Numerical Optimization And Comparative Study Of N-butanol Concentration Stratification Combustion And N-butanol/Diesel Reactivity Stratification Combustion For Advanced Compression Ignition (CI) Engine

Previous studies show that satisfactory engine performance can be achieved by using both the reactivity stratification combustion(RSC)and the concentration stratification combustion(CSC)fueled with n-butanol.However,the different ignition and combustion characteristics between RSC and CSC have not been well explored yet.In this study,the n-butanol/diesel RSC and n-butanol CSC strategies were investigated by integrating the KIVA-3V code and non-dominated sort genetic algorithm II(NSGA-II).For both RCS and CSC,n-butanol was premixed in the intake port.However,diesel and n-butanol were injected into the cylinder for RSC and CSC,respectively.Five important operating parameters were selected as the variables for optimization,including premixed fraction(PF),start of injection(SOI),the initial in-cylinder pressure at the intake valve close(IVC)timing(pivc),the initial in-cylinder temperature at the IVC timing(Tivc)and exhaust gas recycle(EGR)rate.The optimization results show that,for the realization of the clean and high-efficiency combustion,a wide range of SOI and the high PF are introduced in RSC,whereas a wide range of PF and the early SOI are employed in CSC.Due to the lower reactivity of n-butanol than diesel,CSC requires the higher Tivc than RSC.In RSC,the increased PF results in the retarded 50% heat release point(CA50),improved fuel economy,decreased ringing intensity(RI)and lower nitrogen oxides(NOx)emissions.For CSC,with the increased PF or the retarded SOI,the cooling effect from the in-cylinder injected n-butanol becomes weaker,resulting in the advanced CA50,improved fuel economy,and higher RI.Comparing the two combustion strategies,RSC illustrates better performance on the ignition control and RI,while CSC is beneficial for fuel economy and the NOx emissions.The effect of the initial temperature,EGR rate and initial pressure on the engine performance is similar for RSC and CSC.