Characterization of Gaseous Emissions from Blended Plug-In Hybrid Electric Vehicles during High-Power Cold-Starts 2018-01-0428
There is a distinct difference between plug-in hybrid electric vehicles in the market today. One key distinction that can be made is to classify a plug-in hybrid electric vehicle (PHEV) according to its operational behavior in charge depleting (CD) mode. Some PHEVs are capable of using the electric-only propulsion system to achieve all-electric operation for all driving conditions in CD mode, including full power performance. In contrast, some PHEVs, henceforth termed “blended PHEVs”, cannot satisfy the power requirements of all driving conditions with the electric-only propulsion system and occasionally utilize blended CD operation whereby it is necessary to blend the use of the internal combustion (IC) engine with the use of the electric motor(s) to help power the vehicle. This characteristic can result in a unique phenomenon where it is possible for a blended PHEV to drive for miles in electric-only mode at the start of a trip before encountering a rapid acceleration that generates a need for blended CD operation. Under such circumstances, blended PHEVs can have a high-power cold-start where the initial IC engine start occurs under high vehicle torque demand, even when the battery state of charge (SOC) is high. Conventional IC engine vehicles do not experience high-power cold-starts since the initial IC engine start typically occurs under a very low initial torque requirement when the vehicle is stopped, in park or in neutral, and some driving is usually required before a high-power driving maneuver is encountered. Testing of various blended PHEVs found that high-power cold-starts have different emission characteristics compared to conventional vehicle cold-starts.
California Air Resources Board (CARB) staff conducted vehicle tests to investigate the effects of high-power cold-starts on the gaseous exhaust emissions of blended PHEVs. Conditions that triggered high-power cold-starts were characterized from on-road driving and the resulting vehicle speed traces were then used to conduct chassis dynamometer exhaust emission tests. A new methodology was developed to compare the cold-start emissions from the high-power cold-start acceleration cycles to emissions from regulated emission certification test cycles. The results from these tests indicated that high-power cold-starts may be yielding significantly higher exhaust emissions than those observed during the regulated emission test cycles that are conducted for vehicle exhaust emission certification. This paper provides a summary of the high-power cold-start test cycle development, the methodology that was utilized to compare test cycle emissions, and the high-power cold-start gaseous emissions results.