Investigation of Negative Valve Overlap Reforming Products Using Gas Sampling and Single-Zone Modeling

Negative valve overlap (NVO) is a viable control strategy that enables low-temperature gasoline combustion (LTGC) at low loads. Thermal effects of NVO fueling on main combustion are well understood, but fuel reforming chemistry during NVO has not been extensively studied. The objective of this work is to analyze the impact of global equivalence ratio and available oxidizer on NVO product concentrations. Experiments were performed in a LTGC single-cylinder engine under a sweep of NVO oxygen concentration and NVO fueling rates. Gas sampling at the start and end of the NVO period was performed via a custom dump-valve apparatus with detailed sample speciation by gas chromatography. Single-zone reactor models using detailed chemistry at relevant mixing and thermodynamic conditions were used in parallel to the experiments to evaluate expected yields of partially oxidized species under representative engine time scales. Modeling efforts help identify physical mechanisms that further describe experimental findings with regards to anticipated fuel-fraction and temperature fields. For the NVO fueling sweep, end-cycle CO₂ concentrations remained essentially flat, while intermediate species concentrations rose as fueling rates increased. The rate-of-increase was most pronounced for the C₃-C₄ hydrocarbons whose rate-of-increase was greater than the relative increase in fueling rate. Modeling results suggest that oxygen depleted environments coupled with lower heat release temperatures result in slower reforming rates, which yielded higher C₃-C₄ production. For the oxygen concentration sweep with fixed NVO fueling, CO and CO₂ products increased as the amount of oxidizer likewise increased. These increases came at the expense of intermediate hydrocarbon yields.