Energy balance: 48V mild hybrids under real driving operation

FEV investigates the potentials of 48V powernets

28. May 2017 | Engineering Service

Vehicles equipped with a 48V mild hybridization offer a potential to save fuel compared to conventional vehicles. This advantage is leveraged by introducing full hybrid features like a greater amount of regenerative braking, supporting the internal combustion engine by load shifting and adding additional functionalities like sailing and advanced stop/start. The 48V system is also adding power for additional consumers for comfort systems like air conditioning and active suspension. Furthermore, electric supercharging from a 48V system can be introduced for Otto and Diesel internal combustion engines which is increasing the fuel economy and can also be used to meet emission regulations by simplifying exhaust gas aftertreatment systems. In a recent study, FEV evaluated which of these features can be supplied by the powernet only from regenerative braking. For the evaluation, vehicle measurements from a 48V powernet are enhanced with simulations in real driving conditions.

System Layouts and Functionalities

In the course of its study, FEV used a representative C-segment vehicle with a 1.4 liter gasoline engine, 7 speed dual clutch transmission and a conventional 12V powernet as a reference vehicle. The 12V powernet topology features an intelligent controlled alternator, conventional sprocket starter and 12V AGM lead acid battery.
The 48V topologies adopted in the study feature a downsized 1.2 liter combustion engine with the same dual clutch transmission and consist of a belt-driven starter generator (BSG, with integrated power electronics) which substitutes the alternator, 48V lithium ion battery and DC/DC converter to connect and supply the 12V powernet.
Various 48V topologies with different functionalities were considered. Starting from a reference baseline with a conventional powernet and stop/start at 0 km/h, this study analyses the impact of expanding a stop/start system up to engine off sailing as well as various boosting strategies and their corresponding electrical energy demand.


Sensitivity Analysis: Powernet Consumption

Identification of powernet consumption in real driving is very complex due to a large range of applications, vehicle variants and the influence of customer behavior. Based on a systematic approach, FEV set up a comprehensive database containing detailed powernet measurements for different drive cycles, including real world driving.

The powernet consumption was varied according to varying loads. In the high load scenario, all selectable consumers were switched on at maximum stage. During the normal load scenario, just low beam lights, infotainment, radio and automatic air conditioning were activated. Average powernet consumption is between 0.5 and 1.2 kW (depending on environmental conditions and user profile). However peak consumption can be 2 to 3 times higher.

Starting from minimum powernet load, fuel saving slightly increases from 6% up to7 %. This improvement is affected by two aspects: On the one hand stop/start function of the conventional vehicle is limited due to less energy recovery and SOC constraints of the lead acid battery. On the other hand electrical charging using the BSG can profit from intelligent load shift operation. In contrast high load scenarios lead to lower benefits of the hybridization due to further limitation of the 48V functionalities. At the same time the limitation of functionalities like advanced stop/start or engine off sailing can also influence the driving experience negatively.

Average Power Distribution during FEV Cycle

The need for active charge energy, which has to be supplied by fuel, is drastically increasing with higher powernet load. At the same time it is restricting 48V functionalities, like BSG boost, since the energy is not free by recuperation and has to be generated from fuel along the engine and BSG efficiency. The amount of regenerative braking energy is also limited due to the effect of idling in stop phases and therefore not the full regeneration potential can be leveraged.

Fuel and CO2 Saving Potentials in the FEV Cycle

For the FEV Cycle, a real world driving cycle with average load profile was taken into consideration and compared to a reference case of a 12V powernet with stop/start functionality at 0 km/h. All cases were evaluated with (12V and 48V) balanced final SOC and an initial SOC for the 48V systems of 70%. The analyzed real world driving cycle consists of 90 km in urban, extra-urban and highway driving with an average speed of 50 km/h and a maximum speed of 121 km/h. During this drive cycle the 48V system achieves high fuel savings of 6-7% by supporting the powernet with the energy recovered during the braking phases. Due to the high powernet load, very limited energy is available for boosting, resulting in a large share of engine operation above optimal conditions. Due to the highly dynamic and demanding driving cycle, the eCharger is extensively used. This component increases engine low end torque and avoids turbo lag enabling combined “downsizing” and “downspeeding” with a resulting benefit in term of fuel consumption of 1-2 gCO2/km. Sailing functionality (with engine off) during deceleration and downhill phases enables a further fuel consumption benefit of 4%. However, this potential is highly depending on the driver’s anticipation.

Graphics - 48V mild hybrid under real driving operation

Powernet consumption of C-segment vehicle (FEV database)

Graphics - 48V mild hybrid under real driving operation

CO2 emissions along NEDC, WLTP and FEV Cycle for balanced SOCs


Graphics - 48V mild hybrid under real driving operation

48V system – new features and functionalities

Graphics - 48V mild hybrid under real driving operation

Average power distribution during FEV Cycle