In order to minimize pollutant emissions and more specifically nitrogen oxides, a decentralized multivariable robust control has been designed for the airpath of a Diesel engine with a turbocharger, an intake throttle and Exhaust Gas Recirculation. The control system is designed with a fractional-order approach called CRONE (Commande Robust d'Ordre Non Entier) control, via a MIMO open-loop transfer function optimization, which maintains performance and robust stability for a wide set of operating points. The experimental results from a test-bench show the relevance of the proposed approach.