Brake energy recovery, or regenerative braking, as it’s more commonly known, has grown in prominence in recent years, as awareness of electrified vehicles has blossomed. Given the increasing complexity of the brake systems installed in such vehicles, manufacturers have started to try to differentiate their products by adding new ways to interface with the brake system (for example, paddles and push buttons). As the technology has become more commonplace, commentary on the subject has increased, along with the comparison of various vehicles’ regenerative brake ‘feel’.
In this article, I will demonstrate how brake energy recovery works, what is involved in decelerating an electric vehicle, why brake blending is necessary and discuss how some of the different brake interfaces are used to brake the vehicle.
To better illustrate this, in a subsequent post, I will use a real vehicle, and a real braking event. The vehicle in question is a Renault Zoe (R240, 2016). This is a FWD electric vehicle, with a single reduction ratio between electric motor and road wheels – which makes it (relatively) easy to understand how energy flows from the road to the battery.
The Physics of Braking
Before we go much further, let’s recap a few fundamentals (if you know your kWh from your Nm, feel free to jump ahead).
Principally, braking a vehicle involves the conversion of energy. Brake energy recovery means converting some of the kinetic energy of a vehicle into a form that can be usefully deployed later. While it is possible to convert energy in a number of ways (flywheels, air pressure tanks, heat), for this article, we will concentrate on converting to electrical energy. Therefore, we should (re)familiarise ourselves with the major terms. Starting with Force (measured in Newtons), we will derive two further terms, Pressure (force per unit area, measured in Pascals or Bar), and Torque (force multiplied by distance, measured in Nm). Using Torque, we can derive a further term, Power (Torque deployed over time, measured in Watts). From Power deployed over time, we arrive at Energy (measured in Joules, but also referred to as kiloWattHours for electrical energy).
Now, we are able to trace energy back through to force – which allows us to consider mechanical energy conversions. However, as we’re talking about electrical energy, a few more terms are useful. First up is Voltage (measured in Volts) – the potential difference between two points on a circuit. Secondly, we need Current (measured in Amps), – the flow of electrons through a circuit. Next comes Power (measured in Watts, as before), which is voltage multiplied by current. Finally, Energy (measured in Joules, as before, but also called kiloWattHours – a term used widely for electricity supply, as well as EV battery capacity).
So, to recap our list of terms:
- Force – Newtons (N)
- Pressure – Pascals (Pa) or Bar
- Torque – Newton Metres (Nm)
- Power – Watts (W)
- Energy – Joules (J) or kiloWatt Hours (kWh)
- Voltage – Volts (V)
- Current – Amps (A)
Electric Motor Torque
The next thing to discuss is how our electric motor generates torque and power. This is central to understanding regenerative braking and actually not too complicated. For our test vehicle, there are two headline figures to consider – the maximum motor torque of 350Nm, and the maximum motor power of 65kW. In the next instalment, we will dig deeper into the specifications of the vehicle, and review how the brake system works.
Electric motors generate torque based on their rotational velocity. As the velocity increases from zero, there are three distinct phases – firstly a sharp rise in torque, second a flat-lining of torque (where the motor has reached its torque limit) and finally decreasing torque with increasing speed, where the motor has reached its power limit.
Delivering Brake Requests
The shape of this graph is important for regenerative braking – the lack of torque at high motor speeds means additional torque must be generated through friction brakes. As motor (and vehicle) speed decreases, more torque becomes available, and so the brake control system must constantly reassess this and modulate the brake pressure to suit. When the motor speed approaches zero, the available torque quickly disappears, and so the friction brakes must generate sufficient brake torque just as quickly.
This blending together of two different sources of brake torque is essential to a smooth braking experience for the vehicle users. At relatively low deceleration levels, more of the requested braking can be achieved with electrical torque, and so the blending workload is relatively easy, as well as more of the braking energy being recovered. At higher deceleration levels, the blending workload increases, and sharp changes in available electrical torque can cause the friction brakes some difficulty in keeping up. It is worth noting that while friction brakes are capable of generating megawatts of brake power, they are the weak link in the chain when it comes to quickly and reliably achieving a target torque.
Friction brakes, by their nature, mean that as they are used, pad material is constantly being consumed, and therefore the friction pair is constantly evolving. The crucial relationship here for the brake control system is hydraulic pressure to wheel torque – how many extra Bar is needed to generate X Nm of wheel torque? The answer to this will always be somewhat vague, so therefore precise blending control must take account of this.
Driver inputs and vehicle outcomes
Now that we can see how desired braking is delivered by a regenerative brake system, it’s worth discussing how the various driver (or vehicle) inputs play into this.
Let’s consider single-pedal driving. For clarity, this is the idea that when the driver releases the accelerator pedal, a preset amount of deceleration is deployed by the vehicle in response. The driver can supplement this with brake pedal input, but if the driver stays off both pedals, the vehicle will continue to decelerate, and in some cases come to a complete standstill. From a regenerative braking perspective, the brake system responds to this as it would any other request – by using as much available electrical torque, and supplementing it as necessary with friction brake torque. For reasons discussed above, if a lower level of deceleration is requested, more of the brake energy can be recovered. Therefore, single-pedal driving can lead to good outcomes in terms of energy recovery – each brake event is probably going to include an amount of low deceleration/high recovery braking.
If the driver decides to step into the brake pedal during such a brake event, the brake system interprets this as an increase in requested deceleration, and must respond in kind. As before, if there is available electrical torque, this will be utilised, and if not, friction braking is deployed.
If the driver pushes hard on the pedal, and the requested deceleration levels are significant (above 0.5g), then the brake control system must prepare to deal with possible wheel lock. In this scenario, it is typical to reduce or discard the deployed electrical torque, and rely on friction brakes, so that individual wheel slip control (Anti-Lock Braking) can be achieved if necessary. Obviously, this has poor outcomes in terms of brake energy recovery, but vehicle stability should always take precedence.
When tuning the performance of such a system, the engineers’ aim will be to make the brake system performance transparent to the driver, and the vehicle response to any inputs to be consistent, regardless of whatever is happening underneath. This is definitely true of the brake pedal – while it is possible to add sophistication to a regenerative brake system – when a driver stamps on the pedal, the only acceptable response is a good old-fashioned (bowel loosening), nose-to-the-floor stop.
So far, we have covered the fundamental physics of braking, and introduced some terms that we will build upon next time. Next we looked at the way electric motors generate torque, especially as the rotational speed of the motor changes. Building on this, we considered how this can be used to (partially) brake a vehicle, and how blending two brake systems is necessary to achieve satisfactory performance.
We covered how requested deceleration is dealt with by the brake control system, and why lower requests and smoother inputs will lead to better energy outcomes. Finally, we talked about when regenerative braking must be sacrificed for more immediate needs, and why it is necessary to keep braking response consistent, regardless of how the deceleration is generated.
Next time out, we will look into how all this theory can be put into practice. We will work through an example for the Zoe, looking at the contributions from the various sources. We will talk about what the drive layout of the vehicle means for the regenerative braking, and changes if a gearbox is added into the mix. Finally, we will discuss regenerative braking availability, and what redundancy measures are necessary.