Low Temperature Combustion (LTC) engines are promising to improve powertrain fuel economy and reduce NOx 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. The engine’s limited operating range can be improved by taking advantage of electrification in the powertrain. In this study, a multi-mode LTC-SI engine is integrated with a parallel hybrid electric configuration, where the engine operation modes include Homogeneous Charge Compression Ignition (HCCI), Reactivity Controlled Compression Ignition (RCCI), and conventional Spark Ignition (SI). The powertrain controller is designed to enable switching among different modes, with minimum fuel penalty for transient engine operations. A Pontryagin’s Minimum Principal (PMP) methodology is used in the energy management supervisory controller to study the powertrain energy flow and to select the optimum electrification level. The simulation results show 1.4% to 8.8% fuel consumption reduction for the hybrid electric powertrain with the multi-mode LTC-SI engine over a single-mode SI engine in the identical powertrain platform. Moreover, the results for the UDDS driving cycle show the maximum benefit of the multi-mode LTC-SI engine is realized in the mild electrification level, where the LTC mode operating time increases dramatically from 1.5% in Plug-in Hybrid Electric Vehicle (PHEV) to 30.5% in mild HEV.