Regenerative braking settings and drive skills for RWD full electric car
from the EMS member Marco Giannini’s thesis
To maximize regenerative braking, a programmed co-operation of both hydraulic and electric brake is necessary. Optimal braking is crucial in Formula Student racing, so Marco decided to fix the issue.
Marco is an experienced member of ‘Sapienza Fast Charge’ Formula Student Team, based in Rome. Here he explains his work to improve regenerative braking.
Starting point: The Formula Student
In this period all formula student teams work hard to perfectly set the cars of the following year. At the moment, Marco’s team is working on both the design of a new energy storage and inverter settings.
Specifically, the most important issues the team is working on are:
- battery pack voltage (nominal and maximum) and energy required to run endurance race, so the right configuration of cells in series and parallel to comply also with maximum cell’s current rating;
- motor base speed,
- permanent magnet synchronous motor field weakening,
- rules compliance,
- Thermal configuration, linked to all current requirement (direct and regenerative current).
Marco’s work aims at improving the regenerative braking efficiency
In particular, the inverter settings help to achieve such goal. Attention has to be paid to max and nominal regenerative braking related to drive confidence.
The starting point of the study is the configuration obtained from the work made exactly one year ago. This baseline starts and stops only by hydraulic brake pumps.
What’s the main issue
The matter to be solved is that, during the braking time, the load is transferred to the front axle and “pulls up” the rear one so the friction between tyre and ground decreases. A decrease in the braking power in the rear axle means a loss in regenerative energy.
Considering that our car has only rear axle motor, the car cannot be stopped only through the regenerative brake. Hydraulic pumps are needed to reach the best racing braking performance, besides meeting Formula Students requirements.
We need to have different pump settings in front and rear axles, namely more power is needed to the front one.
Brake pumps on board have a telescopic cylinder useful to set different pump action with the same pedal movement.
Since for rules compliance braking has to be applied to all the tyres together without regenerative braking, a difference between front and rear pumps has been set (same modality as in race ispections).
Marco’s team has designed different levels of pedal sensitivity to the regenerative brake, according to the driver feedback in an endurance-type track. Such function is performed by the inverter: it can vary the start/end voltage of the regenerative braking, which are correlated to the positions of the pedal. So the driver can decide at which position of the pedal the electric braking shall start/end.
The end of regenerative brake is not the maximum pedal travel! This allows the pump to start working before the end of the pedal travel is reached, therefore allowing a smooth transition between electrical and mechanical braking. This way, gaps are avoided in the brake power and the rear axle doesn’t get blocked.
An important note concerning ABS made by regenerative braking.
In Formula Student mechanical ABSs are not allowed and the regenerative braking is allowed only over 5kph. If the regenerative axle stops, the tyre and motor speed go below 5kph (375 motor rpm) and, in that case, the inverter arrests the regenerative braking. The tyre, which has not much brake power, is then able to restart and have enough friction to run. This is a real electric ABS.
Marco and his teammates had been testing it for many hours. They made endurance tests with two drivers and one Electrical System Officer that checks the real time energy storage status (voltage, temperature and current).
Defining the best driving style
The best driving style consists in preserving energy and regenerating in the best possible way and at the same time keeping good chronometric time.
A cheap Raspberry Pi has been used as a CAN-data logger with Arduino-based CAN nodes, CAN-BMS and CAN-inverter. A script, realized by the team, then converts all recorded CAN messages by the Raspberry Pi in CSV files editable as Excel files.
By looking at this graphic (made with CSV files edited with Excel), we can see that driverB’s driving style has a better energy usage than driverA and that a long brake is more convenient than a short one with great power brake perfomance. This is coherent with the expectations, since high brake-power involves the hydraulics. Another consideration is that driving over bends at higher speed is worse for energy usage, yet it is better for performance.
Medio tutissimus ibis
Ovidio, Metamorfosi, “you will go most safely by the middle course”
The best performance in terms of chronometer and energy can be found in the middle of the best regenerative performance: this can be done by braking at the limit of the regenerative braking time, driving as fast as possible in curves (because this does not require too much energy) and avoiding a nervous driving style with wavy brakings.
The influence of the initial State Of Charge of the battery
There is another consideration to be made in order to define the best regenerative strategy. If the car starts the race with 100% SOC (State Of Charge), at the start of race, the regenerative brake has to be disabled and this can cause a bad driving feeling to the driver.
Don’t forget the thermal characteristic of the battery
Great attention has to be paid to the thermal design, to avoid serious risk of thermal runaway, i.e. fire and explosion. It can represent a problem because all the connections and cells have to endure current in every moment of the race.
I have no idols. I admire work, dedication and competence (Ayrton Senna)
Q&A to Marco
- Introduce yourself
I’m Marco, from Terni (Italy), I’m a 25 years old student of mechanical engineering at Sapienza University of Rome. I’ve always been fond of cars and motorsport and I entered the e-mobility world thanks to Sapienza Fast Charge, the Formula Student team of Sapienza University. I was reluctant about it at first, but then I discovered all its positive features and I’m building my career in it.
- Why have you picked this topic for the thesis?
One of the most important positive features of electric cars is their ability to recover energy during braking. Together with prof. Martellucci, we believe regenerative braking ( and range ) will play a crucial role in the replacement of the car fleet. Thus, its optimization is essential, as well as the optimization of all other components of the vehicle. More in detail, traditional cars waste all the braking energy, whilst it can be recovered and stored. We have been using a Li-polymer battery for this purpose.
- How impactful will your work be?
Our article explains in detail one of the essential aspects of electric vehicles. It will be useful both to drivers to be aware of the processes happening in the car, and to technical professionals to know how to conduct a detailed analysis on a vehicle.
- How important do you consider EMS association for students?
EMS offers all students the opportunity to approach the world of energy and sustainable mobility through a technical and friendly approach, being a solid and widespread point of reference within the university. EMS allows both the newbies to approach such topics without the reverential fear, and those who are already passionate about it, to deepen their knowledge (as with the present publication). Last but not least, it is certainly possible to participate in EMS events and initiatives, where ideas are exchanged and a network of students and professionals can foster. That will be important to create career opportunities.
