How great would it be if every time you stepped on your car's brake pedal, a little bit of "bonus" gasoline was reclaimed and deposited back into the fuel tank?
Think about it: While driving along, you glance down at the gas gauge and notice the needle reads 1/8 of a tank. So, you step on the brake pedal a few times and … presto! You're back up to a 1/4 of a tank and can drive a few more miles without having to fuel up.
Braking Your Way to Riches?
OK, the scenario above is a bit of an exaggeration, but not altogether inaccurate or impossible with an electric or hybrid electric car. Hybrid vehicles and electric vehicles use "regenerative braking" during which the electric motor flow of electricity from the battery to the motor is reversed. Friction occurring inside the motor as it generates electricity slows the forward motion of the vehicle and generates power that is used to charge the battery. So when the electric motor in a hybrid or EV isn't powering the vehicle or assisting a gasoline engine to move the vehicle forward, it becomes a generator or dynamo and, in effect, generates more of the power the car needs to continue along its way.
Waste Not Want Not: A Perpetual Motion Machine?
Unfortunately it isn't possible for a vehicle to reclaim all of the energy it expends through regenerative braking. and go on indefinitely without refueling. Quite a lot of the energy is lost through friction and heat. However, regenerative braking does boost a vehicle's efficiency. A normal car is only about 20% efficient, meaning some 80% of the energy it expends is wasted as heat created by friction. Regenerative braking could reclaim as much as half of that wasted energy, which equates into more motion produced by the fuel you're paying for instead of using that fuel to create heat that is being dissipated uselessly into the environment. That's a good thing.
What's Old Is New Again
Regenerative braking is nothing new. It was used a century ago to slow trolley cars and electric locomotives and send generated power back into the electric lines. A December 1920 Popular Mechanics article discussing the advantages of electric locomotives over their steam counterparts explains: "Regenerative braking as it is called, is not electric braking. It is electronic speed control. The motors on the down grade produce an electric current when the armatures are revolved and the recovered current is restored either to the railway's power line or to the power company's transmission line. In the latter case, the restored current automatically sets back the power company's meters and credits the railway with the amount of the regenerated current."
A Brake From Convention
All modern cars and trucks use friction brakes to slow down. Disc-type brakes use pads lined with a friction material that clamp down on a steel disc attached to your car's wheel. Drum brakes use brake shoes that expand inside a drum that is attached to the wheel.
In either case, when the brakes are applied the motion created by the expenditure of fuel is being turned into heat. EVs and hybrids use conventional friction brakes in conjunction with regenerative braking. The operator doesn't notice any difference, he or she simply steps on the pedal as usual and an electronic brake controller decides how much or how little regenerative braking to apply in conjunction with the standard friction brakes.
The EV or hybrid vehicle's brake controller keeps track of the vehicle's speed via sensors mounted at the wheels and decides if the motor can stop the car alone or if the friction brakes need to be utilized. The controller might also decide if regenerative braking is necessary based on the amount of charge the battery is holding.
Why can't hybrid cars use regenerative braking alone and forego the additional weight and complication of friction brakes altogether? Regenerative braking isn't as effective at all speeds and, in most hybrids and EVs, only two, not four, of the vehicle's wheels are connected to the electric motor.
Is This Really Significant?
Regenerative braking is one of the unsung heroes of the EV or hybrid powertrain. By generating electricity with the motor, reverse the flow of electricity back to the battery, the vehicle slows down and, at the same time recharges its own battery. The car will subsequently expend less fuel, in the case of a hybrid, or spend less time recharging, as in the case of an EV. In both cases it means conserving more energy and ultimately driving farther on fewer dollars.