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This paper presents results from an experimental programme researching the in-cylinder conditions necessary to obtain gasoline Controlled Auto-ignition (CAI) combustion in a 4-stroke engine. A single-cylinder, variable compression ratio research engine is used for all experiments. Investigations concentrate on establishing the CAI operating range with regard to Air/Fuel ratio and Exhaust Gas Re-circulation (EGR) and their effect on ignition timing, combustion rate and variability, ISFC, and engine-out emissions, such as NOx, CO, and unburned HC. Comprehensive maps for each of the measured variables are presented and in relevant cases, these results are compared to those obtained during normal spark-ignition operation so that the benefits of CAI combustion can be more fully appreciated.