Capacitors Get a Boost from Suzuki
April 04, 2008

By Bill Visnic

Battery talk has become favorite dinnertime discussion throughout the industry.

It’s been that way at least since General Motors Corp. started pumping up the volume about its Chevy Volt — a so-called “extended-range electric vehicle” concept car everyone wants to see come to production. But it won’t unless battery developers now working feverishly to perfect lithium-ion chemistry can whip up the right formula to allow the car to run for 35 or 40 miles purely on battery power.

Lithium-ion is on everyone’s lips, but capacitors, an alternative energy-storage technology discovered centuries ago and used in all manner of electronics from microwaves to computer chips, is rapidly advancing toward practical automotive consideration.

Sure, there are the usual emerging-technology caveats for capacitors, or their bulked-up brothers, ultracapacitors. But because of the useful ways capacitors differ from batteries, the possibilities for hybrid-electric vehicle applications are quite attractive.

Moreover, although capacitors’ potential for automotive hybrids suddenly seems to have a little too much hype for comfort, 2008 nonetheless saw an innovative production application on the outskirts of the auto industry: The motorcycle division of Japanese automaker American Suzuki Motor Corp. recently introduced a motocross dirt bike that uses a capacitor to help drive its fuel-injection system.

Capacitor Enables Dirt Bike's Fuel Injection
Suzuki’s ’08 RM-Z450 dirt bike is the company’s first with fuel-injection to replace the  traditional carburetor. Fuel injection requires an electrical source to drive the injector and other components, but because excess weight is of the greatest concern to motocross racers, engineers did not want to burden the bike with a battery.

As with a carbureted dirt bike, the new RM-Z450 uses an engine-driven magneto to supply the electrical energy for the spark plug. But dirt bikes often are crashed or stalled, and the additional electrical demand of fuel injection means there must be a stored energy source to drive the injector, or else the engine would be difficult to restart solely from the energy supplied by the magneto when the rider kick-starts the engine.

Enter the tiny capacitor, which Suzuki engineers call a “smoothing condenser.”

When the engine is running, the capacitor stores electric output for use when the engine is stopped or stalled, ensuring the fuel injector has power when the rider attempts to restart the engine with the kick starter.

Capacitors are much simpler than batteries because they do not actually produce electrons. In a capacitor, two metal electrodes are separated by a non-conducting material. One of the electrodes accepts the electric charge. That charge is held until a wire connects the two electrodes, then the charged plate passes electricity to the opposite terminal until the circuit is interrupted or the charge is depleted.

Capacitors Are Better, But Weaker
Capacitors can freely accept and discharge the rated capacity with much more speed than a conventional chemical-based battery. There is no possibility for memory effect and the number of charge-discharge cycles is virtually unlimited.

But because the ability to be charged is dictated by the surface area of the electrode, most capacitors’ energy density — how much energy can be stored in relation to size or weight — typically is a fraction of the energy density of a conventional battery. Many scientists and research entities are working on so-called ultracapacitors or supercapacitors that use nanotechnology or other means to greatly enhance a capacitor's energy density.

For Suzuki’s motorcycle, the capacitor’s meager power density is not an issue; it's meant to supply power for the fuel injection only a few minutes after the engine is stalled and the magneto is no longer supplying power. And as with all capacitors, if the charge goes unused, it eventually dissipates. But a Suzuki source said the capacitor retains the charge for a surprising amount of time, as long as overnight.

Because capacitors are unlikely to be able to store enough electricity anytime soon to propel a vehicle for any meaningful distance, most engineers believe ultracapacitors’ best use for automotive applications will be to augment lithium-ion battery packs, taking care of the short, high-energy acceleration bursts while the batteries deliver the endurance to drive the vehicle longer distances. The capacitors’ ability to quickly accept charging also makes them ideal to store the energy from regenerative braking.

Lots of Players, Lots of Believers
Ultracapacitor initiatives abound. At January’s Detroit auto show, for example, AFS Trinity Power Corp. displayed its XH-150, a Saturn Vue Green Line re-engineered as a plug-in hybrid that uses both lithium-ion batteries and ultracapacitors to generate an electric-power driving range of about 40 miles. The ultracapacitors enable a smaller, less-stressed lithium-ion battery pack, AFS said.

An Internet and alternative-energy community cult favorite is Texas’ EEstor, which claims a proprietary formula for the capacitor’s electrodes and dielectric separator to deliver some fantastic claimed performance. Canadian electric minicar maker Zenn purchased a stake, and EEstor also has some respected Silicon Valley and military endorsement — although no automotive application has thus far appeared.

And even BMW AG has been talking about capacitors since it unveiled its Concept X3 Efficient Dynamics in 2005, an optimized hybrid concept that uses so-called Super Caps, which BMW claims approach the energy density of the nickel-metal hydride batteries used in today’s “full” hybrids such as the Toyota Prius.

Hybrids or fully electric vehicles driven by ultracapacitors sounds almost like a Utopia, but researchers at several institutions are working to improve the energy density of capacitors to enable them to better serve automotive applications.

But OEM and independent experts tell AutoObserver they are dubious about the potential for capacitors to completely replace batteries for extended-range driving in vehicles of typical size and weight. Instead, they say a configuration such as AFS Trinity’s is more likely, with ultracapacitors augmenting a chemical battery pack, enhancing the efficiency of the entire package.•

Bill Visnic is a senior editor of Edmunds’ AutoObserver, specializing in product and technical areas. He was senior technical editor at Ward's Automotive Group.

Photos
1 — AFS Trinity XH-150. (Photo from AFS Trinity Power Corp.)
2 — The small, lightweight capacitor that helps power the fuel-injection system of Suzuki’s RM-Z450 motocross motorcycle. (Photo from American Suzuki Motor Corp.)

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