Influence of injection parameters on evaporative cooling and mixture formation for ethanol-fueled knock-limited spark ignition operation using multidimensional CFD modeling

Direct-injected spark ignition (DISI) engines experience knock suppression compared to their port-fuel-injected (PFI) counterparts primarily due to the charge cooling effect. High heat of vaporization fuels, such as ethanol, can produce an enhanced charge cooling effect, thus improving its synergetic application to DISI engines. 3D CFD simulations are first carried out to highlight the fundamental effects from the heat of vaporization and equivalence ratio stratification on knock suppression. The air-fuel mixing process, charge cooling, in-cylinder stratification, and overall performance of ethanol-fueled knock-limited DISI operation are evaluated under an injection parametric study based on the start of injection, injection pressure, and included spray angle. The results show that delaying the injection to later in the intake stroke under a lower injection pressure via a narrower spray angle significantly increases fuel evaporation in the air. Elongated combustion and delayed combustion phasing accompanied by lower knock intensity is observed with an increase in evaporative cooling and equivalence ratio stratification, thus allowing spark advance to match the baseline knock intensity. An increase in fuel conversion efficiency by roughly 3 percentage points is observed under a spark timing sweep carried out for neat and hydrous ethanol operation at 1500 RPM knock-limited conditions by maximizing fuel evaporation in the air.