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Primary diesel and gasoline reference fuels, along with secondary reference diesel fuels across a very broad cetane range were tested in an Ignition Quality Tester (IQT) unit using the ASTM D6890 protocol. Additionally, numerous pure component fuels across a range of hydrocarbon size and structure were evaluated. The reference fuels’ ignition delay (IGD) followed expected trends, however, the diesel PRF fuels in the low cetane range produced DCNs (derived cetane numbers) that were moderately higher (shorter IGDs) than their cetane reference values. From the perspective of fuel size, IGD shows a significant ‘shortening’ - faster nature with increased fuel carbon number. For a given carbon number fuel molecule, normal alkanes showed the ‘fastest’ IGD, with alkenes and branched alkyl aromatics leading to moderately longer IGDs. Cyclo-paraffins had the ‘slowest’ - longest IGDs. Various methods were used to determine the IGD of the various fuels.