Hybrid cars are fuel-efficient vehicles that get fantastic gas mileage through the combined use of new technology and simple physics. Aerodynamic bodies, for instance, create much less air resistance as the car moves down the road, and lightweight materials only further the effort. The tires on hybrid cars are often the low-rolling-resistance type, and even the simple act of stopping the car is more efficient, too. You see, regenerative braking allows the vehicle to capture its own kinetic energy to increase fuel efficiency.
To put it in the simplest of terms, kinetic energy is the energy of motion. Anything that moves, like the wheel of a car, has kinetic energy. In the specific case of the wheels on a hybrid car, or any car for that matter, the kinetic energy is rotational -- and as everyone knows (please feel free to sing as you read the next line), the wheels on the car go round and round.
Most hybrid cars on the market today have regenerative braking systems that make good use of this kinetic energy. Regenerative braking systems can be used by full hybrids, which can operate in all-electric mode for short periods of time, or by mild hybrid vehicles, which make use of advanced systems like idle-stop technology, sometimes called start-stop technology. But regardless of the specific type of hybrid vehicle, full or mild, they both benefit from regenerative braking.
Anytime a vehicle can use energy other than gasoline, including kinetic energy, the fuel efficiency of that vehicle will increase. Up next, let's take a closer look at how regenerative brakes make eco-friendly driving possible.
Automotive Sources of Kinetic Energy
Since kinetic energy is the energy of motion, you could probably guess that cars create lots of it. Capturing some of that kinetic energy for the sake of fuel efficiency in a hybrid car is a little tricky, but regenerative braking is one common method employed by many automakers.
On a non-hybrid car during a routine stop, mechanical braking slows and then stops the vehicle. For instance, if your vehicle has disc brakes, the brake pads clamp down on a rotor to stop the car. If your car has drum brakes, the brake shoe pushes the brake lining material outward toward the brake drum surface to slow or stop the car. In both cases, most of the kinetic energy in the spinning wheels is absorbed by the pads or the drums, which creates heat.
On a hybrid car that uses regenerative braking, the electric motor is used to slow the car. When the motor is operating in this mode, it acts as a generator to recover the rotational kinetic energy at the wheels, convert it into energy and store it in the car's batteries. When the driver of the hybrid car takes his or her foot off of the accelerator pedal, the resistance provided by the generator slows the car first and then the mechanical brake pads can be applied to finish the job. Of course, the mechanical brake pads can also be engaged immediately in an emergency braking scenario.
The car uses the energy stored in the battery to power the electric motor which drives the car at low speeds. Depending on the type of hybrid, the electric motor can either work alone to move the car or it can work in concert with the car's gasoline-powered engine. So regenerative braking, coupled with eco-friendly driving techniques like slow starts and slower overall vehicle speeds, is an important feature on some of some of the most fuel-efficient vehicles on the road today.
For more information about hybrid cars and other related topics, follow the links on the next page.
Related HowStuffWorks Articles
- Dobson, David A., Ph.D. "Energy and the Natural Environment Hybrid Cars- Some FAQs and Answers." Welty Environmental Center. (August 25, 2009) http://www.weltycenter.org/energy3.htm#braking
- Formula1.com. "Renault answer 10 questions about KERS." Jan. 27, 2009. (August 25, 2009) http://www.formula1.com/news/features/2009/1/8887.html
- Henderson, Tom. "The Physics Classroom Tutorial." (August 25, 2009) http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/energy/u5l1c.html
- Hybrid Center. "Hybrids Under the Hood (Part 1)." (August 25, 2009) http://www.hybridcenter.org/hybrid-center-how-hybrid-cars-work-under-the-hood.html#2_Regenerative_Braking
- Physics World. "Vehicles Powered by Electricity." (August 25, 2009) http://www.hk-phy.org/energy/transport/print/hybrid_phy_print_e.html