Experiments conducted with a set of reference diesel fuels in an optically accessible, compression-ignition engine have revealed a strong correlation between hydrocarbon (HC) emissions and the flame lift-off length at the end of the premixed burn (EOPMB), with increasing HC emissions associated with longer lift-off lengths. The correlation is largely independent of fuel properties and charge-gas O2 mole fraction, but varies with fuel-injection pressure. A transient, one-dimensional jet model was used to investigate three separate mechanisms that could explain the observed impact of lift-off length on HC emissions. Each mechanism relies on the formation of mixtures that are too lean to support combustion, or “overlean.” First, overlean regions can be formed after the start of fuel injection but before the end of the premixed burn. Second, during the mixing-controlled burn phase, longer lift-off lengths could increase the mass of fuel in overlean regions near the radial edge of the spray cone. Third, after the end of injection, a region of increased entrainment and mixing upstream of the lift-off length could cause late-injected fuel to become overlean. The model revealed a correlation between the lift-off length at EOPMB and overlean regions from the mixing-controlled burn that closely matched experimentally observed trends. HC emissions associated with overlean regions produced either before the end of the premixed burn or after the end of injection did not correspond as well to the experimental observations.
