Abstract: In order to explore the effects of exhaust gas recirculation (EGR) rates, hydrogen blending ratios, and ignition timings on the combustion and emission performance of a natural gas engine under partial load conditions, combustion and emission characteristic experiments were conducted on a six-cylinder spark ignition natural gas internal combustion engine under different EGR rates, hydrogen blending ratios, and ignition timings. The tests were performed under stoichiometric conditions with the engine speed kept constant at 900 rpm and the throttle opening fixed at 30%. Cylinder pressure signals were collected by a Kistler 6117N piezoelectric sensor, and crankshaft angle signals were measured by a Kistler 2613B crankshaft angle encoder with a minimum counting interval of 0.1℃A, and 101 cycles were recorded. During the experiment, an oxygen sensor was installed on the exhaust side to monitor the oxygen content in the engine exhaust in real time, transmitting the air-fuel ratio signal to the electronic control unit (ECU), which maintained the excess air coefficient φ_a=1 under different EGR rates and hydrogen blending ratios. The results showed that with increasing EGR rates, the peak values of cylinder pressure, temperature, and total heat release rates decreased. When the ignition timing was set at 18℃A bTDC, as the EGR rates increased from 0 to 16.3%, the peak values of cylinder pressure, temperature, and heat release rates decreased by 28.4%, 17.7%, and 53.6%, respectively. Conversely, the peak values of cylinder pressure, temperature, and heat release rates increased with increasing hydrogen blending ratios and advanced ignition timings. As the hydrogen blending ratios increased from 0 to 50%, the peak values of cylinder pressure, temperature, and heat release rates increased by 13.1%, 11.3%, and 27.9%, respectively. As the ignition timing advanced from 12℃A bTDC to 30℃A bTDC, the peak values of cylinder pressure and temperature increased by 27.5% and 10% respectively. The crankshaft angles corresponding to the peak values of these parameters shifted away from the top dead center (TDC) with increasing EGR rates, and shifted closer to TDC with increasing hydrogen blending ratios and advanced ignition timings. The coefficient of variation of the indicated mean effective pressure (COV_IMEP) showed an opposite trend to cylinder pressure with changes in EGR rates, hydrogen blending ratios, and ignition timings. The brake thermal efficiency of engine increased with increasing EGR rates and hydrogen blending ratios. When the hydrogen ratio was 50%, as the EGR rates increased from 0 to 16.3%, the peak brake thermal efficiency increased from 26.4% to 28.2%. When the EGR rate was 0, as the hydrogen blending ratio increased from 0 to 50%, the peak brake thermal efficiency increased from 24.5% to 26.4%. Within a certain range, the brake thermal efficiency first increased and then decreased with advancing ignition timings. The ignition timings corresponding to the peak brake thermal efficiency shifted away from TDC with increasing EGR rates. When the EGR rates increased from 0 to 16.3%, the corresponding ignition timing advanced from 8℃A bTDC to 26℃A bTDC. As the hydrogen blending ratios increased, the ignition timings shifted closer to TDC. When the hydrogen blending ratios increased from 0 to 50%, the ignition timing was delayed from 8℃A bTDC to 16℃A bTDC. As the EGR rates increased, the emissions of CO and NO_x decreased, while the emissions of THC increased. When the ignition timing was set at 18℃A bTDC, as the EGR rates increased from 0 to 16.3%, the CO and NO_x emissions decreased by 32.1% and 88.7%, respectively, while THC emissions increased by 30.7%. With increasing hydrogen blending ratio, the NO_x emissions increased, while CO and THC emissions decreased. When the ignition timing was set at 18℃A bTDC, as the hydrogen blending ratios increased from 0 to 50%, the NO_x emissions increased by 34.2%, while CO and THC emissions decreased by 14.4% and 28.6%, respectively. The emissions of CO, NO_x, and THC all increased with advancing ignition timings. The research findings can serve as a reference for optimization of hydrogen-blended natural gas engines based on stoichiometric air-fuel ratio.