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A stop sign is seen near Peace Tower in Ottawa on December 30, 2009. (BLAIR GABLE)
A stop sign is seen near Peace Tower in Ottawa on December 30, 2009. (BLAIR GABLE)

Rob's Garage

Stop! Why do the brakes on a hybrid feel strange? Add to ...

Rob, I'm finally in the market for a new car and thinking of buying a Hybrid.

I've done some research on the Toyota Prius and have pretty much decided on this little car, but I have a question: How does the "regeneration braking" work? When I road tested the car, the brakes felt "different." The car seams to slow down faster than my last car with the same sort of brake pedal effort.

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Am I imagining the brakes working differently or what?

Thanks for any light you can shed on this for me.


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No George, you're not imagining this phenomenon. The braking system on a hybrid vehicle is definitely different, and will act differently compared to conventional braking systems.

You already know what makes a hybrid different from most other cars on the market, but what most people don't realize is that the most important system in a hybrid vehicle is that which keeps the battery charged. Without a charged battery to supply current to the traction motor, the internal combustion engine must propel the vehicle - negating any advantage that the electric motor might provide.

Because it is still difficult and often inconvenient to find an external means to recharge a hybrid's battery, the engineers designed braking systems to capture the wasted energy created when a car is slowed or stopped. As I have mentioned in earlier columns, brakes slow a vehicle by converting kinetic energy (the car moving down the road), into heat. Rather than waste this energy, car manufacturers have created a method of capturing that energy and re-routing it to create electric current.

Through a scheme of electronic sensors, controllers and modules, engineers have built circuitry that can turn the on-board electric motor into a generator.

If you studied electricity in school, you know that with very few modifications, an electric motor can become a generator and a generator can become an electric motor. The trick is to be able to do this without having to stop, take things apart, re-wire and put it all back together - enter powerful on-board computers (ECU) and electronics.

During a stop, as the driver lifts their foot off the throttle (an electronic drive-by-wire device), the ECU signals the controller relay to stop providing current to the electric motor. At this point the vehicle systems are sensing an imminent stopping process through fuzzy logic programming. Because the ECU is reading that the throttle is now fully closed and the vehicle is beginning the first stages of slowing down, it fires up the charge controller to begin the process of receiving current to the battery instead of sending it out to the motor. A better term to use now is motor/generator or M/G.

Here's where the regeneration (regen) braking kicks into high gear - or low gear more aptly. As the driver begins to press the brake pedal, torque is now being created between the tires and the road. While the car is under light or full acceleration, torque is provided by the engine and M/G, and is delivered to the tires. Under braking the reverse is true. Torque is "felt" backward through the drivetrain. This adds to the ability of the reverse torque to rotate what used to be the motor, which is now a generator.

Back to your brake feel observation. To make a generator rotate takes a considerable amount of force, or torque. In a hybrid system during braking, this reverse torque is being absorbed by the generator as it tries to produce current to send back to the battery. This is a built in, naturally occurring resistance to turning caused by electrical Eddy currents or Foucault currents within the motor/generator.

Stay with me George, I'm not making this up.

This internal electrical resistance is what you are feeling when you press the brake pedal. When I have driven hybrids, the braking action almost feels disproportionate or disconnected from the amount of force you are applying to the brake pedal. In effect, there are two forces acting on the vehicle to slow it down.

We now truly have a back seat driver on board!

Happy Motoring.

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