Not much bigger than a stack of pennies, the "mini engine" is the first engine of its size to deliver power on a continuous basis. Fashioned from steel, the engine is also a prototype for a UC Berkeley endeavor to create an even smaller engine chemically etched from silicon.

"We are at the frontier of research into how to generate power using the smallest of components," said Carlos Fernandez-Pello, a mechanical engineering professor at UC Berkeley who developed the engine with the help of Kenji Miyaska of Fukui University in Japan, UC Berkeley post-doctoral researcher David Walther and graduate students Kelvin Fu, Aaron Knobloch and Fabian Martinez.

At present, the engine, built by Berkeley's Mechanical Engineering Machine Shop, can produce up to 2.5 watts of electricity, enough to power a bicycle headlamp. But Fernandez-Pello and his team are ramping the engine up to produce 30 watts, enough to power a weak light bulb, but plenty to power electronic devices.

Like the engine in your car, the mini engine produces motion from controlled combustion, which takes place when a fuel such as gasoline is combined with oxygen and a spark in a chamber. The released energy drives the movement of a rotor that can be attached to a gear system to make automobile wheels turn or drive other machinery.

The mini engine is designed to run on liquid hydrocarbon fuels such as butane or propane, chemical cousins of gasoline. One shot glass of fuel (a fluid ounce) will keep the motor running for two hours. Once it is optimized, the tiny engine will be able to run 10 times longer than a conventional lithium ion battery, meaning that refueling will be required 10 times less often than changing a battery. The motor and fuel together weigh only a fraction of the weight of a standard battery such as the one powering your digital camera.

Unlike most automobile engines where combustion occurs in a piston/cylinder assembly, the rotary engine has a flat, peanut-shaped chamber and triangular rotor. As the rotor rotates within the chamber, the edges of the rotor act to partition areas of the chamber into smaller volumes where combustion can occur. Combustion acts to further rotate the rotor around the chamber, transferring force to the shaft that is attached to the rotor. Called a rotary engine or Wankel engine after its inventor, this engine design has not been as widely used as the piston-style engine found in most automobiles today, although the rotary did appear in the Mazda RX-7 and is reappearing in the concept cars of tomorrow.

Also like a car engine, the mini engine produces exhaust that contains carbon dioxide and water, about the same amount as is exhaled by one and one half persons at rest. While this is hardly enough to generate substantial pollution, the researchers are now developing a miniature catalytic converter.

The Berkeley team hopes that some day the mini engine can be used to power electronic devices, like computers or robots. The U.S. military is interested in the device as a way to assist soldiers in the field. For example, the engine could power an apparatus attached at the knees to provide an extra boost to soldiers wearing heavy combat equipment. Another military application is a power source for field sensors to track helicopters.

To Fernandez-Pello and his colleagues, the mini engine is an important first step towards designing a much smaller engine made using microelectromechanical (MEMs) technology. The mini engine design is ideal for miniaturizing because of its simplicity, and the components are relatively easy to make using silicon etching technology.