Modern diesel engine injection systems are largely computer controlled. This opens the door for tailoring the fuel rate. Rate shaping in combination with increased injection pressure and nozzle design will play an important role in the efforts to maintain the superiority of the diesel engine in terms of fuel economy while meeting future demands on emissions.
This approach to studying the potential of rate shaping in order to reduce NO formation is based on three sub-models. The first model calculates the fuel rate by using standard expressions for calculating the areas of the dimensioning flow paths in the nozzle and the corresponding discharge coefficients. In the second sub-model the heat release rate is described as a function of available fuel energy, i.e. fuel mass, in the cylinder. The third submodel is the multizone combustion model that calculates NO for a given heat release rate under assumed air /fuel ratios.
When studying the potential of fuel rate shaping, different heat release rates are generated by using Vibe functions. The NO formation, convective heat losses, IMEP, indicated thermal efficiency and fuel rate are calculated for every given heat release rate.
The calculations show that by increasing the length of the injection period and maintaining a smooth and even fuel rate, the NO formation could be reduced with very limited reduction of the indicated thermal efficiency. In the example given in the paper, NO is reduced by about 20% with an increase in the fuel consumption of only 1%.