Recent studies on real-world automobile emissions measurements have not adequately addressed the question of whether driving style affects emission levels. In this study, we hypothesized that given the same experimental conditions and a random selection of drivers, the variability associated with individual driving styles (e.g. intensity or duration of acceleration events) would produce statistically significant differences in measured emissions. To test this driver variability hypothesis, we conducted a field study on 24 drivers in a single vehicle on a specified route under low traffic conditions using on-board exhaust emission and engine operating data analyzers and tested for statistically significant differences in CO and NOx emissions between drivers. Our data show significant (95% level) variations in carbon monoxide (CO) and oxides of nitrogen (NOx) emissions among the 24 drivers under driving conditions where we have controlled for driving route, traffic density and vehicle type. Since the ANOVA tests showed significant differences in emissions between drivers but the frequency of driving modes were very similar, this suggests that the intensity of vehicle operation within a give mode, not the modal frequency, explains the emissions variability between drivers.

Past research has shown on-road emissions patterns unique to HEVs, indicating the need to account for them in emissions models. The main objective of this work is to outline a framework for development of new HEV emissions models based on current knowledge of CV emissions. The premise is that accurate knowledge of the instantaneous (1 Hz) power split between the HEV combustion and electric propulsion sources can be used to modify existing CV emission models for HEV emission predictions that are currently lacking in regulatory models. The HEV power split metric, instantaneous hybridization factor (IHF), was developed and quantified with on-road data collected from a 2010 Toyota Camry HEV operating on a fixed route in hilly Vermont over all seasons. IHF is the second-by-second ratio of electric system power to total system power and accounts for energy storage in the high voltage battery. IHF ranges from -1 to +1 and varies widely with vehicle speed and VSP. Different road types on the driving route were associated with different proportions of binned IHF activity, which represent the three key HEV operating states defined in this study: electric-drive only (EDO), power recovery and electric-drive assist. A more detailed analysis of IHF relationships to tailpipe emissions and road grade is found in a recently published article (Ref 1).

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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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