Hybrid buses on urban trial

Since the spring of 2009, six ethanol-powered hybrid buses have been operating on one of the most heavily trafficked bus routes in Greater Stockholm. During a two-year period, Scania’s new prototype powertrain will be tested to analyse performance and improved fuel economy. This project is the first in which the vehicles have been equipped with series hybrid technology.

Focus on renewable fuel

The Swedish capital already boasts the world’s largest fleet of ethanol-powered buses. In addition to more than 400 ethanol buses, Stockholm Public Transport (SL) also has more than 80 biogas buses. SL’s goal is for half of all the city’s bus traffic to run on renewable fuels by 2011.

“We are also very interested in reducing energy consumption, so we are obviously pleased to join with Scania in this project to test a hybrid bus that runs on renewable fuels,” says Stefan Wallin, Environmental Manager at SL.

Better fuel economy with series hybrid

Fuel savings also mean reduced exhaust emissions. Wallin believes there may be further advantages. “We are interested in seeing whether a powertrain like this will also result in quieter traffic,” he says. “Perhaps we will achieve smoother driving and more satisfied passengers.”

The buses are operating on one of the most heavily used routes in the southern part of Greater Stockholm, resulting in a large number of test kilometres and driving hours. Stop-and-go traffic creates the best prerequisites for the fuel savings that a series hybrid provides, due to rapid charging and discharging of the energy storage system.

Rebuilt ethanol buses

“A series hybrid is at its best when you have stop-and-go traffic,” says Marcus Wik, who manages the project at Scania in Södertälje. “This makes the technology ideal for refuse trucks and city buses.”

For the test series, six three-axle Scania OmniLink ethanol buses were rebuilt. The gearbox was removed and replaced with an electric motor. A generator on the ethanol engine was electrically connected to the electric motor and the supercapacitors, which store the energy generated from braking and provide energy when the bus starts up again to accelerate to 50 km/h.

“The technology is designed to lower overall fuel consumption by about 25 percent, and of course we hope to achieve this,” Wik explains. “But the primary purpose of the project is to assess the robustness of the technology and its potential for fuel savings.”