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

Abstract

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.

Integrating battery degradation in a cost of ownership framework for hybrid electric vehicle design optimization

“…(All values and calculations are in nominal terms). The conventional diesel bus was simulated in Autonomie to obtain the corresponding fuel consumption as 87L=100km(2:7mpg), which is close to the experimental fuel economy obtained on Orion V buses on New York City Transit bus routes by Barnitt, 56 Barnitt and Chandler 57 and Chandler et al 58,59 Results…”

Comparison of Parallel and Series Hybrid Power Trains for Transit Bus Applications

“…A wide range of experimental results have indicated that hybrid power train technologies increase fuel savings for city transit buses (7)(8)(9)(10)(11)(12)(13), as summarized in Table 1. The tested hybrids included both series and parallel buses, and the data were measured for both on-road city driving and chassis dynamometer testing conditions.…”

“…Having two power sources makes it possible to keep the electric motor and battery relatively small, thus reducing costs (5). Although favorable results have been reported in the literature for fuel consumption and emissions of hybrid transit buses (6)(7)(8)(9)(10)(11)(12)(13), many issues related to the combined fuel efficiency and emissions control of series and parallel hybrid power train configurations, particularly when integrated with emerging aftertreatment systems required to meet new emissions regulations, remain unresolved. This is especially true when the effects of different urban drive cycles are considered (14).…”

Highway vehicle electric drive in the United States : 2009 status and issues.

“…For urban buses using a much more complex two-electric machine hybrid powertrain, a well-designed comparative analysis by Walcowitz (2006) indicated dynamometer test cycle savings of 43% at 7 mph "Manhattan" conditions, which dropped to 20% at 23 mph with air conditioning on. Field tests of diesel hybrid versus conventional diesel buses for Manhattan conditions, at about 6 mph where air conditioning was included (and was clearly very important), had fuel consumption reductions varying from 22-28% (Barnitt, 2008). For field tests in Seattle at speeds about twice as high, the reduction was 20% (Walcowitz, 2006).…”