|The gasoline hybrids exhibited an average 4.3% lower fuel economy over the 24-month evaluation period than the conventional diesel buses. Click to enlarge.|
A two-year evaluation (July 2005 through June 2007) of gasoline-electric series hybrid buses in operation with Long Beach Transit (Long Beach, CA) found that on a volumetric basis, the average fuel economy for the gasoline hybrid buses is 3.35 mpg—4.3% lower than that of the conventionally-powered diesel buses also in service with Long Beach Transit.
The study by the Fleet Test and Evaluation Team at the National Renewable Energy Laboratory (NREL) concluded that the difference is likely due to the lower efficiency of a throttled, spark-ignited engine as well as the lower energy content of a gallon of gasoline versus a gallon of diesel. On an equivalent energy per volume basis, the hybrids had an 8.5% mpg increase.
During the evaluation period, gasoline at LBT cost an average of $2.49 per gallon and diesel cost an average of $2.29 per gallon. This lower fuel economy, combined with a higher fuel cost for gasoline, resulted in fuel costs per mile being $.74 per mile for the hybrids as compared to $.65 per mile for the diesels.
|Energy flow in the ThunderVolt hybrid drive. Click to enlarge.|
The hybrids were New Flyer low floor buses equipped with ISE Corporation’s ThunderVolt Hybrid Drive propulsion system (earlier post), which combines a Ford ULEV 6.8-liter V-10 gasoline engine with a 650V, 110 kW (nominal), 145 kW (peak) Siemens ELFA generator and motor system. Dual drive motors each deliver 85 kW (nominal), 150 kW (peak) of power and nominal torque of 220 Nm, peak torque 530 Nm. The LBT hybrids each used two roof-mounted 360V Maxwell ultracap enclosures for energy storage.
ISE says that the typical fuel economy for the gasoline ThunderVolt system can range between 3.5-4.5 mpg US.
The ThunderVolt gasoline hybrid system was certified by the California Air Resources Board to 0.6g/bHp-hr NOx and 3.7g CO, making it a very low emission 40-foot bus drive system. PM is not measured on gasoline systems. By contrast, the diesels in the study are certified to 4.0 g/bHp-hr NOx, 0.05 g/bHp-hr PM and 0.5 g/bHp-hr CO.
As expected with hybrids in general, brake system maintenance costs were dramatically lower for the hybrid propulsion systems with regenerative braking. The hybrids had brake system maintenance costs that were about 90% less than that of the diesel buses with no relines to date on the hybrids. Miles between road call (MRBC) for the hybrids was lower than for the diesels, indicating a lower level or reliability.
Overall, said the report, LBT has been happy with the performance of the original forty-seven 40-ft hybrid gasoline-electric buses over the first two years of service. The transit authority has taken delivery of 15 more in the third quarter of 2007 and has requested an additional 25 for 2008.
The evaluation team selected 10 vehicles from the hybrid group of 47 vehicles and 10 vehicles from the diesel group (MY2002) of 138 vehicles for analysis. The number of vehicles in the study was sufficient to provide some degree of statistical significance to the results obtained, according to NREL.
LBT operates 228 buses out of two facilities and averages almost 40,000 miles per bus annually for both facilities. The average speed of the 40-ft bus fleet is 13.8 miles per hour (mph) with an average of 8 stops per mile.