Multi-objective optimization of the combustion of a heavy-duty diesel engine with low temperature combustion under a wide load range: (I) Computational method and optimization results

By coupling a multi-dimensional computational fluid dynamics (CFD) code with genetic algorithm (GA), the combustion of a heavy-duty diesel engine with LTC (low temperature combustion) was optimized under a wide load range. At each load, a comprehensive optimization of the operating parameters including IVC (intake valve closing) timing, SOI (start of injection) timing, EGR (exhaust gas recirculation) rate, the initial in-cylinder pressure and temperature at IVC was conducted in order to simultaneously minimize ISFC (indicated specific fuel consumption), NOx (nitrogen oxides) and soot emissions, and seek the optimal control strategies. Furthermore, by employing the one-dimensional simulation, the correlation between the initial in-cylinder conditions at IVC and the intake conditions was developed. The optimization results indicate that the range of the operating parameters narrows considerably with increasing load. At low load, both early and late IVC timing can be employed. As late IVC is introduced, high intake pressure and high EGR rate up to 70% are needed to realize low NOx emissions, whereas low intake pressure and moderate EGR rate (around 40%) are necessary for early IVC. For both late and early IVC, the optimal SOI timing is 10–20 °CA BTDC (before top dead center) at low load to simultaneously avoid serious spray/wall impingement and diffusion combustion. At mid load, IVC timing should be advanced to 104–110 °CA BTDC with a moderate EGR rate (40%–50%) and slightly high intake pressure, and SOI is similar with that of low load. In contrast, at high load, the optimal IVC timing is fixed at around 114 °CA BTDC and EGR rate is reduced to about 20%, while a late SOI (2.9 °CA ATDC) is needed to avoid overly high in-cylinder peak pressure and pressure rise rate.