20 Jun 2022

Evaluation of plug-in hybrid vehicles in real-world conditions

Report no. 10/22: Assessing the real-world energetic performance and emissions of Plug-in Hybrid Vehicles (PHEVs) is complex. First, because of the complexity of the powertrain itself, pairing thermal and electric propulsion. Second, because their evaluation results are extremely sensitive to their usage while driving (e.g. trip distance) and before driving (e.g. recharging behaviour). In this context, the present study aims at delivering energy consumptions and GHG emissions data of the PHEVs in real-world conditions and as a function of their use cases. The study is based on an extensive experimental campaign. Two Euro 6d PHEVs were selected to allow a back-to-back comparison between petrol and diesel internal combustion engines. The first purpose of the test campaign is to evaluate and compare the energy consumptions (in terms of electricity and fuel), the CO2 and pollutant emissions of different vehicle configurations: charged PHEVs vs non-charged PHEV; non-charged PHEV vs non-plug-in hybrid electric vehicles (HEV); Diesel vs gasoline; traditional fossil-based fuels vs renewable fuels, etc.

These vehicles were tested in a first step on a chassis dynamometer to accurately control and reproduce experimental conditions allowing the different configurations to be compared and to allow the implementation of advanced measurement systems (engine-out and tailpipe emissions of both regulated and non-regulated pollutants, energy consumptions, AdBlue consumption). In a second step, the vehicles were tested on-road to allow a comparison of the measurements made in the laboratory and assess their representativeness. All the driving cycles performed, either in lab or on-road, were RDE-compliant. Both PHEVs tested show low regulated emissions (well below Euro 6d limits) and unregulated pollutant emissions in the range of Euro 7 proposals1. Compared to the gasoline PHEV, in charge sustaining (CS) mode, the Diesel PHEV shows a 20.5% reduction in tank-to-wheels (TtW) greenhouse gases (GHG) emission, and a reduction of regulated pollutant emissions. On the gasoline PHEV under the operating conditions tested in this program, switching from a standard E10 fuel (mostly fossil-based) to a 100% renewable gasoline blended with 20% v/v of ethanol (E20) fuel has no significant impact on the pollutant tailpipe emissions, or on the TtW CO2 emissions. However, it implies a higher volumetric fuel consumption (+4.5%), linked to the higher oxygen content in E20 (hence the lower energy density). For the Diesel PHEV under the operating conditions tested in this program, switching from a standard B7 fuel (mostly fossil-based) to a 100% renewable HVO fuel also has no significant impact on the pollutant tailpipe emissions. In charge sustaining mode, it decreases by 2% the TtW CO2 emissions, and increases by +8,4% the volumetric fuel consumption, due to the fuels physico- chemical properties (resp. CO2 emission factor and energy density).

These experimental measurements allowed the calibration of energy simulation models of both vehicles, using Simcenter Amesim™ software and its IFP-Drive library. The simulator was calibrated to fit roller test bench results, real road measurements, and climatic cell data. For the latter, elementary thermal models of Heating, Ventilation and Air Conditioning (HVAC) and battery conditioning were added to the vehicle simulator to fit with over consumption and electrical range decrease due to cold or warm ambient conditions. Regarding the other powertrain components, their parametrization relied on a dedicated tool that generates efficiency maps based on engine/motor/battery general description. Special attention was paid to the on-line hybrid control strategy, so that the simulated vehicle behavior remains accurate for various types of driving, including the harshest ones, while still fitting with both electric and fuel consumptions. As this simulator modelled properly the available experimental data, a comprehensive range of real-world uses was forecasted over a wide Design of Experiments (DoE). This DoE spans vehicle configurations, battery capacity, outside temperature, and driving profiles extracted from IFPEN’s clustered trips database. The huge amount of results was then synthetized through an analytical method, since it would be too heavy to re-simulate and generalize day to day patterns.

Finally, a mathematical method of weighting each of the simulated use-cases according to their representativeness of real use was proposed, based on usage statistics in terms of daily distance travelled and temperature. The study is carried out for a wide range of battery sizing and recharging frequency, thus making it possible to determine the weighted average energetic performance and emissions of PHEVs according to these two key parameters, determined respectively by the original equipment manufacturers (OEMs) and the end user. Considering the technology sensitivity to real use conditions and considering the statistical conditions of use in Europe (temperature and daily mileage), this approach allows to quantify the weighted average energetic performance (share of electric drive, fuel and electricity consumption) and TtW CO2 emissions of PHEVs depending on their battery sizing and recharging frequency.

It shows that frequent recharging of PHEVs is a necessary condition for a high electric drive rate: recharging every day a gasoline PHEV having a battery of 15 kWh leads to an average fuel consumption of 2.25 L/100km and a share of electric drive (utility factor, UF) of 77%, whilst recharging it every 3 days leads to a fuel consumption of 4.85 L/100km (+116%) and a UF of 48% (-29 points). By comparison, the non-rechargeable gasoline HEV with a 2kWh battery evaluated under the same conditions shows an average fuel consumption of 7.3 L/100km and a UF of 24%. Compared to this reference HEV, the gasoline 15kWh PHEV vehicle allows a consumption reduction of 69% if it is recharged every day and a reduction of 34% if it is recharged every three days. Furthermore, it is observed that the first kilowatt-hours of battery capacity are the most effective in electrifying the PHEVs: for instance, adding another 15 kWh of battery capacity to the vehicle, leading to a 30 kWh PHEV, would increase by only 10 points the utility factor, from 77% to 87%, if recharged every day; instead, the same 15 kWh battery capacity could have electrified 77% of the mileage of another PHEV, which is more efficient if the total amount of available batteries is constrained.

The assessment of life cycle GHG emissions of PHEVs, adding the vehicle production emissions and the Well-To-Tank (WtT) emissions of energy carriers are not covered in this report, and will be addressed in a further study.

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