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This paper presents an analysis of combustion phasing in a controlled auto-ignition (CAI) engine fuelled with gasoline. Auto-ignition was achieved using an exhaust gas trapping method via negative valve overlap (NVO). Under slightly lean mixture conditions variable intake and exhaust valves timings were applied in order to analyze influence of amount of retained exhaust on auto-ignition timing and combustion duration. Combustion on-set was independent of exhaust valve closing event, which was responsible for amount of trapped residuals. However, it was found that auto-ignition timing was determined by intake valve timing. Combustion duration was affected by both exhaust and intake valve timings. Direct injection allowed for application of different mixture formation strategies including in-cylinder fuel reforming during the NVO phase. When fuel was injected in the late stage of NVO increase of air-fuel ratio (AFR) caused a retard of auto-ignition and reduction of heat release rate.