Modeling Enleanment Emissions for Light-Duty Vehicles

A comprehensive modal emissions model for light-duty cars and trucks is being developed under the sponsorship of NCHRP Project 25-11. Model development has been described previously for vehicles operating under stoichiometric and enrichment conditions. A modal emissions model is presented for vehicles operated under enleanment conditions. Enleanment typically occurs with sharp deceleration or load reduction events, and sometimes during long deceleration. Under enleanment conditions, the air/fuel ratio is lean and incomplete combustion or misfire occurs. Preliminary research indicates that enleanment emissions (particularly for hydrocarbons) contribute significantly to a vehicle’s overall emissions. An enleanment emissions module has been developed on the basis of second-by-second emission measurements generated at the College of Engineering—Center for Environmental Research and Technology’s vehicle testing facility using the Federal Test Procedure, US06, and a specially designed modal emission cycle (MEC01). On the basis of more than 200 vehicles tested and modeled, lean-burn hydrocarbon emissions (HC_lean) account for 10 to 20 percent of the overall HC emissions under the various test cycles. HC_lean emission contributions vary greatly from vehicle to vehicle, ranging from near 0 to more than 30 percent of total HC emissions of individual vehicles. After detailed analysis of the second-by-second emission data over the modal emission cycle MECO1, it was found that enleanment hydrocarbons emissions are mostly associated with rapid load reduction events and long deceleration events. The former is most likely to cause extremely high levels of HC as short spikes, and the latter is mostly associated with longer-lasting HC puffs. A methodology has been developed to characterize and model enleanment hydrocarbons emissions associated with these two events. The model estimates are compared with measurements, with encouraging results.