22 October 2003

King County, WA, which includes Seattle, will buy 235 diesel electric hybrids buses, marking the biggest order ever for this type of urban bus technology. The 60-foot (18-meter) articulated buses utilize the EPSystem parallel hybrid powertrain by GM’s Allison Transmission and are powered by Caterpillar C9 ACERT engines. They will be delivered in 2004 and 2005 by New Flyer Industries of Winnipeg, Manitoba.

The King County Metropolitan Transit Authority placed orders for 213 hybrid buses, and Sound Transit, which runs regional express buses in the King and neighboring counties, will buy 22 hybrids. According to King Metro Transit, hybrid buses cost about $645,000 each, compared to $445,000 for a standard diesel bus. The combined investment is over $150 million, or $47 million more than it would cost to buy standard diesels.

The bus operators expect to recover the cost difference by savings in operating costs over the bus’ estimated 12-year service life. The hybrids are expected to be 20-30% more fuel efficient that a standard diesel bus. The King Metro Transit also hopes to spend less on maintenance, due to less frequent oil change interval.

The hybrid buses are equipped with diesel particulate filters, bringing about 90% emission reduction of PM, as well as HC and CO. NOx is reduced by about 50% due to the use of a smaller engine and advanced engine control.

The hybrid buses will replace a fleet of dual-mode buses that run normally on diesel, but switch to overhead electric wires while passing through a 1.3-mile underground downtown bus tunnel.

Parallel hybrid systems use two sources of power to move the vehicle—diesel engine and battery-powered electric motors. The battery is charged by the engine-generator combination. The engine is also coupled to a drive unit which passes mechanical energy to the wheels. This allows to use a smaller engine in a hybrid system. When the bus accelerates from a stop, the batteries supply ample power for the acceleration, allowing for optimization of engine performance and exhaust emissions. A parallel hybrid bus uses the diesel engine’s mechanical power to maintain speed after the vehicle is underway. In this mode the engine also charges the battery.

In contrast, in a series hybrid system, the bus is always powered by electrical motors. The diesel engine is used exclusively to drive the generator to charge batteries. The braking energy is also recovered from the wheels and used to charge the batteries, thus improving the fuel efficiency.

Parallel hybrids are more complex (and more expensive), but more versatile. For instance, parallel hybrid powertrains may be used in highway trucks, while the application of series hybrids is practically limited to stop-and-go applications, including urban buses and delivery trucks. A previous Allison design generation, the ESSystem, was a series hybrid.

Seattle is the second US city, after New York, to invest in hybrids for urban transit. New York City has been operating a pilot fleet of 10 hybrid buses, with orders placed for 125 hybrids to be delivered starting this December and 200 more in 2005. The New York City buses are supplied by Orion Bus Industries, with series hybrid powertrains by BAE Systems (former Lockheed Martin Control Systems).

GM’s Allison has hybrid pilot bus programs in Austin and Houston, TX; Hartford, CT; Minneapolis, MN; Newark, NJ; Orange County, CA; Philadelphia, PA; Portland, OR; and Salt Lake City, UT.