Effect of ultra-high injection pressure on diesel ignition and flame under high-boost conditions

In this work, we conducted three-dimensional numerical simulations to investigate the effect of ultra-high injection pressure on diesel ignition and flame under high-boost and medium-load conditions. Three injection cases employed in experiments with a multi-cylinder Volvo D12 engine were applied for validation. The simulations were performed using the KIVA-3V code, with a Kelvin-Helmholz/Rayleigh-Taylor (KH/RT) spray breakup model and a diesel surrogate mechanism involving 83 species and 445 reactions. A range of higher injection pressure levels were projected and the injection rates estimated for the current study. Three different rate shapes of injection were projected and investigated as well. All the projected injection events start at top dead center (TDC). Computations demonstrate that high-pressure injection strongly affects engine ignition and combustion. An increase in injection pressure leads to reduced ignition delay time, higher in-cylinder pressure peak, advanced combustion phasing, and faster flame propagation. The study found that the ultra-high pressure injection does not cause the flame lift-off length in the engine to increase, the trend of which seems to be contradictory to the observations obtained from the studies in high-pressure, high-temperature constant-volume vessels. While the burn durations reduced with an increase in injection pressure, the simulations of three different injection rate shapes suggest that the rate-falling injection leads to a shorter, early (10-30%) burn duration angle but a longer, late (70-90%) burn angle. The prediction indicates that the engine has a relatively larger flame area of higher temperature in the late cycle for the rate-rising injection than for the rate-falling one. The existence of higher temperature in the late engine cycle may be beneficial to soot oxidation. On the other hand, the simulations show that higher injection pressure results in a faster NO production rate in the early phase of combustion but leads to a lower NO peak level. The rate-rising injection lowers NO production compared with the other two injection strategies.