By Brad Berman
As we discussed in October, the secret sauce of a good electric car is not how it accelerates, but how it brakes. Yesterday, I had two short rides in the Honda Fit EV at the company’s Twin Ring Motegi complex north of Tokyo—prior to my visit to the 2011 Tokyo Motor Show today. My driving impressions from behind the wheel are embargoed until Dec. 5—but I’m free to report on my discussion with Kunimichi Hatano, the engineer who created Honda’s new regenerative braking system.
Honda’s strategy rests somewhere between what Ford will do with the Focus Electric—make it feel like any conventional car without special modes or gears—and BMW’s strategy. Electric Bimmers will brake in an aggressive and distinct way, according to Aaron Singer, BMW product strategy manager. The Fit EV's braking is perhaps most similar to the one found in the Nissan LEAF, with a couple of key exceptions. Honda’s Mr. Hatano explained that the goal is to make the brake feel comfortable and familiar—yet the Fit EV will have a “B” gear, providing the driver the choice to engage an aggressive motor-only regen setting, as well as an “Econ” mode that removes regenerative braking in a coasting-type function. The current LEAF lacks such driver-managed controls.
Again, I can’t speak yet about the relative level of performance achieved when toggling between Normal, Sport and Econ modes. But what’s important here is the role that Honda’s new electro-servo brake system—the first of its kind—in yielding about 75 miles of real-world range, and123 miles of range (in the forgiving LA4 cycle). The Fit combines a 94 kW motor and 20 kWh battery pack. (Honda is not yet sharing details about the weight of the Fit EV, but it's bound to be lighter than the LEAF.)
So far, the Fit EV is the only Honda car that has been (ahem) fitted with the new system, but it is also slated to go into Honda’s upcoming Accord-sized plug-in hybrid—which I also was able to drive yesterday.
Here’s an excerpt from my conversation with Mr. Hatano.
Many EV drivers like very aggressive regenerative braking. Why not give a unique aggressive feel to regen braking in your electric vehicles?
Honda does not intend to reduce the amount of regenerative braking to achieve satisfactory brake feeling. Obviously, we want to recuperate as much energy as possible. But if you try to recuperate too much energy, you might destabilize the vehicle. The question is how much energy can we recuperate without going over the threshold of losing stability. You have to bear in mind that vehicles are driven under bad weather, in snow, and rain, and when the road surface might not be good. You have to make sure the regenerative braking doesn’t go beyond what is reasonable.
But it can feel different and still be safe, right?
Sure, some prefer a different feeling. Whatever kind of brake feel you want—whether it’s like normal gasoline-powered cars or different—that’s one subject. And the limit of regenerative braking is another subject. The limit is more about preserving stability. Because this is an electric braking system, you can fine-tune the taste in many different ways.
How is the electro-servo braking system different than its predecessors?
In our system, you have two different main parts. In the first part, you create the brake feel. This is quite common with other carmakers. The way our brakes feel might be different, but the system is not very different. However, what is different is the part that actuates the brakes. Traditional hydraulic brakes use a vacuum to activate the brake system, and then pump oil to press the pads against the rotor. A hybrid system may use solenoids. Our new system is different because the medium to apply the brakes is using an electric motor to directly drive the pump that serves the brake fluid.
What’s the advantage?
This system has very high accuracy. This means more efficiency. It’s more faithful to the brake input from the driver. [Honda estimates that the servo-based system increasing efficiency by at least 5 percent.]
When you take your foot off the accelerator, you immediately have a little bit of regenerative braking. Then, you put your foot on the brake—and if you look at the curve on the hydraulic system, it goes up very sharply. But if you look at the yellow zone in the graph, [in the old system] the regeneration doesn’t go up as sharply and it’s delayed. The idea of the new system is to bring the regenerative yellow zone closer to the hydraulic response.
The first part of the slope is very similar to the hydraulic. Then, as you start reaching the regeneration limit, the slope gradually slows down a little and reaches the limit after what the hydraulic system can do. After that, you can keep regeneration system at the highest level longer than the current system. And just before you stop, the drop is even sharper than the old regeneration system.
All of this is possible because of the servo system? Could you have done this just with calibration?
No, you need the accuracy to achieve it.
Does the increased regenerative braking mean slowing down faster?
The braking distance itself is not shortened by this alone. The braking distance is determined by the weight of the vehicle, the speed, and the amount of energy you take out either by regenerative braking or traditional braking, and the tire grip. This system doesn’t mean that you’ll shorten the braking distance. That depends on the caliper or other brake operations.
How does the “B” gear mode affect regenerative braking?
B is for higher regeneration only when the throttle is off. If you are in B mode, it's only regenerating energy through negative torque at the motor level. [There’s no use of the brakes.]
And what’s the effect of Econ mode?
In the econ mode, on the contrary, it reduces the engine brake effect so the car can coast. The idea is to reduce the up and down demands on the engine—by having less engine braking, which means you don’t lose as much as speed.
The B mode is made to suit certain driving situations, like when you’re going down a slope and you see a red light at the end. You put it in B to recuperate energy, so you slow down, which is fine. And if you see the light changes to green, you just put it back into D. It’s a question of responding to all possible driving situations.
Postscript
I can guess that many PluginCars.com readers will like the sound of Honda’s approach—but the nagging question remains: Why is the company only producing 1,100 Fit EVs over the next three years, and not allowing customers who might fall in love with the car the ability to keep it beyond the lease period? Company officials here repeated the same “one step at a time” message—that they want to learn as much as possible from the first customers in order to refine the car into the best possible EV.
I spoke with multiple high-ranking executives to encourage them to reconsider—especially if it looks like the Fit EV is a winner. It’s too early to know, but I have a glimmer of hope that Honda could reconsider its lease-only approach and/or increase production. If indeed the Fit EV represents an improvement in electric car braking, along with controls for higher regen and coasting, it will only be a matter of time before competitors offer similar features in their battery-powered cars. Honda's apparent technology and feature advantage could evaporate in three years, just as the company is collecting and processing feedback from the 1,100 Fit EV leaseholders who have the distinction of driving the car. That would be a shame.
Source;
http://www.plugincars.com/honda-reinvents-regenerative-braking-fit-ev-110596.htmlource;
http://www.plugincars.com/honda-reinvents-regenerative-braking-fit-ev-110596.html
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