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Eye on electronics

Motor, Sep 2003 by Dale, Mike

Supercapacitors can be used to supplement the battery during heavy-duty diesel engine startup. This technology likely will 'trickle down' to auto applications.

Pull into any truck stop and you can hear the sound of diesel engines idling. Many trucks sit parked with their engines idling while their drivers sleep or watch television during their off-hours. There are two main reasons the engine is left running: The first is to provide heat and electricity for the driver; the second is the fear that in cold climates, the engine may not restart after an hours-long shutdown.

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Already, three states have passed anti-idling laws, and more are looking at the idea. There's a new, second-generation type of capacitor coming on the market that may affect the need for trucks to do all of this idling.

While all truck drivers do this, there are problems with the practice. The typical diesel truck engine consumes about a gallon of fuel per hour at idle. While a gallon an hour may not sound like much, it has a lot of implications, and they all have to do with diesel exhaust fumes. Diesel exhaust is a complex mixture of thousands of gases and fine particles that contain more than 40 known toxic air contaminants. These include many known or suspected cancer-causing substances such as benzene, arsenic and formaldehyde. The California Office of Environmental Health Hazard Assessment reviewed more than 30 studies that show that people working around diesel engines are at greater risk for lung cancer.

And it's not only the truck drivers at risk here. Neighborhoods adjoining the truck stops have the noise and pollution to contend with. Mechanics, loading dock workers and even truck stop waitresses live and work in affected areas. A single truck idling six or seven hours per day could burn more than 2000 gallons of fuel in a year's time, all because of the need to keep the cab warm for the sleeping driver and the fear that the engine won't restart.

This problem is being attacked on two fronts. For trucker comfort, a company in Tennessee called Idleair is working on a plan to equip parking spaces at truck stops with what are basically umbilical cords. An air hose fits into the cab window opening and provides heat or cooling to the cab. Cables on the air hose also provide satellite TV, electrical power and phone and Internet access. These same parking spaces have 50-amp, 240-volt connections to power the trailer refrigeration units for those vehicles that have them. The cost to operate a full-service parking space is said to be less than the cost of the fuel an idling truck would have consumed during the same period.

So now the other question is: How do you make sure the engine will restart when the trucker wants to pull out? This is where a new type of capacitor may have a role to play. Typically called UltraCaps or Super Caps, these devices do things that ordinary capacitors don't. To understand the differences, you need to understand the basics about capacitors.

The simplest capacitor consists of two metal plates separated by an insulating material. When a voltage is applied across the two plates, electrons flow from one plate and accumulate on the other. The insulating or dielectric material between the two plates prevents electron flow. How much charge a capacitor can hold depends on the area of the plates, the number of plates and the types of insulating materials used. The closer the spacing of the plates-or, put another way-the thinner the dielectric, the greater the storage capacity (capacitance).

The measurement unit for capacitance is the farad. One farad is equal to the storage of one coulomb (6.25x10 to the 18th power) of electrons stored at 1 volt. Throughout the electronics era of the last century, this has been an impossibly large number. Typical capacitors used in electronic circuits are measured in millionths (micro) or even billionths (nano) of a farad (most are in the range of .01 to 100 microfarads). Using conventional capacitor technology, a 1-farad device could be the size of a small house. But these new Super Caps can have thousands of farads in a package tho size of a soft drink can.

Super Caps get their very high storage values by maximizing the effective area of the plates and by reducing the separation between the plates to molecular dimensions. The key is the use of carbon for the plates. Carbon presents a very large surface area when in a finely granulated or powdered form. The best comparison is a sponge. In a normal capacitor, only the surface is important; in Super Cap technology, all of the inside is effective, as well.

In a typical conventional electrolytic capacitor 1 inch in diameter by 3 inches high, you might have an effective plate area of 10 square meters. In a Super Cap of the same physical size, you might have 10,000 square meters of plate area. Not only that, but the spacing between the plates can be reduced to nanometers, further increasing the effective capacitance.

The first Super Caps had storage in the 1- to 100-farad range and were too expensive for general automotive use. Second-generation units now becoming available range between 1000 and 5000 farads. Because the plates are so close together, the maximum voltage per cap is 2.5 volts. These caps can be strung together in series to achieve automotive voltage levels.

Automotive Industry

Eye on electronics

Motor, Sep 2003 by Dale, Mike

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