Regenerative Braking Systems – By Pete Melville, Hybrid and Electric Vehicle Repair Alliance, HEVRA.
We’ve got two separate challenges when it comes to braking systems for electric vehicles. The first is brake assistance, normally provided by the suction of the engine, or by an engine-driven vacuum pump, and with the absence of an engine, or an engine that runs periodically, this isn’t going to work. The second is regenerative braking – if we can do some of our slowing down without generating brake dust or wearing out the pads, and get a bit of free fuel at the same time, this is our preferable option.
However, regenerative braking isn’t enough on its own. When the battery is too full, too hot, or too cold, it can’t absorb energy at the rate we might need it to. Regenerative braking is not very effective at very low speeds, or for holding a vehicle stationary once it has stopped. It also can’t slow us down fast enough when we really need to slam the anchors on, and it has no provision for individual wheel speeds, which means no ABS or ESP, and with most cars having a single motor and open differential, a loss of traction on one wheel can mean virtually no braking effort.
So, we need a system that combines the friction braking with the regenerative braking and provides some sort of brake assistance. And like most things with electric vehicles, there’s not one-system-fits-all.
System One: Electric vacuum pump…
The first of our three contestants is the electric vacuum pump. This provides a simple, cheap solution and a nice consistent pedal feel, and is used on the cheap and cheerful Renault Kangoo ZE right up to the high-tech BMW i3. Typically, a pressure sensor in the servo will report the current level of vacuum, and the pump will be triggered as required. The major downside of this system is it doesn’t allow for any regenerative braking via the brake pedal. Most manufacturers allow for this by providing strong regenerative braking when the accelerator is released – the driver tends to only press the brake pedal when harder braking is needed. These systems are straightforward to service as hydraulically they are similar to traditional cars. When replacing the vacuum pump, there is often a drain tube which must be at the lowest point to allow moisture to escape. Some models also monitor pump life, and this must be reset using diagnostic equipment if the pump is replaced.
System Two: Electronic servo…
Behind door number two we have our second system, the electronic servo. Favoured by models such as the Nissan Leaf and VW e-Golf, these feature an electromechanical servo, where an electric motor provides the assistance, using a similar concept to electric power steering. This is how the VW system works (the Nissan uses a similar concept but works mechanically rather than hydraulically). When pushing the pedal, the fluid travels out of the master cylinder as normal, but instead of finding its way to the ABS module and down to the wheels, the fluid instead moves into an accumulator. No fluid makes its way to the wheels, instead just moving along the pipe. The system can see the pedal movement and organise the required regenerative brake force. If the friction brakes are needed, the accumulator pushes the fluid back out, and with it unable to return to the master cylinder (because your foot is pushing on it, with the servo’s help), it instead finds its way into the braking system and operates the brakes as normal.
System Three: A break from tradition…
The third and final system is a complete departure from traditional systems and is favoured by Toyota, and also used on the Renault Zoe and Nissan e-NV200. Pressurised brake fluid is stored in an accumulator, and a pressure sensor and pump ensure pressure is maintained at all times (you can often hear a buzz as the car wakes up). Pressing the brake pedal simply tells the system how much brake effort you would like, similar to the accelerator pedal. The car then decides what proportion can be done by the motor, and what proportion needs the friction brakes. When the friction brakes are required, a valve lets fluid out of the accumulator into the brake circuit. Of course, it’s a little unnerving to have a brake pedal that feels like an accelerator pedal, so a device called a stroke simulator (effectively a hydraulic spring) makes the pedal feel like it’s attached to something. In the event of a fault, the hydraulic lines between the pedal and the rest of the system are joined to provide hydraulic braking (this is without power assistance and in the case of Toyota, on the front wheels only).
Most of the electric and hybrid vehicles on the road use a variation of one of these systems. Regardless of what is fitted, as with all safety-critical systems, ensure you understand the system and follow the correct procedures to keep it in safe working order.
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