Other cities—notably Seattle—have found similar, or even worse, performance on fuel consumption with the GM-Allison hybrids, but the findings on PM emissions is a surprise.

The ultra fine particle data were surprising because we expected significant reductions for the hybrid technology. That proved not to be the case.

—Britt Holmén, UConn professor of civil and environmental engineering and lead researcher

In June 2003, CTTransit, which oversees bus service in Connecticut and is operated by the state Department of Transportation (DOT), purchased two of the GM-Allison parallel hybrids. The Connecticut Department of Transportation and the Joint Highway Research Advisory Council retained UConn’s School of Engineering to test the particle emissions of the hybrids as well as two conventional diesel buses manufactured about the same time that have the same emissions standards and, to compare the results.

The conventional diesel buses, manufactured in 2002, used a standard 280-hp Detroit Diesel Series 40 engine. The GM hybrids, manufactured in 2003, used the 280-hp Cummins ISL engine combined with the Allison E^p 40 electric drive, NIMH battery pack and dual electric traction motors.

The researchers used three fuel and aftertreatment configurations: No. 1 Diesel; ULSD; and ULSD plus a DPF (diesel particulate filter). UConn researchers tested all four buses between January and November 2004 on actual roadways, as opposed to using a dynamometer.

To my knowledge, this is the first study to evaluate hybrid bus particulate emissions while the buses were operating on actual bus routes. We found that the only testing variable that reduced particulate emissions was use of a diesel particulate filter, and these reductions of more than 90 percent were similar for both the hybrid and conventional diesel bus types. Without the particulate filter, the particulate emissions from the hybrid buses were not statistically different from the particulate emissions of the diesel buses, regardless of fuel sulfur content.

The good news is that our measurements show that diesel particulate filters can remove even toxic nanoparticles—those with a diameter of less than 50 nanometers—from engine exhaust on both the diesels and the hybrids. My hope is that the hybrid parallel-design technology can be improved to give better fuel economy while maintaining the low-noise and smooth ride performance of these buses.

I suspect the hybrid manufacturer had these parallel-drive hybrid buses programmed for optimal performance rather than optimal emissions benefits because they were capable of freeway commuter-express route speeds of 65 mph and the hybrid bus speeds were not affected by travel over Avon Mountain. These parallel design hybrids are unlike the series-drive hybrids that have been extensively studied previously and are in-service in cities like New York. We hope to test a series-drive hybrid bus in the future using the same experimental methods to test this hypothesis.

—Prof. Holmén

The parallel hybrid drive in the transit bus is the basis for development of the two-mode hybrid drive being developed by GM, DaimlerChrysler and now BMW for applications in light duty vehicles.

Resources:

• On-Board Diesel & Hybrid Diesel Electric Transit Bus PM Mass and Size-Resolved Number Emissions (AAAR 04 Conference)

• Ultrafine and Nanoparticle ELPI Number Distribution Emissions from Hybrid-Electric and Conventional Diesel Buses Using Ultra-low Sulfur Diesel (AAAR 04 Conference)