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Mr. Green Car: Why all hybrids have regenerative braking
By Allen Penticoff
In any review or brochure of hybrid vehicles you will hear mention of “regenerative brakes.” I’ll try to describe this system and why all hybrid vehicles have it.
In short, the electric motor that powers the hybrid or electric vehicle is put to use backwards to supply energy to the batteries during slowing down and braking. Like a lot of technical things, upon doing some research I discovered this system of braking is nothing new. This technology had been used in electric railroad locomotives since the early 1900s. One mountain ore train generated so much power coming downhill with its load that it was a net generator of power to the grid — producing more power descending than it consumed to go up the mountain. Even early electric cars had this feature.
In a hybrid car or truck, whenever you touch the brake pedal, a switch tells the computer that you are switching from being under power to deceleration. Whether being propelled by electricity or gas, it doesn’t matter — the system engages the traction electric motor to produce electricity (it becomes a generator). Since producing electricity takes energy, the vehicle begins to slow down. The kinetic energy that is a motor vehicle in motion is a lot of stored-up energy, and this is a good way to get some of it back.
Most hybrids do not have a lot of storage battery capacity, so this payback of energy through braking with the motor helps extend the ability of the batteries to help the engine with acceleration and gas-free driving distance. In other words, it increases gas mileage.
Depending on the design, the regenerative braking can take some getting accustomed to. The braking action can be quite profound, as the motor becomes a generator, and slowing takes place faster than you actually anticipated. One car is designed so you don’t even have to touch the brake — as soon as you lift your foot off the accelerator, the brake light comes on and it begins to slow. Other early systems had a different feel — like nothing was happening. Drivers would press on the brake pedal, and while the motor was producing drag, it wasn’t much, and the driver would press much harder on the pedal to get the friction brakes to engage — this felt like a lag in brake action. All reported they eventually grew to not notice this.
Of the hybrid cars I’ve driven recently, the Ford C-Max and Toyota Prius, I can’t say I noticed anything different about the braking action. In all hybrids, there is some sort of display showing that as you coast or brake — power is being returned to the batteries. Conscientious hybrid owners take this on as a challenge — to see how much power they can get back through gentle driving. They are rewarded with much increased fuel economy. For the pure electric car owner, this is their “fuel” they are getting back — extending their driving range and lowering charging costs.
A benefit of regenerative braking is that the brake pads on the friction brake system last much longer. Some reports have been of getting 85,000 miles on a set of brake pads. A downside might be the extra complication of the two braking systems — but that does not concern me much. Most new vehicles are quite complex, yet reliable.
There has been talk of putting regenerative braking on non-hybrid cars, but the complications don’t seem to be worth the expense. In 2010, some Formula 1 racecars had been allowed to have KRS energy recapture systems. These are actually flywheel systems that convert braking energy into flywheel speed, which is then used for a burst of power during acceleration. All teams have since abandoned it, although it did prove effective and won a couple of races.
Now that there are a lot of hybrid cars and trucks on the streets, as you see them coast down a hill or brake to a stop, you can envision them saving energy and fuel cost through the use of a rather simple technology.
You can mimic this without the system — by accelerating and braking gently, coasting down hills (just taking your foot off the gas pedal) and you, too, will be rewarded with saving money and reducing pollution.
From the May 29-June 4, 2013, issue