The effects of fuel–air mixing and injector dribble on diesel unburned hydrocarbon emissions

Abstract

The primary objective of this work is a more complete characterization of the sources of diesel unburned hydrocarbon and a better understanding of their minimization over a range of combustion-system-relevant parameters, so as to optimize engine system performance. Measurements of unburned hydrocarbon emissions from a single-cylinder heavy-duty research engine (2.53 L) were acquired with slow and fast flame ionization detector exhaust instrumentation. Temporal lift-off-length correlation analysis coupled with a one-dimensional jet-mixing model was used to interpret and understand the spray-driven mixing-controlled unburned hydrocarbon sources. Separate experiments were conducted in a cold-spray vessel with a newly developed high-speed injector dribble imaging diagnostic, which enables quantification of the dribble event, including rate shape, velocity, duration, and probability. To supplement our understanding of injector dribble, a fuel injector nozzle flow volume-of-fluid computational fluid dynamics simulation methodology and study were conducted to illuminate nozzle geometry, injection and cylinder boundary condition influences on the dribble event. Engine combustion computational fluid dynamics simulations were also completed, incorporating the newly acquired dribble behavioral knowledge. Prior works indicate that fuel injector dribble sources of unburned hydrocarbon can dominate; however, the spray-driven unburned hydrocarbon sources were found to have significance for these engine conditions. Engine unburned hydrocarbon emissions increased with factors that promote overmixing: longer lift-off-length, higher injection pressure, reduced nozzle steady flow, and reduced dilution. Unburned hydrocarbon emissions also increase for lower in-cylinder temperatures (reduced load) and shorter charge residence times (higher engine speed). Dribble quantities were found to increase with injection pressure and nozzle sac volume, with a mean value of 18% the sac volume. The dribble event duration was found to be longer than expected at 1–2 ms, and stochastic in nature regarding hole-to-hole observation. Incorporation of these dribble phenomena into engine combustion simulations was necessary for reproducing measured engine-out unburned hydrocarbon trends. Without modeling effects of dribble, computational fluid dynamics predictions of unburned hydrocarbon levels were between 2 and 20 times low.