King County in Washington State, which includes Seattle, has announced plans to purchase 235 diesel electric hybrid buses, the largest ever order for this kind of urban bus technology.

The 60-foot articulated vehicles use the E System Parallel hybrid powertrain developed by General Motors's Allison Transmission, and are powered by Caterpillar C9 ACERT engines.

They will be supplied in 2004 and 2005 by New Flyer Industries, which is based in Winnipeg, Manitoba.

The hybrids are expected to be 20-30 per cent more fuel-efficient than a standard bus, and operators are likely to recover the cost of buying them over their 12-year service life.

Diesel particulate filters will reduce PM emission by about 90 per cent, while the vehicles' smaller engines are expected to reduce NOx by about 50 per cent.

The hybrids will replace the current fleet of dual-mode buses that usually run on diesel, but switch to overhead electric wires when travelling through a 1.3 mile underground tunnel.

Seattle will be the second American city, after New York, to invest in hybrids for urban transit.