Anyone who has driven an Electric Vehicle or a Hybrid has noticed the brakes feel weird. Even after some getting used to, the non-linear, unpredictable response from “regenerative brakes” can make experienced drivers look like student drivers, especially at parking lot speeds when the problem seems to be its worst.
The issue is that the generator is trying to get as much power as it can from the wheels, while the brakes are trying to figure out how much leftover inertia there is for them to arrest. The result is unpredictable, erratic braking that can leave drivers feeling like the car is going to roll into the vehicle ahead when creeping to a stop at traffic lights or parallel parking.
Honda has solved this problem. The upcoming 2014 Honda Accord Plug-in Hybrid and the battery electric 2013 Honda Fit EV both use the automaker’s new electro-hydraulic braking system in coordination with the generator to stop the car.
Electro-hydraulic braking (Honda calls it Electric Servo Braking) is brake-by-wire. The brake pedal doesn’t send pressurized hydraulic fluid to calipers at each wheel. Instead, it sends an electronic request to a computer that evaluates the pedal pressure and speed of application, matches it against vehicle speed and considers the effect of the regeneration of electricity underway. Then it uses an electric pump to send hydraulic pressure to the brake calipers to stop the car.
The seamless coordination between the brake computer and the generator’s computer produces smooth, predictable braking, a situation that has eluded every hybrid and EV until now.
Electro-hydraulic braking has been done before. Mercedes-Benz applied the technology to many of its cars in the early 2000s, but customers hated the erratic response. When it introduced new versions of those cars, they arrived with familiar, comfortable, conventional brakes. At the launch of Mercedes’ flagship SLR supercar following a morning of tearing across the countryside, I nearly rear-ended the SLR parked ahead of me when the computerized brakes ignored the gentle pressure I applied while trying to park smoothly. After a brief bit of panic and a slam on the pedal, the SLR stopped, just short of the equally expensive supercar ahead.
Why does Honda’s system work better than the one Mercedes used? Honda says it has a new brake pedal sensor that is more accurate at measuring the driver’s intent. It also pushes back using a pedal force simulator, like a force-feedback videogame joystick, giving the driver realistic brake feel instead of the disconnected feeling typical of “by-wire” electronic controls.
Together, regenerative braking and electro-hydraulic braking are perfectly suited to one another, thanks to expert matchmaking by the computer and improved communication courtesy of the pedal force simulator. This doesn’t make hybrids or EVs much more efficient, but it does make it more invisible when they are recapturing energy.
As electric drive technology becomes more commonplace in response to new government Corporate Average Fuel Economy standards, improvements like this one from Honda will welcome drivers of conventional cars to the world of hybrids and EVs with a familiar driving experience. — Dan Carney, Motor Matters
Manufacturer Photo: An electrically power-assisted hydraulic braking system on the 2013 Honda Fit EV utilizes 11.1-inch ventilated front discs and 8.7-inch rear drums, along with a new electric servo brake system that helps to recharge the lithium-ion (Li-Ion) battery pack while the vehicle is coasting or braking.
Copyright, Motor Matters, 2012