In a 1.5 L naturally aspirated direct-injection gasoline engine,a cylinder-independent EGR(Exhaust Gas Recirculation)system is applied.After the introduction of EGR,two methods of EGR transient delay measurement are studied and compared:in-cylinder combustion pres-sure variation and direct measurement of in-cylinder CO2 concentration.The transient delay measure-ments during the process of opening the EGR valve from closed to open across the entire operating range indicate that the EGR transient delay of the 1.5 L naturally aspirated gasoline engine is between 3 to 4 combustion cycles,with some operating conditions reaching 5 to 6 combustion cycles.The du-ration of EGR transient delay is inversely proportional to both engine speed and EGR gas flow rate.Based on the EGR valve orifice flow model,the EGR transient delay measurement results,and char-acteristics,corresponding EGR ignition timing calibration strategies are developed to ensure reason-able ignition timing calibration under EGR transient operating conditions.