When King County Metro Transit turned to GM-Allison for its Two Mode hybrid bus transmission, the Seattle area authority really had no other choice.
Maintenance problems plagued its old dual-mode buses, which had to run on electricity through the 1.3-mile Downtown Seattle Transit Tunnel. The transit authority also wanted to add light-rail service to the tunnel, but overhead wiring needed for those older buses would make this difficult.
Simply switching to regular diesel buses would be too loud for use in the tunnel. Using older diesel buses was ruled out as too toxic.
"It was either make this work or I don't know what," said George Stites, fleet engineering supervisor for King County Metro Transit.
The transit agency never had to find out what. By 2004, it had 235 New Flyer buses using the Two Mode hybrid transmissions in service, becoming the first mass transit fleet to adopt the technology.
King County first considered the Two Mode system in 2000, when GM-Allison was developing it. The authority tested its first bus in 2002, running it nearly 24 hours a day.
But the slope of the tunnel prevented the buses from driving through it on solely electric power, so GM-Allison developed a special mode to minimize noise and emissions.
When approaching the tunnel's entrance, the driver pushes a button to charge the battery. In each of the five stations inside and at the end of the tunnel, the buses run on electric power only. Elsewhere the diesel engine runs at a reduced horsepower, so the buses are no louder than a typical passenger car.
King County appears pleased with the results. It has ordered 265 more hybrid buses, with the first deliveries scheduled for this year.
Said Stites: "It's been a good experience with partners, and the product has been good for us."