Experiments have been performed using a modern heavy-duty direct-injection diesel engine equipped with a common-rail, electronically-controlled high-pressure fuel injection system. The experimental results demonstrate the utility of using multiple or split fuel injections in combination with EGR for giving significant emissions reduction over the entire engine operating range. The mechanisms of emission reduction have been revealed by means of combustion visualization and CFD modeling. Soot is reduced due to the fact that the soot producing regions at the tip of the sprays are not replenished with fresh fuel when each injection is terminated and then restarted. The large reduction in soot possible with multiple injections allows the use of higher levels of EGR than is possible with single injections, so that NOx can be reduced significantly while soot levels are simultaneously kept low.