National Center for Sustainable Transportation

Hybrid-electric vehicles are a growing segment of the vehicle market and their share is anticipated to continue to grow as automakers seek to comply with increasingly stringent fuel economy standards. Hybrid vehicles generally use less fuel and produce fewer associated emissions than their conventional counterparts. However, their emissions patterns are unique; hybrids produce zero emissions during certain operations when the internal combustion engine is off, but produce particulate emission spikes when the engine starts or restarts. The overall air quality implications of more hybrid vehicles on the roads are not well understood.

Researchers at the University of Vermont collected real-time emissions and performance data from a hybrid vehicle operating in a variety of on-road conditions to develop a new parameter that could serve as the basis for future hybrid vehicle emissions models: the instantaneous hybridization factor. This parameter is the real-time proportion of a vehicle’s overall power use that comes from the hybrid propulsion system, and could be used in combination with known conventional vehicle emissions and operating data to derive more exact emissions estimates for hybrid vehicles. This research brief summarizes the findings and research implications from that work.

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