The Big Wheel

To passengers on San Francisco's "Muni" (Municipal Railway) system, the two new vehicles will look--and ride--like ordinary electric trolley-buses. But when they begin rolling up and down the city's hills next year, transportation engineers everywhere will be eagerly watching their progress. The test vehicles will be pushed along part of their routes by a spinning flywheel.

Among the most ancient of mechanical devices, the flywheel works on a simple principle: a rapidly spinning weighted wheel serves as a highly useful reservoir of energy. It has been put to work in a wide variety of ways. As a potter's wheel, it smooths out motion between movements of the foot pedal. On the crankshaft of an auto engine, it prevents uneven rotation that would result from piston strokes. But it is only recently that engineers looking for less polluting means of transportation have begun to give serious thought to tapping the whirling flywheel's energy.

The Electrogyro. Swiss designers pointed the way in the 1950s with a flywheel-powered bus called the Electrogyro, which operated in Europe and Africa for a number of years. To start up, the bus drew electricity from overhead wires to drive a combination motor-generator attached to a 3,300-lb. steel flywheel under the floor of the vehicle. When the motor had the flywheel turning at a speed of 3,000 r.p.m., the driver would break contact with the overhead wires. At this point, the motor would become a generator powered by the heavy flywheel, which would be kept spinning by its own inertia. Electricity from the generator would then turn a conventional drive motor attached to the rear wheels. Electrogyros could not travel more than three-quarters of a mile before the flywheel had to be spun up again, and eventually they were abandoned as impractical.

A flywheel can be kept rotating longer if its weight or its initial rate of spin--or both--is increased. Trouble is, top speed is limited by the strength of the flywheel's material. Had the Electrogyro's wheel been spun much faster centrifugal force would have ripped it apart. In the vehicles being equipped by Lockheed for San Francisco, the flywheels will be revved up to 12,000 r.p.m.--fast enough to drive a fully loaded trolleybus (80 passengers) for six miles. To keep such fast-moving machinery in one piece, say Lockheed engineers, they will use new high-strength steels originally developed for spacecraft and new designs that concentrate the mass of the wheel close to the center.

As in the original Swiss system, the flywheel (housed in a vacuum chamber to reduce friction from the air) will be used to produce electricity for the vehicle's 150-h.p. motor rather than to drive the wheels directly. If the flywheel's speed drops below its normal operational minimum (6,000 r.p.m.), the motor can be operated on power from overhead electric lines. Simultaneously, this power source--or in the future, underground transformers--can also be used to spin up the flywheel again.

Lockheed engineers have incorporated a new twist: regenerative braking. When the driver applies the brakes on San Francisco's steep hills, electrical switching will also turn the drive motor into a generator. In that mode, it will act as a drag, helping to slow the trolley, much as an auto engine does while the car is coasting downhill in gear. At the same time, it will be providing power for the flywheel's generator-motor. With this system, two-thirds of the energy needed to get up the hill should be recouped on the run down.

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