Model-based multi-objective Pareto optimization of the BSFC and NOx emission of a dual-fuel engine using a variable valve strategy

Various advanced technologies, including dual-fuel combustion, have been developed to meet reinforced emission regulations in the automobile industry. In this work, natural gas (NG) was added from 0 to 30% in steps of 10% as the energy fraction of the fuel to evaluate the fundamental effects of the NG substitution ratio (NSR) on the combustion, performance and nitrogen oxides (NOx) emission of a dual-fuel engine. Although the NOx emission significantly improved because of the higher specific heat capacity of air-NG mixtures, the in-cylinder pressure decreased with increasing NG fraction because of a longer ignition delay, which also detrimentally affected the brake specific fuel consumption (BSFC) by decreasing the brake thermal efficiency (BTE) of the engine. The injection timing was optimized using a design of experiments (DoE) approach to minimize the BSFC under various load and NSR conditions. The optimal injection timing was more advanced with lower load and higher NSC conditions. Additionally, the optimal Pareto fronts for improved performance and NOx emission of the dual-fuel engine were obtained from a multi-objective Pareto optimization. These results suggested suitable intake and exhaust valve timing strategies to control the BSFC and NOx emission of a dual-fuel engine.