Exposure to particulate matter (PM) and pollutant gas (NOx) is associated with increased cardiopulmonary morbidity and mortality. Mobile source emissions contribute to PM and NOx emissions significantly in urban areas. Hybrid Electric Vehicles (HEVs) plays an important role to reduce emissions. While sales of electric vehicles has been increasing, electric vehicles have to overcome issues with charging time, driving range, and production/sales cost for more widespread market penetration. Hybrid electric vehicles have a potential to serve as a bridge technology between current internal combustion engine powered vehicles and zero emissions vehicles such as electric vehicles and fuel cell vehicles. Current regulations require emissions to be tested on a chassis dynamometer. However, it is known that on-road emissions can be quite different from that those measured on regulatory driving cycles in the lab. In this study, emissions from two HEVs with different combustion technologies (gasoline direction injection vs port fuel injection) were compared using PEMS (portable emissions measurement system) and tailpipe sensors under cold weather conditions. The study has found the frequency and duration of re-ignition events vary depending on the type of HEV. Prius (PFI HEV) showed more frequent re-ignition events compared to Sonata (GDI HEV) for both city and highway driving conditions. Prius re-ignited almost every one minute while Sonata re-ignited every two minutes on average during the city driving condition. Reignition events affected emissions profile significantly during the city driving condition. As a result, Prius showed higher NOx emissions during the city driving condition while Sonata showed higher NOx emissions during the cold-cold start and highway driving condition. For PM emissions, PFI technology is known to make minimal amount of soot which is shown in the cold-cold start result while GDI technology is more prone to generating soot. This gap is reduced in city and highway driving condition due to more frequent re-ignition events of the PFI HEV (referring to Prius). Future studies should include more vehicles to understand whether the re-ignition events are vehicle specific or technology specific.
