Feasibility study of the combustion strategy of n-butanol/diesel dual direct injection (DI2) in a compression-ignition engine

For n-butanol/diesel dual-fuel combustion, there are two different implementations, i.e., dual direct injection (DI2) strategy and reactivity controlled compression ignition (RCCI) strategy. The DI2 strategy is a novel combustion concept, which is expected to address the inefficient combustion and high pressure rise rate dilemmas raised by RCCI. In this work, the full-parameter optimizations of DI2 and RCCI were first performed at mid load, and the optimal cases were comprehensively compared. The potential benefits of DI2 over RCCI and its optimal operating parameters were determined. The results indicate that DI2 can achieve low nitrogen oxides (NOx) emissions below 0.00094 g/kWh and near-zero soot emissions while keeping high fuel economy near 167 g/kWh comparable to that of RCCI, but it usually yields higher NOx, soot, and carbon dioxide (CO2) emissions than RCCI. In the optimal DI2 cases, very advanced n-butanol injection timing (before-120 degrees CA ATDC), high n-butanol energy fraction (around 0.94), and early diesel injection timing (near-50 degrees CA ATDC) are adopted. Meanwhile, the combination of a narrow n-butanol spray angle (30 degrees<alpha butanol 50 degrees) and a wide diesel spray angle (65 degrees) is preferred for DI2. Compared with RCCI, DI2 exhibits lower combustion losses, however, it suffers from higher exhaust and heat transfer losses. The higher intake temperature, exhaust gas recirculation (EGR) rate, and lower intake pressure are more beneficial for DI2. Moreover, it is suggested that DI2 is easier to achieve lower ringing intensity (RI), and the ignition timing and RI of DI2 are more sensitive to the n-butanol fraction than RCCI.