In-Use Performance Comparison of Hybrid Electric, CNG, and Diesel Buses at New York City Transit

The National Renewable Energy Laboratory (NREL) evaluated the performance of diesel, compressed natural gas (CNG), and hybrid electric (equipped with BAE Systems’ HybriDrive propulsion system) transit buses at New York City Transit (NYCT). CNG, Gen I and Gen II hybrid electric propulsion systems were compared on fuel economy, maintenance and operating costs per mile, and reliability. These comparisons are based upon comparable service years; the second year in service for CNG and Gen II hybrids, and the second and third years in service for the Gen I hybrids. Conventional diesel buses provided a baseline comparison for fuel economy. Both the CNG and hybrid propulsion systems are alternatives to standard diesel buses and allow for reductions in petroleum use and emissions (usually focused on reductions of particulate matter and oxides of nitrogen). The Gen I hybrid buses exhibited 88% and 37% higher fuel economy than CNG and conventional diesel buses, respectively. The average fuel economy for the Gen II hybrid buses was 5.9% lower than the Gen I hybrid buses. Fuel economy decreased for all bus groups during summer operation due to air conditioning load. However, the hybrids exhibited the most dramatic seasonal fluctuation. The average total maintenance cost per mile for the Gen II hybrid buses was 39% lower than the Gen I hybrid buses, while the CNG buses’ average was 5% higher than the Gen I hybrid buses. Total propulsion-related systems maintenance costs per mile were 55% lower for the Gen II hybrid buses than the Gen I hybrid buses, while the CNG buses were 5% lower than the Gen I hybrid buses. The Gen I hybrids experienced a 4.8% traction battery failure rate per year during evaluation year 1, and a 3.3% failure rate per year during evaluation year 2. BAE Systems utilizes lead-acid chemistry Hawker batteries. Hybrid buses are expected to have reduced brake reline frequency because they use regenerative braking. The Gen I hybrid buses accumulated more than two times the mileage of the CNG buses before requiring their first brake reline.