A Study on Optimal Powertrain Sizing of Plugin Hybrid Vehicles for Minimizing Criteria Emissions Associated with Cold Starts 2018-01-0406
Plugin hybrid electric vehicles (PHEVs) have several attractive features in terms of reduction of greenhouse gas (GHG) emissions. Compared to conventional vehicles (CVs) that only have an internal combustion engine (ICE), PHEVs retain the attractive features of regular hybrids (HEVs) in terms of energy buffering and also bring about the benefit of electrifying an appreciable portion of the miles driven. Furthermore, unlike battery-only electric vehicles (BEVs), PHEVs are not range/charging-rate limited on long trips. In terms of criteria emissions (e.g. NOx, NMOG, HC) however, the advantage of PHEVs compared to CVs can be less obvious. Criteria emissions are generally eliminated once the catalyst material in the catalytic box of the vehicle has reached a certain temperature. Consequently, most of the criteria emissions let out by a vehicle happen during the time in between starting the ICE till the catalyst warms up. Once the ICE is started in CVs and HEVs, the ICE remains in operation (or operates frequently enough) to keep the catalyst warm. For PHEVs, however, ICE starts can be few and far between that under certain conditions, it is plausible that the number of cold starts could be more than CVs. This work presents a first-order methodology for analyzing large sets of real-world driving trips in order to infer the frequency of PHEV cold starts as function of: i) all electric range (AER), and ii) electric-only peak power capability of the powertrain. Results of the study are then summarized as guideline design curves for sizing of PHEV powertrains.