This simulation study discusses the application of a multimode combustion engine in a mild hybrid electric vehicle (HEV) with three-way catalytic converter (TWC). Operation in the lean combustion mode homogeneous charge compression ignition (HCCI) results in oxidation of the oxygen storage capacity (OSC) of the TWC. Thereby, the TWC's ability to convert, NOx under lean conditions is removed. Succeeding depletion of the OSC under rich spark-ignition (SI) conditions is required, winch results in significant fuel efficiency penalties. In case of a mild HEV the torque assist from the electric motor is able to extend the residence time in HCCI, thereby reducing the number OSC depletion events. The applied supervisory controller, which decides when to switch between SI and HCCI, is based on the equivalent consumption minimization strategy (ECMS) and incorporates the fuel penalties associated with mode switching and OSC depletion. It is shown that, while the impact of the OSC depletion on drive cycle fuel economy of the mild HEV is still significant, it is much smaller than in case of the vehicle without electric motor. The prolonged operation in lean HCCI mode leads to substantial amounts of tailpipe NOx for all drive cycles tested. In a case study two modifications to the system hardware are introduced with counterintuitive results. First, the HCCI regime is further constrained to conditions where engine-out NOx levels are extremely low. Second, the size of the OSC is significantly reduced, allowing a much faster and less inefficient depletion. Associated drive cycle results show a substantial reduction in tailpipe NOx while fuel economy benefits can be maintained. (C) 2016, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved.