The bus operators reportedly like the new buses—especially the power of the hybrid buses. Mileage accumulation for the 260 CNG buses through May 2005 was more than 9.5 million miles with a general usage rate of 2,500 monthly miles per bus. The 125 hybrid buses accumulated more than 2.6 million miles through May 2005 with a general usage rate of 2,500 monthly miles per bus. Both the CNG and hybrid bus fleets experienced miles between roadcalls (MBRC) rates above NYCT’s required 4,000 MBRC (average 5,000 MBRC for CNG, 7,000 MBRC for hybrid).
The following results and discussion focus only on the selected evaluation operating depots and study bus groups.
Duty Cycle The general duty cycle for the CNG bus evaluation location (West Farms Depot) was an average speed of 6.5 mph for 2004 and 6.3 mph for 2005. The general duty cycle for the hybrid bus evaluation location (Mother Clara Hale Depot) was an average speed of 6.5 mph and 6.1 mph for 2004 and 2005 respectively. The average speeds are comparable between the two evaluation locations. The buses at the two depots were randomly dispatched on all standard bus routes.
Bus Use Bus use is intended as an indicator of reliability and availability for service. The lack of use may indicate downtime for maintenance or purposeful reduction of planned work for the buses. For the detailed study groups, the CNG buses had 15% higher bus use than the baseline diesel buses (CNG had 2,244 monthly miles, diesel had 1,952 monthly miles). The hybrid buses had essentially the same bus use compared to diesel at 3% higher for the hybrid buses (hybrid had 2,461 monthly miles, diesel had 2,385 monthly miles). When compared across depots, the CNG buses had a bus use similar to the hybrid buses, with the CNG buses having a 10% lower rate.
Fuel Economy The CNG buses’ average fuel economy was 25% lower than the diesel baseline buses. Average monthly fuel economy for the CNG and diesel baseline groups is shown in Figure ES-1. This fuel economy difference is typical for a low-average-speed operation for the spark-ignited natural gas engines. The hybrid buses’ average fuel economy was 45% higher than the diesel baseline buses (ranging from 32% to 52% better than the diesel baseline during the evaluation period), as shown in Figure ES-2.
The diesel baseline buses for the hybrid bus evaluation have diesel engines without exhaust gas circulation (EGR). The addition of EGR for emissions control would tend to lower the diesel baseline fuel economy. The eight-month evaluation period does not include summer months, which could have reduced the hybrid bus fuel economy advantage from air conditioning loading and the ability to collect regenerative braking energy into the batteries. The summer-month fuel economy information will be provided in the final results report on this evaluation. The hybrid buses had an average fuel economy 100% higher than the CNG buses.