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NREL Evaluating Fuel and Emissions Performance of GM Parallel Hybrid Buses

The DOE’s National Renewable Energy Laboratory (NREL) is wrapping up a 12-month performance and evaluation test of the GM/Allison hybrid buses in service in the King County (Seattle) Metro (KCM) Fleet.

Preliminary findings, previewed at the SAE Hybrid Vehicles Technology Symposium 2006, indicate that the hybrids deliver decreased fuel consumption of 20%–43% depending upon duty cycle, a 10%–39% reduction in NOx emissions; and a 51%–97% reduction in PM emissions.

The study involved the on-road evaluation of 20 of the 235 KCM hybrid buses in service and 10 of 30 non-hybrid version, as well as extensive chassis dynamometer testing in the NREL labs.

The results, which will be presented at an American Public Transportation Association (APTA) meeting in May 2006, are in contrast to earlier reports in the Seattle Post Intelligencer about the hybrids falling short of a promised 40% improvement in fuel economy. (Earlier post.)

The hybrids in question are 60-foot articulated buses equipped with the GM/Allison EP 50 two-mode hybrid system—the precursor of the hybrid system currently under development by GM, DaimlerChrysler and BMW.

The buses, by request of King County Metro (KCM) were up-powered with Caterpillar C9 8.8-liter diesel engines.

Dynamometer testing. NREL ran a set of seven cycle (driving pattern) tests on the buses, including custom KCM cycles, the Manhattan cycle, the Orange County Transit Authority (OCTA) cycle, and multiple sets of a more generic Central Business District (CBD) Cycle. All involve heavy stop and go driving at varying speeds, different rates of acceleration, number of stops and distance.

Not surprisingly, the hybrids deliver the greatest benefit in lower-speed, sharp stop-and-go driving (best represented by the Manhattan cycle). Going upgrade and supporting air conditioning can cut the benefit sharply.

On the emissions side, the PM results were particularly impressive.

Chassis Dynamometer Test Results
Cycle Average Speed Hybrid Reduction in Fuel Consumption (g/mile) NOx Reduction (g/mile) PM Reduction (g/mile)
Manhattan 6.8 mph 42.9% 38.7% 92.6%
OCTA 12.3 mph 33.7% 28.6% 50.8%
CBD 12.6 mph 32.8% 26.6% 97.1%
CBD with Air Cond. 12.6 mph 26.7% 17.0% 97.1%
KCM 23.4 mph 23.4% 17.8%
KCM w/o Grade 23.4 mph 24.2% 16.9%
KCM w/o Grade w/AC 23.4 mph 20.2% 10.1%

On-road testing. For the on-road component, NREL is analyzing maintenance and refueling records for a 12-month period (April 2005–March 2006). The team is evaluating fuel economy, warranty claims, mileage accumulation and service calls.

They also evaluated buses from different transit depots (which have different drive cycles.)

On the fuel economy side, NREL is finding approximately a 26% improvement with the hybrids (3.19 mpg vs. 2.54 mpg) when comparing comparable duty cycles. On the maintenance side, the hybrids appear to go 28% longer between road service calls.

NREL is also wrapping up a comparable study of the series-hybrids in use by New York Metro.

In summary, the findings seem to refute assertions that the GM hybrids delivered “underwhelming” fuel economy—worse than the buses the hybrids replaced—although the benefits tended to be below the 40% that apparently had been the target. (GM now describes its hybrid buses as providing significant—and un-quantified—fuel economy benefits.)

The findings also indicate the importance of the nature of the duty cycle (and, presumably, driver training) in shaping the benefits ultimately provided by the hybrids.



Robert Schwartz

I had thought that King County was not happy with the buses and that NYC had DQed GM.


According to NREL, it was a combination of things--as in the actual driving cycle and the expectations. But King County Metro supported the NREL study. It will be very interesting to see the full report when it comes out, and also to compare it to the upcoming evaluation of the series hybrids that are in NY.

As an aside, ISE Corp was also at the Symposium--they're the ones that make gasoline, diesel and fuel cell serial hybrids. According to the presenter from ISE, they consider that they achieve comparable fuel economy savings to the parallel hybrid--which they say is about 30%.

They have different arguments for why buyers should go with them. :-)


I imagine that a series hybrid could make a lot of sense for inner-city stop-and-go duty. Big batteries and a small constant speed genset that runs at optimal load or not at all.


Did they do any emissions testing using biodiesel?


Yes, the Orion series buses being implimented in NY make way more sense. They have a smaller diesel, and a larger motor. Once they can be fitted with a long life Lithium Ion battery, I expect they will provide the 40% improvement in MPG in stop and go routes. Plus the benefit of no cloud of foul smelling smoke as the bus pulls away from the bus stop or school.

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