Partially premixed combustion (PPC) with gasoline fuels is a new promising combustion concept for future internal combustion engines. However, many researchers have argued the capabilities of research octane number (RON) and Motor Octane Number (MON) to describe the autoignition behaviour of gasoline fuels in advanced combustion concepts like PPC. The objective of this study is to propose a new method, called PPC number, to characterize the auto ignition quality of gasoline fuels in a light-duty direct injected compression ignition engine under PPC conditions. The experimental investigations were performed on a 4-cylinder Volvo D4 2 litre engine. The ignition delay which was defined as the crank angle degrees between the start of injection (SOI) and start of combustion (SOC) was used to represent the auto ignition quality of a fuel. The ignition delays of primary reference fuels PRF (blends of n-heptane and iso-octane) were used to develop a reference curve where a PPC metric for gasoline could be based on. The PPC number of a specific gasoline is defined as the octane number of the PRF, which has the same ignition delay as gasoline under the same operating condition. Twelve different gasolines, having RON values between 55 and 95, were tested at two different operating conditions of 0% exhaust gas recirculation (EGR) and 40% EGR levels namely Case0 and Case40 respectively. The intake pressures of Case0 and Case40 were 1.5 bar and 1.8 bar respectively with a constant inlet temperature of 110
C. The PPC numbers of all gasolines were measured and the relevancy of the other indices such as RON, MON, Octane Index and HCCI number were assessed. When the indices were compared, PPC number showed consistence and continues correlations with ignition delays in both conditions. Results also revealed that the spray target and piston geometry gave a big impact to the auto ignition quality of fuels.