GM's Two Mode hybrid system scores with transit agencies

Sales of General Motors' Two Mode hybrid SUVs started slowly this year. But as cities look to make their mass transit systems more environmentally friendly, GM has had increasing success selling the Two Mode system for use in diesel buses.

More than 1,000 buses powered by the GM-Allison Two Mode transmission have been delivered to transit systems. Late last year, three transit authorities agreed to buy a combined 1,700 more.

"Hybrid is an incredible tool for transit authorities," said Tim Grewe, GM's chief rear-wheel-drive Two Mode engineer.

First used on bus fleets in 2004, the Two Mode system allows transit authorities to decrease emissions, improve fuel economy and lower maintenance costs. Cities from Kelowna, British Columbia, to Istanbul, Turkey, have purchased buses with the transmission.

The hybrid technology is called Two Mode because it offers hybrid efficiency in both city and highway driving.

Two Mode buses combine a diesel engine with the hybrid system, which includes a 600-volt battery pack and an electronically variable transmission with two 100-kilowatt motors. The engine and electronic motors work in parallel, so either or both can power the wheels.

At low speeds, the electronic motors help accelerate the vehicle, eliminating the black poof of sooty diesel smoke that most city dwellers associate with buses moving from a standstill. At higher speeds, power from the internal combustion engine does not need to be converted into electric energy as in a series-hybrid setup, increasing efficiency.

The system also uses regenerative braking. As the bus slows, its kinetic energy is converted into electricity to recharge the battery. This also reduces brake wear.

The technology can greatly improve fuel efficiency, too. On a Manhattan-style route, featuring frequent stops and starts, the Two Mode hybrid system increased fuel efficiency nearly 75 percent compared with a conventional diesel bus, according to a study by the National Renewable Energy Laboratory. On suburban routes that involve more highway driving, it produced at least 30 percent greater fuel efficiency.

The savings on maintenance and fuel economy can add up. Although King County Metro Transit in Washington state paid about a $200,000 premium on the 235 buses it purchased in 2004, fleet engineering supervisor George Stites said he expects to recoup the added costs within the 12-year life of the vehicles.

But more important to many cities are the environmental benefits. The Two Mode diesels dramatically cut nitrogen oxide and particulate emissions.

"Public transit is already the greenest option available for daily riders traveling to work, school and recreational activities, as our services provide them with the opportunity to reach their destinations and leave their cars at home," Faye Moore, general manager of the Southeastern Pennsylvania Transportation Authority, said in a statement when the Philadelphia area system agreed to purchase 400 buses.

"The hybrid-diesel buses allow us to be even more energy-efficient and environmentally conscious."

GM engineers designed a smaller and more complex Two Mode system for use in its SUVs. With a higher power-to-weight ratio, the SUV versions feature four additional fixed-gear ratios. They also have an onboard computer system that adjusts in real time to optimize fuel efficiency.

Although these changes signify an upgrade, GM's Grewe doesn't think they're needed on the hybrid buses. He says they were made because the SUVs might go off-road or haul heavy loads, situations buses won't face.

"It's just not necessary," Grewe said. The bus version is "an excellent system. It still is the best system out there."