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Hybrid auto engineers put the brakes on energy loss
Regenerative braking boosts vehicle efficiency
Dexter Sumner
4/20/2004

The very idea seems like an exercise in faulty logic. Indeed, it would be nonsensical to think that one could gain by losing? If one were to break any whole, one would always come up with pieces or always come up with less...
right?

Try applying this line of thinking to the act of driving a car. Think of momentum as the whole. Braking slows you down, resulting in a loss of momentum, which is, well... a loss. The car slows down: end of story.

Needless to say, this conventional thinking has dominated the science of automobiles since before their invention, when passenger vehicles were still horse drawn. But this may no longer be the case. Japanese auto designers have taken a new tack on the subject, finding energy gains from the loss of momentum.


One would think that the unique, inherent qualities of hybrid cars (cars using multiple power supplies, such as internal combustion and battery) like fuel efficiency and the potential of fuel cell technology, would be enough to put them over the top as far as innovation goes, but Japanese designers have raised the bar higher still through an ingenious braking scheme that challenged conventional designs … and conventional thinking. Regenerative braking has emerged as a premier technology, and has brought an advance in fuel efficiency.

The Concept

The concept was sitting there the whole time just waiting for someone to notice it. Slowing a vehicle down, as contrary to intuitive thinking as it is, can create energy. It’s not so odd to consider, however, when you realize that braking occurs due to friction, and friction produces energy in several forms, the two most obvious being heat and sound.

The main concern of a hybrid designer is to supply the needs of the electrical motor, and this is the foundation of the “regenerative braking” concept.


The goal for hybrid vehicle designers is fuel efficiency. Hybrids utilize an electric motor to assist the gasoline partner, creating a fuel-efficient technological tag team. At the crux of the plan is a battery, the charging of which is a main concern. Generally, this is achieved through a complementary exchange between the two motors. When the electrical motor is off line, electrical energy generated by the gasoline-powered engine charges the electrical motor’s battery. When the battery is charged, the gasoline components rest.

This energy waltz between gasoline and electrical sources brings to light a realization: Greater fuel efficiency can be gained through increasing the electrical engine’s work load. This back-and-forth requires working the electrical system harder, which in turn means a much stronger and more reliable central battery is needed. It’s got a bigger belly, so needs more food. With this challenge in mind, hybrid designers have become electrical energy prowlers, constantly hunting for non-gasoline sources of recharging power. One place to seek is internal energy sources. These are valuable in part because consumers are reluctant to plug into external sources. It was not long before braking came to be seen as a source of significant energy loss ripe for capture and redirection. The process has been coined “regenerative braking.”

Heat-Energy Catharsis

Heat-energy catharsis refers to the heat and energy that that are generated, only to be lost in the process of braking. It doesn’t take an expert to spot the inefficiencies of the internal combustion engine. The most noticeable are the massive heat discharges: Just touch an exhaust pipe or open the hood of a running car - you’ll be burned if you’re not careful.

And there are more energy gaps. There are residual currents, or extra electricity, generated by alternator functions, and wasted turbine potentials whenever the vehicle is manually downshifted. Heat-energy catharsis, however, is the big one. The compression and use of heat is what powers the combustion engine in the first place. Engine designers say that the potential energy lost through the catharsis is the reason combustion engines aren’t more efficient. Only 45 percent of their total energy output is captured. This is a tremendous waste, but it represents a tremendous opportunity.

Regenerative braking can compensate for some of this loss by tapping into the torque effect present in braking. This energy originates from the twisting and grinding forces that occur in the wheel hub. A similar type of action occurs during high engine revs during downshifts, which also creates energy. A modern exercise treadmill offers a good illustration of the concept. They have electronic consoles, yet they do not need to be plugged in. As a user begins exercising, force is applied to the rollers (or turbines) causing them to spin. This in turn creates an electrical current powering the display consoles. The same scheme is the basis for regenerative braking.

How does it work?

As an electric-powered vehicle slows down, a small alternator motor attached to the brake pads spins under the force created by the braking action and sends the electrical energy created to the 42-volt battery used in most hybrid schemes. However, the state of the technology today requires a gradual slow down of the vehicle before coming to a complete stop. If an electrical vehicle comes to a sudden stop, it captures less energy, or sometimes no energy at all. The best technique is to allow the vehicle to come to a halt without actually braking. So another small benefit to the braking concept is increased brake life.

Where are they found?

Several Japanese hybrids are now fitted with the new regenerative braking system. They include the likes of Toyota’s Prius, the Honda Civic and the Honda Insight. Lexus has put the system into its RX 400h SUV. New York City’s bus system, and Washington, D.C.’s Metro-rail have both introduced limited regenerative braking systems for their mass transit vehicles. European transit authorities are now using regenerative braking systems as well.


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