A Method to Reduce Cold Start Emissions while Shortening Fast Idle Catalyst Light-off Time 2023-01-0247

For vehicles with internal combustion engines, tailpipe emissions heavily rely on the aftertreatment system, typically a catalytic converter. Modern three-way catalysts (TWC) can very effectively convert the unburnt hydrocarbons (HC), CO, and NO_x into non-harmful gases such as H₂O, CO₂, and N₂ when the catalyst brick reaches a relatively high temperature. However, before that catalyst light-off temperature is reached, the emissions conversion efficiency is low, leading to high tailpipe emissions. Due to this light-off temperature requirement of the catalytic converter, the emissions from the engine cold-start period contributes a significant portion of vehicle overall emissions. One of the major reasons for high emissions during cold start is low combustion chamber wall temperatures, lower than the initial boiling temperature of gasoline fuel. This results in fuel film formation, and significantly incomplete evaporation prior to combustion. In this study, an approach to increase the fuel evaporation rate and fuel-air mixing for reduced cold start emissions while attaining fast catalyst light-off time is explored by using CVVD (continuously variable valve duration) & CVVT (continuously variable valve timing) mechanisms for both the intake and exhaust valvetrains. Early exhaust valve closing (EVC) and late intake valve opening (IVO) can be used to create negative valve overlap (NVO) for trapping hot exhaust gas residuals to facilitate fuel vaporization and reduce engine-out emissions during the engine cold-start and warm-up periods. In addition, early exhaust valve opening (EVO) timing can be employed to ensure fast catalyst light-off time. In this paper, a spark-ignited combustion engine is considered, and the engine-out emissions during the cold fast-idle period are studied. Both numerical simulations and engine testing are conducted to analyze the potential improvement of fuel vaporization and cold start emissions reductions with the proposed NVO approach.