Spark assisted compression ignition (SAC!) combustion is a potential means of extending the high load limit of homogeneous charge compression ignition (HCCI) while maintaining high thermal efficiency. In these experiments, an HCCI engine equipped with fully-flexible valve actuation was used to explore the effect of spark assist in controlling peak heat release rates. Fueling rate was held constant at 19 mg/cycle with a net indicated mean effective pressure (IMEPn) of similar to 6.5 bar. The fraction of flame heat release was varied from 18% to 34% by controlling spark timing and unburned gas temperature (T-u) via changes in internal and external exhaust gas recirculation (EGR). Internal EGR was adjusted by varying the duration of negative valve overlap (NVO). This strategy also allowed mixture composition and equivalence ratio (phi = 1.0) to be maintained. It was found that combustion phasing (CA50) could be held constant while reducing peak heat release rates by similar to 40% and ringing intensity by similar to 75% with no penalty in thermal efficiency. Heat release analysis showed that the effects of SACI were consistent with changes in estimated laminar flame speed near the time of spark and a relatively constant temperature at the onset of end-gas auto-ignition. For the range of NVO investigated, potential variations in thermal and compositional stratification were found to have a weak effect on burn characteristics, affirming that temperature and spark timing were the primary variables affecting SACI burn rates under the conditions studied. (C) 2013 Elsevier Ltd. All rights reserved.