Hybrid Buses: How They Work
Tuesday, April 24, 2007 3:44 PM
By now, you may have noticed our sleek new hybrid buses rolling along the streets of Houston.
An easy way to recognize them: Look for the "smiley face" on the front. A curved line that appears as an upturned mouth makes it appear as if the hybrid is smiling.
"A hybrid bus is a happy bus," quipped Chris Collet, product sales manager at Allison Electric Drives at GM Allison Transmission.
Our new hybrids - equipped with the GM Allison 2-Mode Hybrid system - are also the only buses right now that have two bike racks attached to the front bumper. You'll also see a hump on the roof of the hybrids - the giant battery pack.
Most of us know that hybrids are good for the environment, reducing the emission of toxic gases and cutting back on the fuel buses consume.
But how does a hybrid work?
In a traditional bus, whenever a driver pushes the brake pedal, the energy that was operating the bus goes to heating up the brake shoes as it slows the bus to a stop.
In a hybrid bus, there are two sources of power: the small clean diesel and the electricity in the batteries. The hybrid system has huge, powerful generator motors.
"We use the generators to generate electricity, so we're not using the brake shoes as much to slow the bus down. Now that we've generated electricity, we're going to save it in energy storage - in the batteries," explained Collet.
As the bus driver pulls away from a bus stop, the energy saved in the battery pack is used to run the motors in the hybrid system to help re-accelerate the vehicle with power from the small clean diesel.
"We put the energy in, and then we take it out to re-accelerate the vehicle," said Collet.
Highly sophisticated software manages the energy, ensuring that the charge level in the batteries is optimum for long battery life. Batteries never have to be plugged in to recharge, as on other hybrid systems.
The 40 hybrid buses METRO is in the process of rolling out have the GM Allison Hybrid EP40 system, the most advanced two-mode parallel hybrid for transit buses, according to GM Allison.
This two-mode hybrid has an infinite number of gear ratios, making it continuously variable, said Collet. Think of the traditional car most of you drive: Most automatic transmissions have four drives or gear shifts you drive in. When you shift from second to third gear, it's like walking up the stairs.
With the continuously variable transmission - or CVT - it's like walking up a ramp. "You just put it in ‘drive,' and the software will pick the gear ratio to minimize emissions, minimize fuel consumption and minimize noise," said Collet, who is pictured on the left riding one of our hybrid buses.
Engineers designed the software so that the hybrid transit buses METRO has can run on any route - from a slow, multi-stop route to an express route on the freeway.
Houston has the third largest fleet of hybrid buses from GM Allison. Seattle has 235, while Hawaii and Washington, D.C. each has 50. We are in the process of getting a total of 40 new hybrids, bringing our inventory to 44. (We have had four retrofitted hybrids on the road since 2002). New York has close to 500 of the British Aerospace Engineering hybrid system. Yosemite National Park's entire fleet of 18 buses has had GM Allison Hybrid systems since April 2005. So far, 701 GM Allison buses have been delivered in 54 cities worldwide, including in Germany, France and China.