is an active area of research with advanced approaches now being proposed. One example is the use of real-time models to regulate the burn angle during transient manœuvres. This paper describes the application of a control-orientated combustion model to control directly emissions during transients. The model is used to optimize and constrain the NOx emissions directly, rather than controlling an inferred variable such as the burn angle. This has the benefit that calibration engineers will be able to set the emissions trade-off directly.