Multiple combustion modes switching to realize full-load efficient energy conversion of n-butanol/diesel dual direct injection (DI2) engine

[Display omitted] •Optimal combustion modes and intake conditions for the DI2 engine under various loads are identified.•Inherent reasons for the multi-mode transition of DI2 under different loads are analyzed.•Effects of fuel injection parameters on the combustion and emissions of DI2 are studied.•Optimal stratified characteristics and key factors driving ignition for DI2 are revealed. The dual direct injection (DI2) engine system involves the in-cylinder co-direct injections of two fuels with the potential to address the operation limitations of traditional dual-fuel engines at part load conditions. However, for the DI2 engine, its optimal fueling strategies and combustion modes under different load conditions are still unknown. By selecting n-butanol and diesel as the test fuels, this study devotes to identifying the optimal combustion schemes of the DI2 engine at different loads using three-dimensional engine simulation combined with an improved genetic algorithm, and further revealing its optimal fuel stratification and auto-ignition characteristics. The results showed that for low load, the optimal DI2 strategy is to pre-inject a low proportion of diesel (near 20%) into the cylinder early in the compression stroke and then introduce n-butanol into this diesel atmosphere. On the contrary, the fuel injection sequence is reversed at mid load, in which n-butanol is first injected into the combustion chamber to form a partially premixed n-butanol/air mixture, followed by a late diesel injection at −50 to −20 °CA after top dead center (TDC). As for the high-load operation, n-butanol is still preferred to be partially premixed, but the diesel injection event is delayed until after TDC. The main factors driving the ignition under low load are the fuel reactivity and temperature, while at mid load the ignition is primarily initiated by fuel reactivity, however, the effect of temperature is highlighted under high load.