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A mechanical alternative to the battery which utilizes braking energy too is the so-called gyro drive. Originally the gyro bus was developed in the early 1950s by the Oerlikon engineering works in Switzerland and presented at the German Transportation Expo in Munich in 1953 in combination with an electric drive motor, as alternative to the trolleybus. A hydrogen-filled housing contained a flywheel which was wound up with the aid of electric energy. For this purpose the bus had to be docked onto a “charging” station for two minutes, enabling it then to travel up to six kilometers; the flywheel supplied the electric traction motor with energy via a generator.
In contrast, the gyro drive which Daimler-Benz studied in the late 1970s with grants from the German Federal Ministry for Research and Technology and in cooperation with Bosch, MAN and several university institutes involved a combination of flywheel and diesel engine. An O 305 standard regular service bus and two minibus variants with large, high windows served as test vehicles.
The flywheel stored the braking energy and the excess output of the diesel engine not required for drive purposes. Diesel engine and flywheel worked together, electronically controlled, to achieve optimum system efficiency. This enabled the vehicles to make do with a smaller diesel engine than usual. For short distances – for example in pedestrian malls or routes through tunnels – the flywheel, noiseless and emission-free, could take over to drive the vehicle alone.
The 17-seat city bus was fitted with a 65 hp diesel engine. The flywheel weighed 115 kilograms and took up 750 Wh of energy. By means of two electric motors, connected by power collecting gears, and a four-speed power shift transmission, the flywheel gave energy back to the drive unit. In the O 305, by contrast, four hydraulic motors transferred the energy from the flywheel to the drive unit. With 1500 Wh and a weight of 226 kilograms the gyro drive was designed with twice the power of the city bus. The diesel engine developed 130 hp.
At the time of testing, the gyro drive had the advantage over the lead battery of a much smaller weight and a longer life. However, its capacity was not nearly as high that of efficient batteries. With the diesel engine switched off, the gyro bus only managed a distance of around 1000 meters, including starting off twice at bus stops. Larger flywheels would have required too much space. The gyro drive could not compete with new, lighter battery types like the zebra battery, the nickel-metal hydride battery or the lithium-ion battery.
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