Modeling Diesel Spray Flame Liftoff, Sooting Tendency, and $NOx$ Emissions Using Detailed Chemistry With Phenomenological Soot Model

A detailed chemistry-based CFD model was developed to simulate the diesel spray combustion and emission process. A reaction mechanism of $n$-heptane is coupled with a reduced $NOx$ mechanism to simulate diesel fuel oxidation and $NOx$ formation. The soot emission process is simulated by a phenomenological soot model that uses a competing formation and oxidation rate formulation. The model is applied to predict the diesel spray lift-off length and its sooting tendency under high temperature and pressure conditions with good agreement with experiments of Sandia. Various nozzle diameters and chamber conditions were investigated. The model successfully predicts that the sooting tendency is reduced as the nozzle diameter is reduced and/or the initial chamber gas temperature is decreased, as observed by the experiments. The model is also applied to simulate diesel engine combustion under premixed charge compression ignition (PCCI) conditions. Trends of heat release rate, $NOx$⁠, and soot emissions with respect to EGR levels and start-of-injection timings are also well predicted. Both experiments and models reveal that soot emissions peak when the start of injection (SOI) occurs close to TDC. The model indicates that low soot emission at early SOI is due to better oxidation while low soot emission at late SOI is due to less formation. Since $NOx$ emissions decrease monotonically with injection retardation, a late injection scheme can be utilized for simultaneous soot and $NOx$ reduction for the engine conditions investigated in this study.