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).
Holmén, Britt and Robinson, Mitchell, "Instantaneous Hybridization Factor (IHF) Development for Hybrid Electric Vehicle Energy-Emissions Analyses Using Real-World, On-Board Data" (2020). Transportation Research Center Research Reports. 5.