In this paper, we present a supervisory control strategy for a hybrid electric vehicle which best exploits the NO/sub x/ characteristics of such a mixed-combustion mode engine, while optimizing fuel economy and meeting driver's demand. This strategy is an extension of our adaptive equivalent consumption minimization strategy (A-ECMS) control strategy, suitably modified to explicitly minimize NO/sub x/ emissions. The HEV configuration, coupled to this control strategy, allows to very effectively manipulate the operating points of the engine to be primarily constrained in the HCCI regime, with very low NO/sub x/ emissions. This results in dramatically reduced NO/sub x/ emissions during actual driving conditions, while retaining the high fuel economy of CIDI engines and hybrids and maintaining the performance envelop of the vehicle. In summary, the control strategy automatically exploits the relatively small region of HCCI operation to minimize NO/sub x/ emissions.