Abstract
The ability to regenerate energy when braking is a valuable advantage of hybrid and fully electric vehicles. How much energy that can be regenerated depends mainly on the car driving and the capacity of the driveline. Detailed studies of possibilities for brake energy regeneration in real world driving are needed to better understand the potential gains of car-electrification since test cycles do not take individual driving or elevation into account. This study has analysed the potential for regeneration in Swedish car driving by applying a model for a normalized vehicle to a highly detailed and representative data set of individual car movements for privately driven cars in Sweden. The share of energy at the wheels used for braking was found to range from 12% to 63%, with an average of 30%. Engine braking could however reduce the amount of recoverable energy to about 16%. On average 42% and 89% of the potentially regenerable energy is available below 10 and 40 kW, respectively. Drivers with lower average speed have in general a higher share of the energy at the wheels potentially available for regeneration. This is however not an important factor to determine the total yearly energy/cost savings. Instead the yearly mileage is shown to be a more relevant indicator on total energy savings from regeneration. The results are compared to the NEDC and WLTP test cycles.
Highlights
Electrification of vehicle drivelines stretches from simple stop/start systems, over different variants of HEV and PHEV, to fully electric vehicles
The average share of braking energy losses, for the here used movement patterns, is 30%. This is close to the share of 29% and 27% for the normalized car following the NEDC and the suggested WLTP [9] test cycles respectively, Fig 2b
Even though test cycles are designed by using data from real world driving they will unavoidably introduce flaws into the regeneration analysis by neglecting elevation
Summary
Electrification of vehicle drivelines stretches from simple stop/start systems, over different variants of HEV and PHEV, to fully electric vehicles. Benefits from hybridisation in terms of energy from regeneration has been analysed on test cycles [1]. As the NEDC, used for emission certification and fuel use labelling, are often not very representative for real world driving [2]. They do not in general include altitude profiles of the driving. Martins et al use a powertrain model of a PHEV to analyse available energy from regenerative braking for different driving cycles, showing braking energy can represent up to 70% of useful motor energy for some urban driving conditions and about 40% and 18% for suburban and motorway conditions, respectively [3]. The aim of this study has been to analyse the possibilities for regeneration in Swedish driving by utilizing a highly detailed and representative data set of individual car movements
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