Numerical simulation of the effects of water injection on combustion characteristics of biodiesel engines

Existing studies on in-cylinder water injection (WI) are primarily focused on parameters such as water-to-fuel ratio (W/F) and injection timing. However, systematic and quantitative analysis on WI plume and in-cylinder flow field interaction, mixing process, and NOx formation mechanisms is lacking. Moreover, such studies are even more limited in biodiesel direct-injection compression ignition engines. To address this research gap, a three-dimensional numerical model was established and validated. The steady-state operating conditions in this study were a W/F of 0.6, an injection pressure of 30 MPa, and an engine speed of 1600 rpm. The structure of the in-cylinder flow field was adjusted by varying the injection position (40/50/60 mm) and injection angle (15°/30°/45°). The effects on water mist evaporation, homogeneity index, combustion phasing, and emissions were analyzed. A parameter map suitable for engineering design was derived. The results indicate that an outward shift of the injector position combined with a larger injection angle reduces the low-velocity core in the central recirculation zone, enhances water evaporation, and improves mixture homogeneity, as reflected by the higher HI. Consequently, the ignition delay (CA0-10) was extended to 23.01 °CA, the combustion duration (CA10-90) was shortened to 8.88 °CA, the peak temperature was reduced by up to 127.98 K, and NOx emissions were reduced by up to 48.45%. Comprehensive multi-indicator analysis of evaporation rate, wall film formation, combustion phase characteristics, and NOx emissions indicates that the optimal nozzle arrangement is 60-45. Additionally, an injection position of 50–60 mm and an injection angle of 30°–45° are recommended.