Computational Optimization of Syngas/Diesel RCCI Combustion at Low Load in Different Engine Size

Syngas is considered to be a promising alternative fuel for the dual-fuel reactivity controlled compression ignition (RCCI) engine to reduce the fuel consumption and emissions. However, the optimal syngas compositions and fuel supply strategies in RCCI combustion are significantly affected by engine configurations, which have not been investigated yet. In this study, by integrating the KIVA-3V code and the non-dominated sort genetic algorithm II (NSGA-II), the optimizations for a 0.477 L single-cylinder engine with shallow/wide piston bowl (Engine A) and a 1.325 L single-cylinder engine with conventional omega-type piston (Engine B) under the syngas/diesel RCCI combustion were performed. The optimized operating parameters include the fuel-supply strategies, syngas compositions, and intake conditions. The results indicate that the fuel-supply strategy is flexible in Engine A due to the shallow/wide piston bowl and the relatively small cylinder bore. The single injection strategy with high premix ratio is able to achieve the high combustion efficiency in Engine A, and the injection timing is restricted around -35 °CA ATDC to realize the desired combustion phase. In addition, the injection pressure is restricted around 100 MPa to avoid the fuel impingement and wetting wall phenomenon. For Engine B with the larger displacement and omega-type piston, the double injection with early pilot injection timing is necessary to improve the combustion efficiency of the near-wall syngas. The pilot injection ratio is controlled in a narrow range to avoid the local high-temperature combustion region and provide sufficient high reactivity fuel in the piston bowl. In order to reduce NO_x emissions and improve combustion efficiency, higher initial pressure and temperature are applied in Engine A with the shallow piston bowl. For improving the fuel efficiency, high H₂ ratio in the combustible gas with low diluent ratio are recommended for both engines.