New combustion processes known as Controlled Auto-Ignition (CAI™) for gasoline engine and Homogeneous Charge Compression Ignition (HCCI) for Diesel engine are the subject of extensive research worldwide and particularly at IFP. Because of the thermo-chemistry conditions of the charge, the thermal NOx formation is in principle much less than with flames typical of the conventional engines. Indeed, these new combustion processes bring NOx to virtually zero “1 digit” ppm while maintaining a very high thermodynamic efficiency of the combustion.
One major issue in the development of CAI combustion for gasoline engines remains the limited engine speed and load range that can be operated in CAI combustion mode, while maintaining near zero NOx and acceptable noise emissions. One of the most promising ways to increase the CAI combustion range lies in the formulation of dedicated fuels, optimised to enhance combustion initiation by modifying and controlling the auto-ignition characteristics and/or the fuel physical and chemical properties.
To select the most relevant fuel formulations, a new procedure has been set-up that allows a better characterization of the fuel impact on engine operating in CAI, for various conditions of speed and load. The use of this methodology for a large set of fuels, formulated according to their auto-ignition characteristics, volatility and chemical composition has underlined the complexity of the action of fuel and has enabled to point out the most relevant fuel characteristics for a large range of operating conditions.