Abstract: A numerical study was conducted to investigate the impact of pilot-injection and pilot-injection coupled exhaust gas recirculation（EGR） on combustion and emissions of a heavy-duty diesel engine under high load conditions.The pilot-injection parameters and the coupled EGR rate were optimized to improve the indicated thermal efficiency and reduce the NO_x emissions.The results indicate that,under high load conditions,the pilot-injection and pilot-injection coupled EGR have an impact on combustion and emissions for the heavy-duty diesel engine.The optimization of pilot-injection parameters and the coupled EGR rate improves the indicated thermal efficiency and reduces the NO_x emissions.Comparing with single injection,using only the pilot-injection scheme increases the indicated thermal efficiency and reduces the soot emission,but worsens the NO_x emissions.Coupling the pilotinjection scheme with an EGR strategy,with a pilot-injection ratio of 20% and an EGR rate ranging from 10% to20%,ensures a favorable combustion phase,which improves the indicated thermal efficiency and reduces local high temperatures and NO_x emissions in the combustion chamber.The optimal solution finally chosen is a pilotinjection scheme（with a pilot-injection ratio of 20% and a pilot-main injection interval of 8°CA） coupled with an EGR strategy（with an EGR rate of 20%）.This solution results in a 57.4% reduction in NO_x emissions and a 2.28%increase in indicated thermal efficiency,compared with the original engine.