Technology▼Solutions Buses for the future
COURTESY OF BAE SYSTEMS
Systems, the company that makes the drivetrains for the Orion bus. Fuel-cell buses have the potential The United States is leading the world gent, the adage “if you can make it to be even less polluting, but they are in bringing hybrid buses into comthere, you’ll make it anywhere” apyears away from being commercially mercial service, thanks largely to big plies, Lynch says. For that reason, viable, Lynch says. The environmental purchases by the cities of New York many transit companies copy NYC performance of hybrids is “pretty and Seattle. Hundreds of diesel–elecTransit’s purchasing decisions, she comparable” to buses running on tric buses are slated to begin carrying says. compressed natural gas (CNG), Lynch fare-paying passengers this year, and At least a dozen other U.S. cities says. For a city with existing CNG terthey are likely to inspire additional are already testing or investing in hyminals, CNG buses are less expensive, purchases by many other municipalibrid buses, and they are being emshe says. But hybrid buses are less exties around the country. pensive if the city needs to But the jury remains out build a CNG terminal, she about which hybrid design adds. The U.S. Transporis best. tation Research Board New York City (NYC) is estimates that building a well on its way to amassfacility for fueling 100– ing the world’s largest fleet 200 vehicles costs $6−10 of hybrid electric buses, million. observers agree. NYC One of the reasons that Transit began testing hyNYC Transit opted to embrids in 1998, and the brace hybrid buses in such agency evaluated 26 buses a big way was to avoid from various manufacturhaving to build more CNG ers before deciding on the terminals, although the Orion VII model, which is city is still purchasing CNG built by a division of buses, Lowell says. New York City is amassing the world’s largest fleet of hybrid–electric DaimlerChrysler. The first However, CNG buses buses. This Orion model features a serial architecture that can be of 325 new 40-foot buses may not remain competiupgraded to run with a fuel cell. began arriving in NYC late tive with hybrids, claims last year. braced in Brazil, Chile, Japan, Mexico, Larry Stone of BAE Systems. As hyOn the opposite coast in Seattle, and New Zealand, according to the brids become more common and the Wash., the King County Metro Transit experts interviewed for this article. price drops, the fact that hybrids are and Sound Transit authorities ordered Although European countries have much more fuel-efficient than CNG 235 hybrid buses from New Flyer, a also investigated hybrids, the Eurobuses will become a factor, he says: Canadian bus manufacturer, last Ocpean Union is now focusing mainly “Our test data…show miles-pertober. “We needed a bus that would on fuel-cell buses, through two Euroequivalent-gallon of 5.3 for hybrid, take us in a different direction,” says pean Commission programs, CUTE 3.5 for diesel, and 3.1 for CNG.” Ron Sims, King County executive. The (Clean Urban Transport for Europe) The appeal of both hybrid and extra-long 60-foot articulated buses, and ECTOS (Ecological City Transport CNG buses is likely to be greatest in which have two connected sections System), which together have ¤21 cities and areas unable to comply that allow passengers to move million in funding. with the Clean Air Act and therefore throughout the bus, are expected to Compared to standard diesel busat risk of losing federal highway begin arriving in late spring. es, hybrids reduce emissions of nitrofunds, Lynch says. Because of the curThese 560 new hybrid buses are gen oxides by 50–60%, carbon dioxide rent fiscal crisis at the municipal and significant because U.S. transit auby 20–30%, and particulates by 30%, state levels, transit authorities are thorities regularly buy only 5000 new says Dana Lowell, NYC Transit’s assisputting off optional purchases, she buses each year, says Sheila Lynch, tant chief maintenance officer. With a explains. executive director of the Northeast diesel particulate filter, the particulate While it is true that “everything Alternative Vehicle Coalition. They are emissions drop to more like 90% beabout our decision making was drivalso expected to fuel more hybrid low standard diesel buses, adds Tom en by clean air concerns,” NYC Tranpurchases, she says. Because NYC’s Webb, product marketing manager sit’s purchase was not motivated by reliability tests are notoriously strinfor the transit bus division of BAE federal funding rules, Lowell says. 16A ■ ENVIRONMENTAL SCIENCE & TECHNOLOGY / JANUARY 1, 2004
© 2004 American Chemical Society
Although the agency does receive federal funds, it uses local funds to buy buses, he explains. The main reason that hybrid buses are less polluting is that they have an electric motor, which either partially or fully drives their wheels. All hybrid buses also have a second source of onboard energy, usually a conventional automotive or truck engine that runs on diesel fuel. In the case of the Orion VII, it is a 5.9-liter Cummins engine similar to the one used in a Dodge RAM pickup. The diesel engine inside the New Flyer buses earmarked for Seattle is the 8.8-liter C 9 made by the tractor manufacturer Caterpillar. Most hybrid buses also feature a regenerative braking system that uses the drive motor to slow the bus and convert the kinetic energy of the vehicle into electricity to charge the onboard energy storage system. A few manufacturers also offer diesel–electric buses with no regenerative braking, but many experts do not consider these to be true hybrids. The main difference between today’s hybrid buses is in the design of the drivetrains that propel them. The Orion buses use a series architecture for their drivetrain that BAE Systems, its creator, claims is ideal for buses because of its ability to perform well at low speeds. The hybrid bus drivetrains made by Eletra of Brazil also use a series architecture, as do the ones from ISE Research Corp. of San Diego, Calif. The series architecture features two large motorsone acts as a generator, and the second is the electric traction motor that actually turns the wheels, says Peter Savagian, director of hybrid power transmission systems for GM’s Power Train division, which makes drivetrains for New Flyer hybrid–electric buses. The series architecture converts mechanical energy to electric energy and then back to mechanical energy, he explains. “In a series architecture, the power flows along a single path from engine to wheels,” Webb explains. Because the diesel engine is totally disconnected from the wheels, the power control system can operate the engine at the best possible fuel efficiency and emissions point, no matter what the vehicle’s speed, he explains. Alternatively, some buses use a parallel architecture, which is much more like a conventional automotive
drivetrain, Savagian says. The motor that serves as a generator also drives the wheels through mechanical gears and shafts, and a small electric motor sits between the engine and transmission. The parallel architecture is more efficient overall—while the “path efficiency” of a series architecture might be 68%, the parallel path is more like 85% efficient, he explains. This allows parallel hybrids to use smaller engines, Savagian says. Among the bus manufacturers featuring parallel architectures is Hino of Japan. “A parallel architecture would probably be more appropriate for an over-the-road truck,” says Lowell, who stresses that he does not think that any of the architectures in question are preferable for city buses. General Motor’s new hybrid GMC Sierra truck, which is a passenger vehicle, uses the architecture. However, most transit buses make frequent stops and spend their time traveling at low speedsthe national average is 12 miles per hour—at which series drivetrains are most efficient, says Webb. Moreover, the series architecture allows the bus to accelerate smoothly, regardless of vehicle speed, and has the potential to be more reliable and require less maintenance because it does not use clutches and brakes, he says. New Flyer offers buses that have series drivetrains, but the buses that Seattle purchased from the company feature a “parallel/series hybrid” designed by GM’s Power Train division. These hybrid drivetrains combine the best of the serial and parallel architectures, Savagian claims. They are configured to run in both series and parallel modes. For this reason, they are more efficient than a pure series architecture, but they take advantage of the smoothness of the series architecture at low speeds and during gear shifts, Savagian explains. On the other hand, a major advantage of the series architecture is that it can be upgraded to accommodate fuel-cell busesthe internal combustion engine in the bus can simply be replaced by a fuel-cell stack, Lynch says. BAE has built demonstration fuel-cell buses using this approach, Webb says. Neither the parallel nor parallel/series architecture can work directly with fuel cells, Savagian notes. Another key difference between bus manufacturers is in the design of their onboard energy storage systems.
The New Flyer buses that Seattle will use have high-tech nickel–metal– hydride batteries, while the Orion buses use conventional lead–acid batteries. Webb acknowledges that the energy density of the nickel–metal– hydride batteries is higher, but he says his company chose to stick with the tried-and-true lead–acid batteries because they’re proven to work in buses and cost less to install and replace. The lead–acid batteries are also widely and readily recyclable, he says. Hybrid buses may always cost more than conventional diesel buses because the hybrids contain extra parts, according to a report on future bus systems by the International Energy Agency (IEA). These parts include the motor and large battery packs. Battery packs currently need to be replaced several times over the operating life of a hybrid bus, which IEA estimates adds $20,000–50,000 to the lifetime costs of operating the bus. Because of the high purchase cost, hybrid buses made in developed countries appear unlikely to make inroads in developing countries in the near future—particularly countries like Indonesia that pay much less for fuel, says Lewis Fulton, a senior transportation analyst with IEA. But the situation could be different for hybrids that are designed and manufactured in the developing world. Brazil’s Eletra buses, which are produced in cooperation with Volvo and Marcopolo, are being sold for U.S.$150,000, the IEA reports. This is less than half of what a hybrid bus designed to do the same job would cost in the United States. The World Bank is testing Eletra buses in Mexico City and Santiago, Chile, with funding from the Global Environment Facility, says Paul Procee, an environmental specialist with the World Bank. Indeed, the standard-size Orion hybrid buses used in NYC cost $385,000, which Lowell says is $100,000 more than a standard diesel bus. The 60-foot New Flyer buses purchased for Seattle cost $645,000. Lowell expects the NYC buses to use 30% less fuel than standard buses, which he estimates is likely to result in a fuel savings of $1800 per year. “There is no economic payback— the payback is in the reduction of emissions,” Lowell says. That, he acknowledges, is hard to quantify, but very important to the citizens of his city. KELLYN BETTS
JANUARY 1, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 17A
