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A Range-Extended Electric Vehicle (REEV) usually has an auxiliary power source that can provide additional range when the main Rechargeable Energy Storage System (RESS) runs out. The range extender can be a fuel cell, a gas turbine, or an Internal Combustion Engine (ICE) bolted to a generator. Sizing the powertrain for a REEV is primarily to investigate the relationship between the capacity of the main RESS and the power rating of the range extender. Worldwide harmonized Light vehicles Test Procedures (WLTP) introduced a Utility Factor (UF) which is a curve used to calculate the weighted test results for the Off-Vehicle Charging-Hybrid Electric Vehicle (OVC-HEV) from the measured Charge Depleting (CD) mode range result, and the Charge Sustaining (CS) mode Fuel Consumption (FC). Therefore, the RESS capacity, the range extender power rating, the control strategy, and the UF are the key factors affecting the weighted FC of a REEV on the test cycle.

The aim of this study is to evaluate the Fuel Economy (FE) over the driving cycle for a 48 Volt P2 technology vehicle with different component ratings (battery and electric machine) in the hybrid powertrain, using simulation and Design of Experiments (DoE) tools. The P2 architecture was selected for this study based on an initial assessment of a wide number of possibilities, using the Ricardo “Architecture Independent Modelling (AIM)” toolset. This allows rapid evaluation of different powertrain options independently of a defined hybrid control strategy. For the vehicle with P2 architecture, a DoE test matrix of battery capacity and electric machine power rating was created. The test matrix was then imported into the simulation environment to perform the driving cycle FE simulations. Then, a 48 V P2 Hybrid Electric Vehicle (HEV) FE emulator model was created and interrogated using model visualisation and optimisation methods.

Maximising the recovered regenerative braking energy during the deceleration can significantly reduce the Electric Vehicle (EV) energy consumption and increase the range. Compared with the Front Wheel Drive (FWD) or Rear Wheel Drive (RWD) EV, an All Wheel Drive (AWD) EV with 2 electric machines (e-machines) has more control degree freedom when developing the regenerative braking control strategy. By implementing the regenerative braking at the front axle, rear axle, or at the front and rear axles simultaneously, the amount of recovered kinetic energy will be affected. Furthermore, the e-machines at the front and rear axle in the AWD EV can have different sizes or be the same. Therefore, the ratio between front and rear e-machine power rating should also be investigated to understand its effect on the amount of recovered energy during deceleration.

Recovering as much braking energy as possible, and then fully reusing it, can significantly improve the vehicle powertrain efficiency, hence reducing the CO2 emissions and fuel consumption. A 48 V mild hybrid system recovers less braking kinetic energy than a HV (High Voltage) hybrid system due to the reduced peak power/current rating. However, the cost of the 48 V mild hybrid system is significantly less than the HV hybrid system which gives the 48 V mild hybrid system a much better cost-benefit ratio. The 48 V mild hybrid system can have several different system layouts (e- machines at different positions, or have numerous e-machines at different position combinations). The aim of this study is to investigate and explain how the system layout affects the powertrain system efficiency and CO2 benefit. Simulation models are used to predict the CO2 of three such configurations.