In this paper, a theoretical study is presented where fuel rate shaping is analyzed in combination with EGR as a method for reducing NOx formation. The analytical tools used include an empirically based model to convert fuel rate to heat release rate, and a zero dimensional multizone combustion model to calculate combustion products, local flame temperatures and NOx emissions at a given heat release rate. The multizone model, which has been presented earlier, includes flame radiation and convective heat losses.
Several geometrical shapes of the fuel rate are tested for different combustion timings and EGR rates. It is found that the fuel rate giving the lowest NOx formation varies with the injection timing. In order to lower the NOx emissions at normal and advanced injection timings, the fuel rate should have a rather long duration, and start at its maximum level followed by a slow decay. At later timings, however, the fuel rate should be more even and, in some cases, have its maximum rate at the end of injection.
The reason why maximum fuel rate, at the beginning of the injection period, gives lower NOx formation is that this retards the start of injection for a given position of the center of the heat release. Thus, the time for NOx formation in the first combusted zones is shortened. The initial fuel rate for low NO formation is, however, strongly limited by the local temperatures. As has been found by others, too high initial flow rate increases NOx emissions as it produces very rapid combustion, high pressure rise and high local temperatures, which promotes NOx formation and increases the engine-out emissions despite the effect of retarded injection.