Stockholm cleans up with ethanol-fuelled hybrid buses

The Swedish capital's air is set to become cleaner still, as full scale trials begin of six ethanol-fuelled hybrid buses. City bus operator Stockholm Public Transport (SL) already has some 400 ethanol-fuelled buses and expects the hybrid versions to cut fuel consumption by 25 per cent.

During the two-year trial, the Scania OmniLink low-entry buses will serve one of the more demanding and heavily used lines in Stockholm. SL is running the trial in co-operation with Scania and another operator, Swebus. The trials are also supported by the Swedish Energy Agency and the Swedish Environmental Protection Agency.

"These operational trials will give us valuable experience of how hybrid technology shapes up in real life, as well as of its environmental effects," said Göran Hammarberg, Scania's head of bus development.

Scania has chosen a series hybrid solution for these buses, a technology which the company believes offers the greatest benefits in stop-go city driving.

In a series hybrid powertrain, there is no mechanical connection between the internal combustion engine (ICE) and the wheels. Electrical power is generated by a diesel engine adapted to run on ethanol, and the bus is driven by an electric motor that doubles as a generator when braking. Energy is stored in supercapacitors with considerably longer service life than today's batteries.

(Fellow Swedish bus-maker Volvo prefers parallel hybrid technology where both the ICE and the electric motor can drive the wheels. Parallel hybrids are generally more efficient, as they need not convert mechanical energy to electrical and back again, but series hybrids require less complex control systems.)

"Long-term, hybrid technology will help us reach even more ambitious environmental objectives for urban traffic," said Hammarberg.

He said that Scania regards ethanol as the most attractive renewable fuel available for city traffic today, taking into account factors such as availability, infrastructure and proven technology. The company will have one of its ethanol-hybrid buses on display at the UITP World Congress and Mobility & City Transport Exhibition in Vienna next month, he added.

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