Typical for the gas-powered buses was a box-like hump on the roof. It contained the gas cylinders, whose contents were compressed to a pressure of 200 bar. The specially designed steel cylinders wrapped in aramid fiber can even withstand 500 bar. The engine was based on the horizontal in-line six-cylinder OM 447 hA, a conventional turbocharged diesel engine that was converted to a spark-ignition engine. The power was regulated by means of a throttle valve. The big advantage of the natural gas-powered vehicles: they did more than twice as well as required by the future Euro 2 emission standard, two years before this standard went into effect. The range of the new natural gas-fired buses was about 300 kilometers.
Mercedes-Benz O 405 NG – Gas bottles on the roof
That same year the prototype of the O 405 GNTD dual-powered articulated bus premiered: this low-floor articulated bus had a diesel engine and an electric wheel hub drive. It could also be designed as a trolleybus. The diesel engine was fitted in an unusual position in this dual-powered bus: transversely, at the rear. This way it made room for a door with a low-floor entrance behind the rear axle. The unaccustomed position was possible because the GNTD made do without a transmission and differential: The energy produced by the diesel engine via a generator (or, alternatively, fed in from the electricity grid) was transmitted to two each air-cooled wheel hub motors on the center and rear axles. Together these motors developed 300 kW and simultaneously serve as generator brake and thus as a service brake.
Mercedes-Benz O 405 GNTD