Past research showed on-road emissions patterns unique to hybrid electric vehicles (HEVs), indicating the need to account for them in emissions models as projected HEV sales increase over the coming decades. This work defines and characterizes a variable that quantifies HEV operating behavior to inform future development of new HEV emissions models based on current knowledge of conventional vehicle (CV) emissions patterns. Instantaneous hybridization factor (IHF), was quantified using on-road data collected from a 2010 Toyota Camry HEV. IHF is the 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). Relationships between IHF and vehicle specific power (VSP), road type and road grade were examined. Negative VSP resulted in regenerative braking operation (IHF = −0.01 to −1) 90% of the time. IHF identified the VSP range where HEV operation was highly variable (VSP = −1 to 8 kW/ton) when driving at speeds below the ICE-off threshold (42 mph). VSP and IHF together account for 76–86% of the variability in HEV CO₂ emissions in this study. CO₂ model results using VSP computed with the measured real-world road grade (R² = 0.86) gave improved fits over the no-grade VSP model (R² = 0.69). This study establishes one framework for calculating the instantaneous HEV power split, confirms the need to include road grade in VSP for accurate modeling of vehicle emissions, and identified the need for significant improvements in on-board diagnostic (OBD) scantool measurement requirements for HEVs in three areas: (1) temporal resolution (sub-second to capture transient events such as ICE restarts); (2) simultaneous data logging capability for multiple controller area networks (i.e., engine and HEV parameters together); and (3) improved data precision.

On-board emissions and vehicle operating data were collected by the UVM Transportation Air Quality Laboratory during real-world driving of one HEV and one CV, on a 50 km (32 mi) route over a variety of roadways in Chittenden County, Vermont using the custom Total On- board Tailpipe Emissions Measurement System (TOTEMS). Both vehicles were model year 2010 Toyota Camry of similar driving performance, but different engine size, vehicle weight (due to HEV battery pack) and transmission type.  The dataset includes records of time-aligned 1Hz data, including emission rates (# or mass per sec) of criteria, air toxics and particle number (PN) and associated engine and hybrid system operating parameters.  Detailed lists of the parameters collected and data collection protocols are available in published UVM Transportation Research Center research reports (Holmén et al. 2014).  Sampling conditions for a total of 28 CV and 33 HEV sampling runs were conducted over an 18-month period under ambient temperatures ranging between -13 and 35 °C.  Road grade on the driving route was measured independently at sub-meter resolution and varied between -13.2 to +11.5%.

The "Data Dictionary" EXCEL file summarizes the contents of the database, with units for each parameter and descriptions of all acronyms. 

The UVM TRC Research Report, Holmén et al. 2014, is linked below via URL to U.S. Department of Transportation National Transportation Library where PDF of the report may be found if the UVM Transporation Research Center website is not available (https://www.uvm.edu/cems/trc/trc-research-reports).