The effects of the air-fuel ratio on a stationary diesel engine under dual-fuel conditions and multi-objective optimization

Highlights: • The large stationary engine was modeled and modified to dual-fuel engine. • The characteristics of dual-fuel combustion were investigated. • The effect of air-fuel ratio under dual-fuel combustion condition are investigated. • The optimal design points for improving fuel economy and NOx emission are suggested. • Helpful methodology for conceptual design of virtual engine system was suggested. -- Abstract: The fundamental combustion characteristics and engine performance of a dual-fuel engine, fueled with diesel and natural gas, were investigated using a commercial 1D simulation program (GT-Power). Furthermore, the effects of air-fuel ratio under dual-fuel combustion conditions were studied, and optimized design points that minimize brake-specific fuel consumption (BSFC) and nitrogen oxides (NO_X) emissions simultaneously were determined using a multi-objective Pareto optimization method. The engine model was validated based on experimental results and previous work under various engine loads. The thermal efficiency of the engine, which was modified from a diesel to a dual-fuel engine, showed a decreasing tendency as the natural gas substitution ratio (NSR) increased due to changes in the combustion characteristics. The decline in thermal efficiency in dual-fuel combustion could be improved somewhat by optimizing the injection strategy. Under dual-fuel combustion conditions, an optimum air-fuel ratio value for BSFC could be determined, especially for higher NSR conditions. However, NO_X emissions showed a tendency to increase with increasing air-fuel ratio. The relationship between the trends was more than a simple tradeoff, so multi-objective Pareto optimization was conducted to minimize both BSFC and NO_X emissions, with start of injection timing and air-fuel ratio as variables. Ultimately, optimal design points were determined.