Coupling effects of engine speed and injection timing on spray dynamics, combustion and emissions in direct-injection spark-ignition n-butanol engine

N-butanol is considered one of the most ideal alternatives for reducing carbon emissions in the transportation sector. However, its inherently poor atomization and low volatility pose significant challenges for mixture preparation and combustion control in engines. To elucidate the intrinsic coupling between injection timing and engine operating conditions, this study numerically investigates the effects of injection timing on in-cylinder flow characteristics, fuel–air mixing, combustion behavior, and pollutant formation of a pure n-butanol-fueled engine under different engine speeds. The results reveal the transition of the dominant mixture-formation mechanism from spray-momentum-dominated mixing at low speed to intake-tumble-dominated mixing at high speed, and link this transition with the speed-dependent recommended injection timing. The recommended SOI timings are −300 °CA at 1200 and 2600 rpm, and −320 °CA at 4000 rpm under the present combustion-emission compromise. At 1200 rpm engine speed, spray momentum dominates mixture formation because of the weak intake-induced flow, whereas at 4000 rpm engine speed, the strengthened tumble flow becomes dominant and requires earlier injection to provide sufficient evaporation and mixing time. Quantitatively, at 2600 rpm, −300 °CA increases IMEP by up to 4.85 % compared with the excessively delayed −260 °CA case, while at 4000 rpm, −320 °CA increases IMEP by 9.59 % and ITE by 3.46 percentage points relative to −260 °CA. In terms of emissions, at 1200 rpm, −300 °CA reduces soot and HC emissions by 9.5 % and 90.0 %, respectively, compared with −320 °CA. However, at 4000 rpm, excessively delayed injection at −260 °CA increases CO, soot, and HC emissions by 199.4 %, 839.3 %, and 7843.1 %, respectively, relative to −320 °CA, although NOx emissions decrease by 51.7 %. The combustion and emission characteristics are found to be strongly correlated with mixture homogeneity: a more uniform mixture significantly improves combustion efficiency but simultaneously increases NO x formation due to elevated combustion temperatures. This study elucidates the underlying mechanism governing the interaction between engine speed and injection timing optimization for n-butanol, providing theoretical guidance for achieving clean and efficient combustion of n-butanol fueled engines.