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Exhaust Gas Recirculation (EGR) is an important parameter for control of diesel engine combustion, especially to achieve ultra low NOx emissions. In this paper, the effects of EGR on engine emissions and engine efficiency have been investigated in a heavy-duty diesel engine while changing combustion control parameters, such as injection pressure, injection timing, boost, compression ratio, oxygenated fuel, etc. The engine was operated at 1400 rpm for a cycle fuel rate of 50mg. The results show that NOx emissions strongly depend on the EGR rate. The effects of conventional combustion parameters, such as injection pressure, injection timing, and boost, on NOx emissions become small as the EGR rate is increased. Soot emissions depend strongly on the ignition delay and EGR rate (oxygen concentration). Soot emissions can be reduced by decreasing the compression ratio, increasing the injection pressure, or burning oxygenated fuel.

A sequential port injection, lean-burn, fully electronically-controlled compressed natural gas (CNG)/Diesel dual-fuel engine has been developed based on a turbo-charged and inter-cooled direct injection (D.I.) diesel engine. During the optimization of engine overall performance, the effects of pilot diesel and pre-mixed CNG/air mixture equivalence ratio on emissions (CO, HC, NOx, soot), knocking, misfire and fuel economy are studied. The rich and lean boundaries of the pre-mixed CNG/air mixture versus engine load are also provided, considering the acceptable values of NOx and THC emissions, respectively. It is interesting to find that there is a critical amount of pilot diesel for each load and speed point, which proved to be the optimum amount of pilot fuel. Any decrease in the amount of pilot diesel from this optimum amount results in an increase of NOx emissions, because the pre-mixed CNG/air mixture must be made richer, otherwise THC emissions would increase.