Characterization of High-Power Cold-Start Emissions Part 2: Impact of Hybrid Topologies and Powertrain Sizing on Tailpipe Emissions Performance

The current work is the second installment of a two-part study designed to understand the impact of high-power cold-start events for plug-in electric vehicles (PHEVs) on tailpipe emissions. In part 1, tailpipe emissions and powertrain signals of a modern PHEV measured over three drive cycles identified that high-power cold-start events generated the highest amounts of gaseous and particulate emissions. The trends in emissions data and powertrain performance were specific to the P2-type hybrid topology used in the study. In this second part of the study, the effects of different PHEV hardware configurations are determined. Specifically, the tailpipe emissions of three production plug-in hybrid vehicles, operated over the US06 drive cycle, are characterized. The approach compared the tailpipe emissions of the test vehicles on the basis of the hybrid topologies and corresponding engine operational characteristics during a high-power cold-start event. Analysis of test results showed differences in the engine startup strategy for different hybrid configurations. Time-resolved tailpipe emissions of CO, NOx, total unburned hydrocarbons (THC), and particulates varied depending on the engine load during the cold-start. The likelihood of experiencing a high-power cold-start on the US06 was dependent on powertrain characteristics including e-motor size and battery state of charge. The results are discussed in detail in terms of the specific regulated air pollutants and the impact of the startup strategy implemented. Lastly, vehicle dynamics including drag and inertia forces were found to be much lower for the smaller power-split hybrid test vehicle, which reduced its propensity to experience a high-power cold-start event. The findings provide insights on how to manage high-power cold-start events in relation to the type of hybrid configuration utilized as well as their capability to meet upcoming emissions targets.