with significant fuel economy penalties that have not yet been reported. CNG propulsion has already achieved the 2007 emissions certification level for NOx at 1.2 g/bhp-hr and CNG engine manufacturers are working to meet the 2010 certification levels early. Both Cummins and Deere have reported the availability of 0.2 g/bhp-hr NOx levels in their heavy natural gas engines by 2006.
The development of diesel hybrid bus propulsion systems is exciting for people in the transit industry because the systems offer improved fuel economy during a time of fuel economy penalties for emissions control. These systems also offer the promise of a clean propulsion alternative to CNG. Many transit agencies are concerned about the cost of converting a facility to support CNG bus operations, and are therefore, much more comfortable sticking with diesel fueled vehicles.
One issue yet to be resolved for hybrid propulsion in transit is the lack of emissions reduction recognition by EPA. A hybrid propulsion system can significantly reduce the overall emissions of the vehicle simply by increasing the fuel economy. Currently, a diesel engine is certified as a stand-alone engine and not as part of a hybrid system. Therefore, there is no recognition of the emissions reduction of a hybrid propulsion system. The benefit is that the engine will be certified to the current emissions standard, and the bus will operate at a lower emissions level. The downside is that there is no way for the transit agency to get credit for the emissions reduction because of the hybrid propulsion system. The California Air Resources Board (CARB) has recognized this savings and provided hybrid bus propulsion systems with a 25% blanket reduction in emissions that can be used in the state implementation plan for emissions reductions. Currently, EPA does not recognize this benefit.
Additional benefits gained from CNG and hybrid propulsion systems in transit applications are reductions in petroleum use and increased fuel economy (for hybrid propulsion).
Project Design and Data Collection
AVTA evaluation projects focus on using a standardized process for data collection and analysis, communicating results clearly, and providing an accurate and complete evaluation. The evaluation in this report uses diesel baseline buses operating at the CNG and hybrid depots for bus usage, fuel economy, and roadcall rate comparisons, but not for maintenance costs. The much older diesel bus maintenance costs represent maintenance outside of the warranty period and both the CNG and hybrid buses have warranty costs expended by the manufacturers or NYCT during the evaluation period (as discussed later). This evaluation also uses the CNG buses as the baseline for the hybrid buses for all operations and maintenance activities.
All 40-foot buses at West Farms and Mother Clara Hale Depot were dispatched randomly on all routes. There were no restrictions on the CNG or hybrid Orion VII buses at the two depots in this evaluation report.
NYCT expects the new CNG buses to have mature diesel-like reliability and operating costs. The CNG buses do not have restrictions other than only operating from the two CNG depots. The hybrid buses were expected to be slightly less commercial than the CNG buses due to the lack of