This paper reports on the third part of a continued study (SAE Papers 961182, 971636) to develop the Ignition Quality Tester (IQT™). Past research has shown that this automated laboratory/refinery apparatus can be used to accurately predict the cetane number of middle distillates and alternative fuels using small sample volumes (< 50 mL). The paper reports on the main objective of a study performed by Advanced Engine Technology Ltd. (AET), in co-operation with its research partners. The primary research objective of this work is to further the understanding of fuel preparation (fuel air mixing) and start of combustion processes in the IQT™. Key to this understanding is the manner in which single molecule compounds and full boiling-range diesel fuels behave during these processes.Insights are provided into the manner in which the American Society for Testing and Materials (ASTM) D-613 primary reference fuels (PRFs) undergo fuel preparation and start of combustion in the IQT™. The effect of oxygen concentration on the ignition delay of blends of PRFs (n-cetane and heptamethylnonane) is compared to the ignition behaviour of ASTM D-613 Check Fuels under similar conditions. The study has led to an improved understanding of the relationship between the fuel preparation/start of combustion behaviour of the reference compounds that define the cetane number scale relative to the intended test fuels.Also addressed are issues specific to combustion research based on the IQT™ test apparatus. Areas covered include: (1) blending approach to estimate the cetane number (CN) of a sample, and (2) the development of calibration procedures.