Hybrid Cars and Regenerative Braking ~ Hybrid Car Review
Hybrid Car Review: Hybrid Cars and Regenerative Braking

Friday, March 20, 2009

Hybrid Cars and Regenerative Braking

Hybrid electric vehicles (HEV), no matter what type they are, have several things in common. First is the two motors. The internal combustion engine (ICE) runs on gasoline just the way every other car does. It may be a diesel-hybrid or it may be a gas hybrid, but you're not going to find anything unfamiliar here.

The second motor is electric, and it runs off of a battery pack. The battery pack, depending on its size, can be used to power the electric motor, which in turn can either 'assist' the gas engine or it can move the car by itself.

When the car can be moved by either power source, we call it a full hybrid. If the electric motor is incapable of moving the car by itself, in other words, it is only there to assist the ICE, we call it a mild hybrid.  Full hybrids get better miles per gallon than mild hybrid cars.

And if the car can only be moved by the electric motor, while the ICE is there to assist the electric motor, we call it an electric range extended vehicle (E-REV).

But no matter what type of hybrid car it is, the electric motor does not get powered by gas or diesel. It gets its power from the battery pack. The battery pack needs to be recharged (filled) and there are several methods of doing so.

Ways to Recharge Your Battery in a Hybrid Car
The most obvious, and yet the most difficult to do, is to plug in. Yes, you can store a lot of energy by plugging in, but you need very large battery packs (We're talking 8 feet long!) to store enough energy to move a car 40 miles or so. Those batteries are heavy and take up a lot of space, not to mention very expensive.  Also, you can only recharge with a plug when your car is stopped.  It takes a long time to fill up a battery right now, which is why plug-in hybrid vehicles (PHEV) are going to be last on the scene.

Another way to recharge the battery is to use the gas motor to power the battery pack. This is obviously not the best method, since the point of a hybrid is to maximize the use of the electric motor, not the gas motor.

The last way, and arguably the best way to recharge the battery pack is by using regenerative braking.

Regenerative Braking in Hybrid Cars
Normally, when you slow down a car, you use the brakes. You push on the brake pedal, which applies force to the brake pads, which are pushed against the wheels and slows the car down through friction. Simple, easy and cheap, but also wasteful.

Regenerative brakes, on the other hand, use the motion of the car to push (apply torque) onto the electric motor shaft during braking and coasting. That motion turns a magnet over a coil of wires which produces electricity. That electricity is then stored into the battery pack.

How much energy you 'reclaim' or 'regenerate' this way is dependent on a lot of factors.  How efficient is the system, how big is it, how many wheels are involved, how good is the motor control, how fast are you going, is the battery already charged, etc...
And that's it. Obviously, this is harder to do than it is to explain. By the way, this is exactly the same principle used in a hydroelectric dam.

Pros and Cons of Regenerative Braking System

One obvious pro for regenerative brakes is the power generation.  It's a cheap source of energy that would otherwise be wasted to friction.  Also, brakes tend to last much longer since they are used less in cars with regenerative braking, making your normal maintenance costs come down.  As with oil changes, the less you use your brake pads or ICE, the less you have to change out your brake pads or oil.

Another big plus is the cost.  Regenerative braking is based on very old technology (see the link to hydroelectric power above).  There's nothing really new about it.

The first con is the same as one of the pros; the initial expense of installing the regen brakes.  Even though its based on old tech, it still adds to the cost of building the car. Also, regenerative braking, even in the best conditions, cannot recharge the batteries by itself. Only a minor percentage of the recharging is done by the brake system.  It's recycled energy, but it's not a lot of energy.  Most of the recharge comes from the gas motor. 

Combining Power Generation
In a typical hybrid, the battery pack is recharged both by the gas engine and by regenerative braking.  In start and stop traffic, regenerative braking will be the better source, while if you're in a high speed, not as much braking going on, situation, you can imagine the gas motor is necessary to recharge the battery. 

This is why there is a 'myth' that hybrids are only fuel efficient in city driving, but if you're doing a lot of highway driving, you're better off without a hybrid.  A diesel, for instance, might be a better choice, but most high efficiency hybrids will use an Atkinson engine to increase fuel efficiency by utilizing both the electric motor to assist the ICE even in high speed situations. 

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