Going head to head: NREL compares hybrids and diesels in study using Seattle's King County Metro transit fleet

Diesel Progress North American Edition, April, 2007 by Bill Siuru

Hybrid electric buses have long been touted as being cleaner and more fuel efficient than conventional diesel buses. Now a study conducted by the U.S. Dept. of Energy's NREL provides some qualitative data to support that belief, as it evaluated similar diesel and hybrid articulated buses operating under similar duty cycles.

King County Metro Transit (KC Metro) in Seattle, Wash., operates 235 hybrid electric articulated buses in its fleet of 1400 standard and articulated buses, trolley buses and streetcars that service the 2134 sq.mi. Seattle/King County region. This is the largest use to date of the GM Allison EP50 parallel hybrid propulsion system, and 20 more hybrids are currently being added to the KC Metro fleet.

The diesel-hybrid articulated buses are identical to the conventional diesel versions except for the Allison EP50 hybrid system. All of the 60 ft. New Flyer buses are equipped with Caterpillar C9 engines rated 330 hp with 1150 lb.ft. of torque, and all are fitted with diesel particulate filters.

The GM Allison EP50 system incorporates a drive unit with two electric motors and replaces the standard transmission. Each motor provides 75 kW continuous power and up to 150 kW peak output and can also act as a generator during braking, storing energy in nickel metal hydride (NiMH) batteries.

The hybrid buses also have a silent mode to minimize noise and engine emissions while operating in Seattle's 1.3 mile downtown transit tunnel. The EP50 system's Hush mode allows the vehicle to operate in an electric-only state in the station areas of the tunnel and in a reduced engine power mode in between stations.

The hybrid buses replaced a fleet of 236 Breda dual-mode buses that were in service for 14 years, two longer than expected. The 60 ft. articulated buses had two operating modes, a diesel engine for operating outside the tunnel and electric propulsion powered by a catenary system while in the tunnel. Fifty-nine of the Breda buses were refurbished as electric-only trolleys and returned to service on a catenary system.

The NREL evaluation was conducted over a 12-month period from April 2005 through March 2006 and compared with 10 buses of each type. The duty cycles on downtown service routes were similar. The average speed was 12.4 mph for the diesels and 11.6 mph for the hybrids for the first six months and 12.3 and 10.5 mph, respectively, during the second six months. During the second six months, the tunnel was closed to install a light rail system and all buses used surface streets in downtown Seattle. During the latter period the Hush feature was deactivated.

The study found that the hybrid buses demonstrated an average of 27% better fuel economy than their diesel counterparts. The differential ranged from 24 to 30% during the evaluation period (a 0.67 mpg difference). The diesel buses used B5, a blend of 5% biodiesel and 95% ultra-low sulfur diesel, while the hybrid buses used only ULSD. The fuel cost and fuel economy combined resulted in a fuel cost per mile that was 22% lower for the hybrid buses, a $0.17 per mile difference.

Bus usage is one indicator of the reliability and availability of buses in service. The hybrid and diesel buses had similar cumulative average monthly mileage totals--3096 for the hybrids and 2948 miles per bus, respectively. The hybrid buses and the diesel buses had similar total maintenance costs per mile. The cost for hybrid buses was 4% less ($0.02 per mile difference) than the diesel buses. Considering both fuel and maintenance per mile, the total operating costs for the hybrid buses were 15% lower than those of the diesel buses ($0.19 per mile difference).

Another maintenance performance indicator is miles between road calls--that is a failure of an in-service bus that causes the bus to be replaced on its route or results in a significant schedule delay. The hybrid buses had a rate that was 13% less for miles between propulsion road calls. Propulsion-related items include transmissions, nonlighting electrical (charging, cranking and ignition systems), air intake, cooling, exhaust, fuel, engine, electric propulsion and hydraulics.

The NREL's ReFUEL facility conducted chassis dynamo-meter testing of one conventional and one hybrid bus. The hybrid showed reductions in N[O.sub.x] emissions from 17.8 to 38.7% depending on the driving cycle used. Four cycles--Central Business District (CBD), Orange County (OCTA), Manhattan (MAN), and a custom test cycle developed from in-use data of the King County Metro (KCM) fleet operation were used.

For more details on the NREL study and evaluations, see the following websites:

http://www.nrel.gov/vehiclesandfuels/fleettest/pdfs/40585.pdf

http://www.nrel.gov/docs/fy06osti/39996.pdf

Bill Siuru, PhD, PE, is a Diesel Progress field editor based in Temecula, Calif.