Previous research in our laboratory has shown that NOx emissions can be sharply reduced by PREDIC (PRE-mixed lean DIesel Combustion), in which fuel is injected very early in the compression process. However some points of concern remained unsolved, such as a large increase in THC and CO, higher fuel consumption, and an operating region narrowly limited to partial loads, compared to conventional diesel operation.In this paper, the causes of PREDIC's problem areas were analyzed through engine performance tests and combustion observation with a single cylinder engine, through fuel spray observation with a high-pressure vessel, and through numerical modeling. Subsequently, measurable improvements were achieved on the basis of these analyses.As a result, the ignition and combustion processes were clarified in terms of PREDIC fuel-air mixture formation. Thus, THC and CO emissions could be decreased by adopting a pintle type injection nozzle, or a reduced top-land-crevice piston. Fuel consumption was also improved by application of EGR or the addition of an oxygenated component to the diesel fuel. The operating region could be extended to higher load conditions by MULtiple stage DIesel Combustion (MULDIC), in which the first stage combustion corresponds to PREDIC. Furthermore, by combining PREDIC at low loads with MULDIC at high loads, Japanese Diesel 13 Mode NOx emissions could be reduced to 1 g/kWh.