2009 Mini E: Regenerative Braking Test

Engine braking wouldn't use fuel. Except for the rev matching on the downshift, the ECU should cut fuel to the engine on engine braking until quite low speeds. Well, on any car made in the past decade and a half, I'm pretty sure.

Pretty cool, the engine braking on the MINI. Makes sense to me, but will require most drivers to retrain themselves. Think about it--normally, when we break, that's just gas turned (in a not-very-efficient manner) directly into (useless) heat!

Great post Dan! The Mini may have come upon a good combo of electric drive and regeneration here. Informative and interesting. Cheers IL.
Kevin

Interesting to see how strong the regenerative braking is on the Mini. I remember seeing this technology on electric RC cars when I was younger, always wondered when the technology would make its way over to full-sized cars.

That said, I can understand how 0.25 g would be annoying for some drivers, especially those used to driving traditional automatics. Correct me if I'm wrong, but I believe that's the equivalent of downshifting roughly 1-2 gears everytime you lift off the throttle.

Nice post Dan. One thing you didn't fully explain is that you don't necessarily have to completely lift your foot from the accelerator (sorry you called it the "gas" pedal!) pedal to use the regen. If that were the case, it would be annoying to be hit with the .25g all the time you were driving. By slightly easing up on the accelerator you get the regen and the car will gradually slow down to give you a smooth deceleration. In your tests you were just completely pulling off the accelerator and that's not how you would normally drive the car. Sure if you were coming up to a traffic light or driving the car really hard you would do that, but in normal driving you are not feeling the .25g too often.

I very frequently let people that are curious about the car take it for a drive. As you would expect they are surprised when they back off the accelerator but after only a few miles of driving they have it down pat. I have probably given 40 or 50 test drives in the past 6 months and after a few miles everyone that I let drive it liked how the regen worked. It took me a couple of days of driving to really get used to how the deceleration works and to use it optimally. Now I almost never use the brakes and as you said I can probably get 100,000 miles out of the brake pads. I like how I can control the car with the single pedal but the most enjoyable use of the regen is under hard driving and cornering.
Tom
MINI-E #250
http://minie250.blogspot.com

Very interesting post.

It simply is the most fun car I have driven in 32 years.

I just past the 6 month mark and have 8100 miles on the odo. The Mini-E has been flawless and has done everything my previous cars have done. In the 6 months we have taken 1 trip that was outside the range of the Mini-E and that was 300 miles to Paso Robles. We took the wifes Ford Escape.

Funny think is the previous year we did the same trip and also took the Escape and left the other gas car at home.

The regen is very easy to get used to and you can coast, feather a stop, or completely lift for the max regen.

The slot car is the perfect comaparison. I remember as a kid having the pistol grip controler that used a resistor with a metal slide against it, and the accell and decell is very similar to that.

Can't wait for this car or similar from BMW to go into production. I want the high performance of this car as oppossed to the Leaf or Volt.

Best part of the car? It drives on sunshine that I harvest from my roof.

Cheers
peder

I can just see some old lady forgetting where the brake pedal is and crashing, then suing BMW.

Seems like really nice tuning/engineering choices by BMW/Mini on the regen. The ability to recapture that kinetic energy and put it back in the battery is one of the huge efficiency advantages that EV's and hybrids have over traditional vehicles. It only makes sense to design the regen system to recapture as much of that energy as possible. Any use of the friction brakes is wasted energy that you can't get back.

you would think BMW could re-program the regen braking to work using the brake pedal, or add a 2-stage brake pedal. this would allow a more fluid crossover from gas engines to electric while still maintaining the benefits of regen braking.

it's also more intuative (and hence, safer) for people to brace themselves over the brake pedal, and as deceleration need increases, they can more-easily apply more pressure. that wouldn't be the case for the electric pedal, where you will need to move your foot to a different pedal should the need arise.

I figured I explained the controllable nature of the regen in the "slot car" post. But yes, you get verying degrees of decel--up to 0.25 G--as you ease off the throttle pedal.

And there are blended systems in the market (and on the way) where the brake pedal is drive-by-wire and a computer decides whether regen, traditional brakes or both will do the job the driver is requesting.

But proper brake feel is very hard to produce. The BMW's blended system felt awful, but the Volt felt OK. But the Volt test was a VERY limited 35-mph drive in a highly-managed setting orchestrated by GM, and the Volt in question had software that was only 65% final. The jury is still out on this point.

And I don't know if I'm comfortable with the idea of brake-by-wire. In this Mini, the traditional brakes are fully mechanical and direct-acting.

Okay, someone can correct me if I am wrong, but don't regular hybrids do the same thing, but only with the brake pedal? In my fathers lexus rx400h, i hardly feel the conventional braking (with the rotors). and he hasnt had to replace brakepads for its entire 75,000 mile lifetime.
I'm just saying you can technically do the same thing in regular hybrids with pressing the brake pedal instead of letting off the gas.. am i wrong? seems like the mini just condenses it into one step, and i think thats really cool.

I bet it feels very similar to riding a motorcycle in 1st gear. Motorcycles, at least 600cc sportbikes, have significant engine braking. Especially so if you're easing off the throttle at 15k rpms :)

In everyday traffic, I really only use the brakes to slow down the last 10mph.

Konocar400h, with hybrids, the initial brake pedal travel should activate stronger regen. Hybrids are tuned to drive more like automatic cars, so they'll coast along just like an automatic when off the throttle. It's not until you get on the brakes more significantly that the mechanical brakes come into action, in addition to the regen.

And hybrid systems like that are very complex with throttle input factoring into ICE engine output, transmission operation, and electric motor output blended to be as seamless as possible. Similarly, off-throttle and braking actions impact engine output, transmission response/gear changes, regen, and friction braking all blended to be seamless.

An EV system like the Mini's can be much simpler, especially with a single speed gearbox. The accelerator pedal is a simple map to the electric motor, providing thrust or regen as required. Similarly the brake pedal can be a simple, conventional braking system with little or no interaction with the motor (may an emergency interrupt so the motor can't be working against the brakes in even of motor/controller malfunction).

Not saying simpler is always better or the easiest to transition to, but it's certainly cheaper and quicker to implement.

Kinda off topic, but I wonder if the clubman body would have been a more practical application for the Mini E and its components. a Clubman body would have either 1. given more space for cargo or even a third seat, or 2. allowed for more batteries to be stored in the car for more range. that said, extra batteries would probably lead to more weight and therefore range reduction, so more cargo space would probably be the best benefit.

@konocar400h: Traditional hybrids have regenerative braking, too, but the amount they recapture is significantly less because of the diminutive size of their batteries. The battery in the Mini E is about 20 times larger than that of a Prius. As a result, the regen action doesn't create near as much drag.

But @txmatt1 is right, too. The Mini E is an electric car demonstration project, converted from a standard mini. The brake system carries over intact, so the regen system had to be stand-alone.

But the result is pretty darn fun. I could get used to this. In fact, I already have.

Could aggressive regenerative braking actually be dangerous in wet/slippery conditions? I know its FWD, but let's say the back slips out in the middle of a turn (oversteer). For most drivers, they would lift off the pedal, which in most cars is not horrible.

However, with the Mini E, that would be equivalent to pressing on the brakes which would exacerbate the situation and send the car into a spin.

Could aggressive regenerative braking actually be dangerous in wet/slippery conditions? I know its FWD, but let's say the back slips out in the middle of a turn (oversteer). For most drivers, they would lift off the pedal, which in most cars is not horrible.

However, with the Mini E, that would be equivalent to pressing on the brakes which would exacerbate the situation and send the car into a spin.

@brn,

"Interesting test. I'm curious as to how fast an ICE Mini would stop with engine breaking. Sure, it'd waste a lot of fuel, but.."

How does engine braking waste fuel? When you engine brake, that's pretty much the MOST efficient use of your car. The ECU usually shuts down or minimizes fuel injection and you use the car's momentum to turn the engine while at the same time provide resistance. And you don't wear the brakes either.