Fuel Economy and Cost
NYCT buses use Jet A diesel fuel, which is designated as aircraft fuel. NYCT and other transit bus operators in the area use Jet A diesel fuel because of its availability in the city. This fuel designation is slightly higher grade than diesel #1. As mentioned earlier, NYCT is using ultra low sulfur diesel fuel at less than 30 ppm sulfur content for its Jet A diesel fuel. This sulfur level is expected to be less than 15 ppm by 2006.
CNG fuel is provided at West Farms by a compression station operated and serviced by Trillium USA. The CNG study fleet fuel consumption and economy is shown in Table 9 and Figure 15. The fuel economy for the CNG buses is shown in diesel gallon equivalent units based on an energy conversion of CNG to diesel. The CNG study group has a 25% lower fuel economy than the non-EGR diesel buses. Based on the duty cycle at West Farms (average speed between 6.3 mph and 6.5 mph), this lower fuel economy is within typical expectations based on previous studies4. The low average speed of the operation is the key to this significantly lower fuel economy. Figure 15 shows the difference between the CNG and diesel study group fuel economy is staying generally the same over time.
The one diesel bus that was removed from the evaluation (bus 416) is listed in the table with a fuel economy of 2.04 mpg. This bus had a new DDC Series 50 EGR engine installed in place of the original DDC Series 50 engine instead of an in-chassis rebuild. This vehicle showed a fuel economy that was consistently 10% lower than the rest of the diesel baseline study group at West Farms Depot.
The hybrid study fleet fuel consumption and economy is shown in Table 10 and Figure 16. The fuel economy for the hybrid buses is 45% higher than the non-EGR diesel buses. This higher fuel economy for the hybrid buses is expected; however, the data period is only eight months. The last four months of the 12-month evaluation period are summer operation in New York, which requires significant load for air conditioning, and the heat may inhibit brake regeneration at times. As shown for the CNG and diesel baseline groups, the hybrid buses and the baseline diesel group have similarly shaped fuel economy curves. The hybrid bus fuel economy has fluctuated between 32% and 52% higher than for the diesel buses in the same time frame at Mother Clara Hale Depot.
Using this data between the two study depots, the hybrid buses have fluctuated between 80% and 120% higher fuel economy than the CNG buses based on diesel equivalent units.
During the evaluation period, diesel fuel at NYCT was an average of $1.70 per gallon for ultra low sulfur diesel fuel with sulfur less than 30 ppm. The diesel fuel cost has gone up significantly since the end of the evaluation period for this report (currently $2.26 per gallon in September 2005). This difference in average diesel fuel cost will be accounted for in the final results report for this evaluation at NYCT.
4 For example, DART’s LNG Bus Fleet, Final Results, 2000, NREL, NREL/BR-540-28739 (www.nrel.gov/vehiclesandfuels/fleettest/pdfs/28739.pdf) and Alternative Fuel Transit Buses, Final Results,”, 1996, NREL, NREL/TP-425-20513 (www.eere.energy.gov/afdc/pdfs/transbus.pdf)