Low temperature combustion (LTC) engines, including homogeneous charge compression ignition (HCCI) and reactivity controlled compression ignition (RCCI), are promising to improve powertrain fuel consumption and reduce NO, and soot emissions by improving the in-cylinder combustion process. However, the narrow operating range of LTC engines limits the use of these engines in conventional powertrains. In range extenders, the engine is decoupled from the drivetrain, which allows the engine to operate in a limited operating range; thus, range extenders offer an ideal application for realizing the advantages of LTC engines. In this study, the optimum fuel consumption improvement of an experimentally developed 2-1 multi-mode LTC-SI range extender is investigated in a light-duty battery electric vehicle (BEV) platform. In the optimization framework, the mode-switching fuel penalty, exhaust aftertreatment catalyst light-off temperature, and engine noise, vibration, harshness (NVH) are considered. The engine operation modes include HCCI, RCCI, and conventional spark ignition (SI). The simulation results show in the city driving cycle, the single-mode HCCI and RCCI range extenders offer 11.0% and 5.4% fuel consumption improvement, respectively over a single-mode SI range extender. These improvements increase to 12.1% and 9.1% in the highway driving cycles. In the multi-mode operation, up to 1.4% higher fuel economy is achieved over different driving cycles, compared to single-mode HCCI/RCCI operation. As the driving cycle average demanded power increases, the contribution of the RCCI mode predominates the HCCI mode. (c) 2017 Elsevier Ltd. All rights reserved.