Typical DI diesel engines operate with fuel injection taking place within a range of about 30 crank angle degrees before top dead center, at the end of the compression stroke. When injection takes place far earlier, at the beginning of the compression stroke, another form of combustion occurs, which we termed PREmixed lean Diesel Combustion, or PREDIC. With PREDIC operation, self-ignition occurs near top dead center and NOx emissions are drastically lower. When ignition occurs, the fuel-air mixture is thought to be nearly homogeneous, with only slight heterogeneity. Appropriate fuel spray formation is very important for successful PREDIC operation. Using a single-zone NOx formation model, calculations showed that the mean excess air ratio in the PREDIC combustion zone was 1.87, which resulted in very low (20 ppm) NOx emissions. Conventional combustion at the same conditions resulted in a mean combustion zone excess air ratio of 0.88. The extremely low PREDIC NOx emissions are the result of a very lean fuel-air ratio and gentle heat release pattern. One shortcoming of PREDIC operation is a lack of ignition timing control, which opens up the possibility of higher fuel consumption and combustion speed(knocking), and limits the engine operating region. To this end, several species of inert gases were mixed with the intake air. The use of CO2 showed the best promise in terms of ignition timing control.