Battery electric, fuel cells, hybrid or hydrogen-fueled internal combustion engines — which is the best for buses? Information to make this decision is coming from the many applications of these technologies now underway. When it comes to advanced technologies to replace diesel power, mass transit is leading the way.

Hybrid Electric Buses

Hybrids gain most of their improved fuel economy by shutting down the engine rather than idling and through regenerative braking. Thus, hybrids provide the greatest fuel savings when the duty cycle includes frequent starts and stops interspersed by short travel distances — like the duty cycle for urban transit buses and shuttle buses. Indeed, many of the today’s hybrid cars and SUVs have higher EPA city mileage ratings than highway mpgs.

Hybrid electric buses (HEBs) have moved from the demonstration to commercial production stage. There are about 700 hybrid buses in regular service in North America with more than 40 transit agencies. Another 400 deliveries are planned in 2006. The New York City Transit (NYCT) is operating 325 Orion buses equipped with BAE Systems’ HybriDrive diesel-electric propulsion system. On the other side of the country, King County Transit in Seattle is operating 214 60-foot-long New Flyer articulated buses powered by GM’s Allison Electric Drive system. Other agencies are operating, or have ordered, HEBs including those with ISE Corp.’s ThunderVolt hybrid drive systems.

The ThunderVolt system is interesting because it uses a Ford V-10 gasoline engine showing that gasoline engines could again compete with diesel engines. With the boost from the electric motor, a gasoline hybrid can supply diesel-like torque needed in a transit bus, while offering the fuel economy of a diesel engine with very low emissions. Ultracapacitors rather than batteries are used in more than 80 buses integrated by ISE, offering high power rates, light weight and reasonable cost. According to ISE, high-power ultracapacitors last longer than typical batteries. Ultracapacitors sometimes have had trouble sustaining charge during hill climbing, limiting their applicability on routes with steep grades.

Recently, the FTA published an Analysis of Electric Drive Technologies For Transit Applications: Battery-Electric, Hybrid-Electric and Fuel Cells. It provides a good overview of the current status of electric drive technologies for transit applications. The study, by the Northeast Advanced Vehicle Consortium, concludes that pure battery buses are not viable for most transit applications, and are seen as niche vehicles. Likewise, fuel cell buses are a long-range solution, 10 to 20 years away.

Thus, the report focuses on HEBs and cautions that while hybrids have demonstrated millions of miles of real-world service, they are still an immature technology relative to diesel and CNG buses. Since there is no lifetime data on durability, long-term system and subsystem reliability, and lifetime costs are still sketchy. Most of the report’s results are based on experiences with the 40-foot BAE Systems-Orion buses, 40- and 60-foot GM Allison buses (with New Flyer or MCI chassis) and the 40-foot ISE-Nova buses.

HEBs are attractive for transit agencies looking for clean buses because they use the existing fueling infrastructure. Hybrids, unlike CNG buses, do not require major infrastructure modifications. They may require lifts or cranes for battery packs and safety equipment for working with high voltage electrical systems. Lead-acid batteries do require reconditioning, while nickel metal hydride (NiMH) batteries do not. NYCT reconditions lead-acid batteries once every six months. It takes 20 hours and the charging units cost around $50,000. The relatively lower infrastructure investment results in many transit operators putting five or fewer hybrid buses in their fleets for evaluation before making a large-scale commitment, something that is less attractive with CNG.