Reaction-space analysis of premixed and direct injected fueling in the context of homogeneous charge compression ignition combustion under positive and negative valve overlap conditions

摘要

Full-cycle computational fluid dynamics (CFD) simulations with gasoline chemical kinetics were performed to determine the impact of breathing and fuel injection strategies on charge thermal and compositional stratification, combustion and nitrogen oxides (NO_x) emissions during homogeneous charge compression ignition (HCCI) combustion. The simulations examined positive valve overlap (PVO) and negative valve overlap (NVO) breathing strategies, along with port fuel injection (PFI) and direct injection (DI) fuel injection events. The charge mass distribution was analyzed prior to ignition using ignition delay as a metric to quantify reactivity. The stratification in reactivity arising from different distributions in fuel-oxygen equivalence ratio (Φ_(FO)), oxygen molar percentage (x_(O2)) and temperature (T) was determined. Under PVO conditions, early DI results in a relatively uncorrelated Φ_(FO)-T distribution prior to ignition and the level of stratification in Φ_(FO) is comparable to that of the PFI case due to better mixing with PVO. The reactivity stratification and burn duration for the PVO valve events with PFI and early DI are nearly identical, dominated by wall-driven thermal stratification. As a result, the combustion characteristics and NO_x emissions are all nearly identical for PFI and early DI under PVO conditions. For NVO valve events with DI during recompression, it was found there remains significant stratification in Φ_(FO) prior to ignition and that this distribution is directly correlated with the distribution in T, which serves to increase reactivity stratification. For this approach however, the latent heat absorbed from the hot residual gas during fuel evaporation reduces the overall thermal stratification compared to PFI, which in turn serves to reduce reactivity stratification. Due to the two competing effects, the reactivity stratification and burn duration are similar for DI and PFI injection strategies when NVO valve events are employed. However, the increased stratification in Φ_(FO) prior to ignition with DI during NVO results in an order of magnitude increase in NO_x compared to the PFI strategy due to higher local burned gas temperatures.