Category: Car

I got an interesting question about the blue “Aim bar” in the new driving screen. Confusion arose how, when increasing speed, the maximum braking torque aim bar actually shrinks, while common sense would dictate it should stay the same or even grow a bit.

Well, common sense is not always right! Assume we are running at a speed where the motor itself can apply it’s maximum braking torque. The power this potential braking would generate is the torque multiplied by the angular velocity. So, as the speed of the vehicle goes up, by definition, the power regenerated with this maximum torque goes up too. Pretty quickly we will hit the limit of the battery: 43kW, and that is under ideal circumstances. At any speed above that, you’d have to actually decrease torque not to go over this maximum, and this is exactly what the power management does and what is displayed through the blue bar..

Consequently, if you “follow” the bar coming down from i.e. 120 km/h, you will find you’ll easily hit that maximum with only a little bit of braking pressure. While speed bleeds off you’ll find yourself braking harder and harder following the blue bar, chasing the maximum power. That goes on until you reach the maximum torque the motor itself can apply. For a short moment, this is a fixed value. As the car decelerates further, now at a constant rate, at constant pedal pressure and with decreasing power generation, it reaches the point where the motor is simply turning too slowly to apply its maximum generating torque and the ability to brake through the motor collapses. I you don’t do anything, the friction brakes will kick in. This is the “traffic light effect”.

Hope this helps.

We get a lot of questions about the braking system. Here is how it really works.

• Coasting without braking is not a braking function and is performed entirely by the EVC (the motor management computer);
• As the driver starts pushing the brake pedal, the requested torque is computed by the UBP (braking computer) based on main cylinder pressure and pedal position. This requested torque is passed to the ESC (the ABS computer that controls the oil valves to the friction brakes);
• The EVC permanently sends messages to the UBP stating the maximum torque of the motor. This is determined by gear, SOC and temperature;
• The UPB requests the EVC to apply braking torque. In principle this is the same as the driver requested torque, up until the maximum the motor can deliver;
• The EVC sends the truly applied torque to the ESC. The ESC computes the difference and applies friction braking for any difference.

Note that if you put the car in N, no motor braking is possible and the ESC will command all braking through the friction brakes.

In normal operation this means braking is almost entirely regenerative, with the following exceptions:

• very fast braking: the hydraulic system is faster than the electrical system (see below for some more details);
• very powerful braking: the hydraulic brakes are more capable than the electrical system;
• related to the above: if fast, powerful braking is applied, it’s hydraulics all the way as the car might want to apply any form of EPS which requires individual control of all 4 wheels;
• when the electrical system cannot apply the requested torque (max charging power reached, you can see this in the driving and braking screen);
• at very low speeds, when the motor simply cannot brake.

Here is a revealing graph: yellow (hard to see, hidden under purple and red) line  is the driver requested torque. The blue line is the regenerative torque and you can see the hydraulic system (red line) compensating for any difference. Note that the final cut over is at a very low energy state (roughly last half second before full stop, 10% of the time, 1% of the energy).

*) Other than this graph shows, when serious braking is applied very fast (3rd bullet above), and therefore, the hydraulics kick in immediately and substantially, they are not released anymore and replaced by motor torque. So, for economical, max regen braking, it is better to not only push the braking pedal not too deep, but do do it gently too.

ps: look here for a description of the computers in the Zoe.

pps: there is a ton of information in this document on scribd.

ppps: Here is some more info on braking.

Today I have spoken with a Renault specialist and to make a long story short: my definition of friction braking is WRONG. In normal operation, the brake system is entirely “fly by wire” and does not perform friction braking as CanZE reports. I will fix this, but I doubt it will make this weekend-release. Long story short, Zoe uses far more motor braking than anticipated. To be continued.