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Study compares particulate emissions from hybrid and conventional buses

17 September 2005

A new study conducted by the University of Connecticut (UConn) found that particulate emissions from hybrid diesel-electric buses and from comparable conventional diesel buses were statistically identical. Thus, the hybrid-electric powertrain had no effect on particulate emissions, in terms of both PM mass and particle numbers.

The study tested two parallel hybrid buses utilizing the GM Allison Ep40 Electric Drive transmission coupled with a 280 hp Cummins ISL diesel engine. As a baseline case, the researchers tested two conventional diesel buses powered by the DDC Series 40 engine, which had the same 280 hp power rating and was certified to the same PM emission standard as the hybrids. All four vehicles were 40-foot low-floor buses on New Flyer chassis.

The hybrid buses were purchased in June 2003 by CTTransit, which oversees bus service in Connecticut and is operated by the state Department of Transportation (DOT). UConn’s School of Engineering was contracted by the Connecticut DOT and the Joint Highway Research Advisory Council to test and compare particle emissions from the hybrids to those from conventional buses.

In the first study of its kind, UConn researchers tested all four buses on actual roadways, as opposed to using dynamometer testing in the laboratory. The buses were tested between January and November 2004 during highway driving on I-91, city driving on Farmington Avenue in Hartford, and high-grade driving over Avon Mountain. The testing equipment was installed onboard: SMPS (by TSI) and ELPI (Dekati) analyzers for particle number measurements, sampling filters for PM mass, as well as the Horiba OBS-1300 analyzer for gas emissions, exhaust flow rate and other parameters. The exhaust gas sample was diluted using an ejector dilution system, using purified, conditioned air. The system was configured to measure number emissions of combined solid and volatile particles (i.e., no thermodenuder was used).

Tests were performed for both bus types in three fuel/aftertreatment configurations:

  1. No 1 diesel fuel/diesel oxidation catalyst (DOC)
  2. Ultra low sulfur diesel (ULSD)/DOC
  3. ULSD/diesel particulate filter (DPF)

In each of the configurations, the hybrid emissions—PM mass and numbers—were not significantly different from the conventional diesels. The only testing variable that reduced particulate emissions was use of the DPF, which produced emission reductions of more than 90% for both the hybrid and conventional diesel bus types. DPFs were also effective in removing nanoparticles, those with a diameter of below 50 nm, from engine exhaust on both the diesels and the hybrids.

These results show that the low PM emissions, often considered an advantage of hybrid buses, are attributed entirely to the use of particulate filters, rather than the hybrid powertrain itself. Today, most hybrid diesel-electric buses sold in North America are equipped with DPFs, while most conventional diesel buses are not. All new diesel bus engines in the USA and Canada are expected to be fitted with DPFs from 2007.

According to CTTransit data, fuel economy for the hybrids was only 10 to 15% better compared to conventional diesels, thus falling short of the 50% improvement claimed by the bus suppliers. This may be perhaps explained, among other factors, by the higher proportion of suburban driving in Connecticut. The fuel economy advantage in hybrid buses is linked to the braking energy recovery, which is most effective in urban stop-and-go duty cycle.

Source: University of Connecticut