This simulation study explores the potential synergy between the HCCI engine system and three hybrid electric vehicle (HEV) configurations, and proposes the supervisory control strategy that maximizes the benefits of combining these two technologies. HCCI operation significantly improves fuel efficiency at part load, while hybridization aims to reduce low load/low speed operation. Therefore, a key question arises: are the effects of these two technologies additive or overlapping? The HEV configurations include two parallel hybrids with varying degrees of electrification, e.g. with a 5kW integrated starter/motor (“Mild”) and with a 10 kW electric machine (“Medium”), and a power-split hybrid. The engine is a dual-mode, SI-HCCI system and the engine map reflects the impact of HCCI on brake specific fuel consumption. The new HEV supervisory controller, called e-HCCI, is proposed considering two additional criteria: (i) maximize the time spent in the HCCI operating region, and (ii) minimize SI-HCCI mode transitions. The relative fuel economy improvements of using a dual-mode SI-HCCI engine over an SI engine for a conventional powertrain (non-hybrid) is 17%. When the dual-mode engine is used in the “Mild” HEV using the e-HCCI strategy, the percent improvement jumps to 35%. This indicates a strong synergy, and creates a very compelling case for the “Mild” parallel HEV w/ HCCI. Gains on a relative basis diminish somewhat with the “Medium” hybrid and significantly more with stronger power-split hybrids, as the latter typically moves engine load above the HCCI knock limit.