Buses Go Green with Hybrid Electric Vehicle Technologies

Frequent starts and stops interspersed with short travel distances represent the typical duty cycle for transit buses in urban areas. It's an environment in which hybrid electric vehicle technologies provide the greatest fuel savings. Increasingly, the advanced technologies like hybrid drive that have achieved such a high profile in personal light-duty transportation are making meaningful inroads in mass transit.

Hybrid electric buses (HEBs) have now moved from demonstration to volume production. There are over 1200 HEBs in revenue service in North America with many more on order. Most are concentrated in Seattle, New York, and San Francisco with smaller numbers spread throughout the country. They are also in service in Europe and Asia.

New York City Transit (NYCT) is operating nearly 400 40-foot Orion VII buses equipped with BAE Systems' HybriDrive diesel-electric hybrid propulsion system. Five hundred more are on order. The HybriDrive system uses a downsized diesel engine operated at an optimal, essentially constant speed for best fuel economy. The 5.9 liter, 260-horsepower Cummins ISB engine is connected to a generator that produces electricity for its electric-drive motor and batteries. The electric motor drives the vehicle and acts as a generator to capture energy during regenerative braking. Lead-acid batteries supply additional power during acceleration and hill climbing, storing energy recovered during regenerative braking and idling. Other cities such as San Francisco and Toronto are also using, or will be using, HybriDrive-equipped buses soon.

On the other side of the country, King County Transit in Seattle is operating 235 60-foot New Flyer articulated buses powered by Allison's EP50 parallel hybrid-propulsion system, with more on order. The EP50 system's two electric motors are integrated into the automatic transmission. Each motor, which also operates as a generator, provides 75 kilowatts of continuous power and up to 150 kilowatts of peak power. This parallel hybrid system blends outputs from the 330-horsepower Caterpillar C9 diesel engine and the electric motors to provide propulsion power. Energy is stored in nickel-metal-hydride (NiMH) batteries charged by the diesel engine and regenerative braking. The two-mode hybrid system used in GM's 2008 Chevrolet Tahoe and GMC Yukon hybrid SUVs is a smaller version of this system. The KCT hybrid buses have a HUSH mode for reduced engine power and electric-only operation to minimize noise and engine emissions while operating in Seattle's 1.3-mile downtown transit tunnel.

These large NYCT and KCT fleets provide concrete evidence that hybrid transit buses save fuel and reduce pollution. The U.S. Department of Energy's National Renewable Energy Laboratory evaluated the hybrid buses against nearly identical diesel and compressed natural gas buses operating over a 12-month period on the same service routes. In New York City, the hybrid buses demonstrated an average improvement in fuel economy of 34 percent compared to standard diesel-powered buses. In contrast to CNG buses, the hybrids demonstrated fuel-economy improvements ranging from 60 percent to 120 percent, based on diesel gallon-equivalent units. Likewise, the KCT hybrid buses demonstrated significantly better fuel economy — on average 27 percent higher — with differences ranging from 24 to 30 percent during the evaluation period. As an added benefit, regenerative braking reduced brake maintenance significantly.

ISE Corp.'s ThunderVolt Hybrid Drive System has been installed in many different transit buses. The ThunderVolt system is very versatile since it can be used with a variety of gasoline and diesel engines as well as with hydrogen internal combustion engines (H2ICE) and fuel cells. This series hybrid system can use either batteries or ultracapacitors to store electrical energy.

Hybrid drive has also been retrofitted to remanufactured buses. For example, the City of Elk Grove, Calif., has 17 e-tran buses, remanufactured Gillig 40-foot transit buses. Here, the ThunderVolt system uses a standard 6.8 liter Ford Triton V-10 gasoline driving a Siemens ELFA 140 kilowatt continuous generator, a package that supplies electricity to two 85-kilowatt electric motors driving the rear wheels. Maxwell ultracapacitors are also used for storing and dispensing electrical energy.

The Regional Transportation Commission of Southern Nevada is buying 50 StreetCar Rapid Transit Vehicle (RTV) articulated buses for operation on new Bus Rapid Transit (BRT) corridors in downtown Las Vegas and to nearby Boulder City. These 62-foot StreetCar RTVs will be built by Wrightbus in Northern Ireland and powered by the ThunderVolt system. Here, a 330-horsepower Cummins ISL diesel engine is used with Siemens ELFA electric and hybrid-electric drive components, powered by advanced Cobasys NiMH batteries. Wrightbus has distinguished its efforts previously by converting one of London's famous double-decker buses to hybrid power.

Today, hybrid buses are about 60 to 80 percent more expensive than conventional diesel buses. KCT's hybrid articulated buses cost $645,000 each compared to $445,000 for a diesel version. Batteries account for part of the higher initial and future operating costs. The lead-acid battery pack used in NYCT's Orion VII buses has an initial cost of $25,000, but replacement costs are less than half, since only the batteries are replaced and not the packaging and componentry. NiMH battery packs like those used in GM Allison buses cost between $35,000 and $45,000. The cost of NiMH batteries will drop as more are produced, but high-priced materials like nickel present challenges on the road to making these affordable mass market products. Lead-acid batteries are not expected to drop in price because of their very mature technology.

The cost of hybrid electric buses at present is high compared to the cost of diesel or natural gas variants. Volume production of hybrid systems, still a maturing technology, will result in significant cost reductions. Helping here is the fact that hybrid systems can be used in heavy-duty truck and other applications. For example, BAE Systems' HybriDrive technology will be used to power the U.S. Army's Future Combat Systems' Manned Ground Vehicles family of eight ground combat vehicles. Also, while capital costs for HEBs are higher, this can be offset by lower operating costs, especially if diesel fuel prices increase substantially.