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Bus makers Scania and Mercedes-Benz have revealed plans for hybrid vehicles.

More and more city authorities are introducing CO2 reduction targets and public transport must play a part in improving city air quality. Cleaning up the NOx from bus fleets is a priority, but they will have to wait several years before suitable vehicles are in service.

Scania’s parallel hybrid will be ready for production first and will run on ethanol. Mercedes is sticking with conventional diesel for its series-hybrid, but is seeking financial support from governments.

Scania says ethanol is the most cost-efficient fuel on the market when factors such as availability, infrastructure and reliability are taken into account. It will build 12 ethanol hybrid buses for trials in Stockholm next year.

Scania R&D chief Hasse Johansson said: “Depending on the production method, running on ethanol can reduce fossil CO2 by up to 90 per cent. The hybrid concept can improve fuel economy and emissions by 25 per cent.”

Scania has spent three years developing its ethanol hybrid bus concept, which uses heavy-duty supercapacitors instead of batteries.

The firm has built more than 600 ethanol-powered buses since 1989 for operation in Swedish cities. It allows its modern engines to run on ethanol.

Mercedes-Benz will unveil its first prototype diesel-electric hybrid Citaro bus later this year. Pilot operation will begin early next year and series production starts in 2009. But the company wants governments to put up cash to support start-up financing.

Andreas Renschler, the Mercedes-Benz board member responsible for buses, said: “These technologies are up to a third more expensive than conventional drivetrain concepts. We need subsidies to make them profitable.”

The diesel engine in the Citaro series hybrid operates as a generator to produce electrical power. This is supplemented by regenerative braking. The electrical power is stored in lithium-ion batteries on the roof.

Four electric wheel-hub motors on the centre and rear axles of the vehicle supply the power to the wheels. They have a combined peak output of 320kW, sufficient for an articulated bus even under strenuous operating conditions.

When arriving, waiting at and departing from bus stops, the bus can operate in electric-only mode.

The hybrid Citaro runs on a downsized 4.8-litre engine instead of the usual 12-litre in-line six-cylinder engine. This reduces the powertrain’s weight from about 1,000kg to 450kg. It also dispenses with the automatic transmission.

“It’s a logical step on the way to a fuel-cell-powered urban bus,” said Mercedes.

Other DaimlerChrysler operations have already put series hybrid buses on the roads. Since 2003, its Orion bus brand has won contracts to supply US cities with some 1,500 series hybrid buses. Japanese commercial vehicle subsidiary Fuso began testing of its Aero-Star hybrid bus in 2002.