So it’s easy: just pull out the engine and the fuel tank, pop in an electric motor and some batteries, and hey presto – a retro EV! Ah yes… but what about braking?
From a sustainable engineering perspective, one of the satisfying aspects of an EV is regenerative braking (“regen”). An internal combustion engine (ICE) vehicle can’t turn kinetic energy back into petrol – it just heats up the brakes. An EV, however, can turn the momentum you are burning off when you brake back into electricity. Even better, it can do this without you even touching the brake pedal (if you want it to).
The principle behind regen braking
To understand how regenerative braking works, first you need to have an idea of how the motor works. All the motors we use at Electrogenic are AC motors, because AC motors are easier to control and have less maintenance than a DC motor. It also makes regenerative braking much easier.
The motor has a set of windings – the “stator” – arranged around a spindle with a magnet on it (permanent or induced). The stator is stationary because it is bolted to the vehicle. The motor controller creates a “flux wave” that travels through the stator windings in sequence (think “Mexican wave” in a football stadium), creating a rotating magnetic field that drags the magnet in the rotor around behind it. The rotor is attached to the wheels, and the car moves. The speed of the rotating flux wave is determined by the frequency produced in the motor controller (which responds to a signal from the throttle pedal).
Regenerative braking works in exactly the opposite way. To switch into regen mode, the frequency of supply to the stator is reduced so the flux wave in the stator rotates slower than the rotor. The rotor is being dragged around by the wheels and is now travelling faster than the flux wave, so the forces in the motor reverse and it begins to generate. The generated electricity is pushed back into the batteries. The electrical resistance in this process slows the car and the batteries get re-charged a little, which extends the vehicle’s range.
How is regenerative braking used in a car?
To make regenerative braking work in your R-EV, we put a pressure transducer in the hydraulic brake line. This gives a signal to the motor controller that is proportional to how hard you press the brake, and the motor controller instructs the motor accordingly.
When you press the brake pedal, the two wheels that are not driven by the motor will be slowed by the brakes. The two wheels that are connected to the motor will be slowed by the motor regen, and then as your speed reduces, increasingly by the brakes themselves. The proportion of regen to mechanical braking varies according to the set-up, but the diagram to the right gives a general idea.
The practical effect of this is that it’s like having servo brakes. The motor adds braking force beyond that created by your foot on the pedal and the car slows more quickly.
How much energy is recovered? It depends on your journey and how you drive. You won’t get much regen on a motorway journey and you get more regen if you are braking at speed, rather than crawling along in traffic (the energy is from recovered momentum, and momentum is proportional to the square of your speed). Some Tesla owners happily report stats, however, and there is a consensus that in general driving it can be as much as 15%, maybe 20% recovery. This can certainly be a useful range boost.
So regen is great? Yes it is, but there is one caveat. The car slows because you are forcing electricity back into the batteries. What happens if the batteries are already full? Then the regen won’t work.
If you have been reading some of our other technical posts, you will know that there is a maximum level and rate at which the batteries can accept charge and that the Electrogenic Battery Management System will step in to make sure you don’t over-charge, or try to force energy into the batteries too quickly. So if your car is fully charged, you swing out of the driveway and do an emergency stop, the re-gen won’t help you. This is true of all EVs – just ask Elon Musk! This is only a temporary phenomenon – once you are a few miles down the road, you will have used enough energy and the regen will be working again.
So should I enhance my brakes?
So when planning your electric car conversion, should you also upgrade the brakes?
We keep our 1963 Beetle completely original (apart from the motor!), just to illustrate this point. It has the original drum brakes all-round, and stops on a sixpence. Most conversions add little, if anything, to the net weight of the vehicle. It depends on the number of batteries of course, but for most cars the biggest factor is whether the original engine was made of aluminium or cast iron. So you don’t necessarily have to add extra stopping power because of the weight, and many of our customers are content with the regen braking as the only brake enhancement.
On the other hand, extra stopping power is almost never a bad thing.
Don’t forget the handbrake!
One thing that can be overlooked and which can be pretty poor on some old cars (e.g. the Beetle), is the handbrake. The handbrake is important on an EV. You can’t “leave the car in gear” if you’re parking on a hill, and holding the car on the motor on a hill-start uses a lot of energy. So the hand brake needs to be in thoroughly good working order.
You need a mechanical (cable-operated) handbrake for the MOT, but if you’re sticking with drum brakes, we can add an additional hydraulic mechanism. If we fit discs, it’s easy to make the handbrake work well anyway.
And lastly… regen without braking?
The accelerator knows when you take your foot off it. What the car does next is a matter of programming. You can coast along, like you would in 4th gear on an internal combustion engine, or the controller can switch on the regen and actively slow you down. This doesn’t really do anything for your range, but it means you can drive the car like a dodgem in traffic, so you aren’t constantly having to hit the brakes. You just lift your foot off the accelerator and you slow down.
We can fit both modes on a switch for you, if you like!