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Cleaner Bus Fleets in New York City
The Department of Transport's bus system carries over 114 million people annually through a mix of local and express bus services provided by seven private, franchised bus companies.

With its more than seven million inhabitants New York City is one of the largest metropolises in North America. The following information on its bus system is taken from the official New York City Department of Transport website:

The Department of Transport's bus system carries over 114 million people annually through a mix of local and express bus services provided by seven private, franchised bus companies. This system has a fleet of over 1,280 buses, making it the 9th largest transit bus fleet in the United States and Canada. There are a total of 82 local and express routes that operate in Queens, Brooklyn, the Bronx, and Manhattan.

Although seven private bus operators provide the service, the City of New York owns most of the buses, subsidizes the cost of operations, and covers the general financial risks of providing bus transportation. The City also owns two bus depots.

The City of New York began subsidizing capital purchases for the private bus companies in 1974. In 1986, the Department of Transportation created a Surface Transit office to monitor the quality of franchised bus service and to manage the City, State and Federal subsidies. In 2000, the subsidies totaled US$ 151,861,090, while fare revenue was US$ 107,034,591.


The Metropolitan Transportation Authority operates North America's largest transportation networks and it comprises five agencies, one of which is New York City Transit. In the year 2000 it started a Clean Fuel Bus Program, which is envisaged to give New York the world's cleanest bus fleet.

Its goals are to ( 1 ):

  • 1. Reduce Bus Fleet Emissions: Achieve levels below current U.S. mandates
  • 2. Improve Service: Improve equipment reliability; Achieve quieter operation
  • 3. Reduce the Cost of Operations: Improve fuel economy; Reduce maintenance costs; Avoid infrastructure costs

The program is technology neutral, and combines several different approaches that are expected to achieve worthwhile results ( 1 ): thus CNG buses are used as well as hybrid buses, and clean diesel technologies.


The program is executed by the MTA agency New York City Transit, with the support, including financial, of the city government. The governor also directed the Department of Environmental Conservation to work with MTA and the environmental community to develop new emissions performance standards for all MTA buses to ensure they meet or exceed those achieved by Compressed Natural Gas (CNG) buses ( w1 ). The MTA is a public-benefit corporation chartered by New York State.

The program is designed to give cost-effective emissions reductions as quickly as possible. In order to achieve this, the Metropolitan Transportation Authority (MTA) 2000 - 2004 Capital Spending Plan includes US$ 304 million for Clean Fuel Programs ( 1 ).

In a press release featured by the EPA ( w1 ) it is stated that a revised and expanded MTA Capital Plan is to include $250 million for the enhanced Clean Fuel Bus program. The plan provides for the purchase of 550 new clean fuel buses, requires conversion of all existing diesel buses to clean technology and calls for construction of a new heavy-duty testing laboratory to fully test all developing clean-fuel technologies for future implementation.


The program is an initiative of the local government in an explicit attempt to set an example of environmental standards. It takes a mostly technological approach, aiming to replace or retrofit the existing diesel bus fleet with cleaner technologies. The initiative goes voluntarily beyond mandatory emission control standards.


The program is "technology neutral" ( 1 ), i.e. not fixed upon using one particular technology. Instead, various technological approaches are taken which promise to be effective, and these are also compared against each other. Three different technologies are being used ( 1 ):

  • 1. Expand CNG bus operations: Purchase 300 buses and convert 2 depots to CNG
  • 2. Expand hybrid bus programs: Purchase 250 hybrid buses; Develop hybrid articulated and coach buses
  • 3. Expand the use of clean diesel technologies: Retire all 2-stroke diesel engines by 2003; Convert entire fleet to reduced sulfur fuel; Retrofit 3,500 buses with catalyzed exhaust filters

In all diesel bus depots low sulfur diesel with less than 30-ppm sulfur will be introduced. Given this change in diesel fuel, all diesel buses in the fleet will be equipped with some type of after-treatment technology such as CRTs, Catalysts, Urea Injection, or other devices, no later than December 31, 2003. Furthermore, it is planned to purchase 300 additional CNG buses and 250 additional hybrid-electric buses.

CNG Buses

Regarding the CNG buses, the following costs were found to arise ( 1 ):

Item Cost
Maintenance: US$ 0.20/mile more than diesel buses
Fuel: US$ 0.16/mile more than diesel
  • US$ 5 million/depot for fuel station (30 bus/hr capacity)
  • US$ 10-40 million/depot for safety modifications
  • Significantly higher costs for constrained urban sites, especially for multistory depot buildings

The following lessons have been learned with regard to CNG ( 1 ):

  • 1. CNG buses work - they can be used to successfully provide passenger service
  • 2. CNG buses are only 50-75 % as reliable as comparable diesel buses
  • 3. CNG buses are 41% less energy efficient than diesel buses in urban service
  • 4. CNG buses are significantly more expensive to operate than diesel buses

One reason for the high CNG fuel costs in New York is the very high costs associated with the limited space available for refueling facilities there and the fact that the existing maintenance facilities need extensive modification to accommodate CNG buses. Other transit systems have experienced much lower costs.

Hybrid Buses

Hybrid Bus Lessons Learned - Operational ( 1 ):

  • 1. Bus operators and passengers like hybrids quiet, smooth operation; excelent acceleration/smooth braking; "feels" like a standard bus; little or no operator training required
  • 2. Able to be used on all NYCT routes
  • 3. Bus does not roll back on hills
  • 4. Performance can be customized

Hybrid Bus Lessons Learned - Technical ( 1 ):

  • 1. Battery equalization and periodic battery "conditioning" are both required
  • 2. Programming must deliver a stable control system
  • 3. Some early component failures - required redesign
  • 4. Catalytic exhaust filter durability to be determined; key to emissions performance
  • 5. "Cleaner" small diesel engines are needed, with hybrid-specific engine programming
Cleaner Diesel Technologies

The CRTTM particulate filters used in the NYCT program require low sulfur fuel for their operation. Therefore two factors have to be considered in an assessment of the environmental effect: the factor of cleaner fuel and that of the filter itself. The following information is taken from ( 2 ):

  • Fuel effects: going from baseline low sulfur diesel (LSD) to ultra low sulfur diesel (ULSD) on the Central Business District (CBD) Cycle resulted in: 76% average reduction in THC (total hydrocarbons), 29% average reduction in CO, and9% average reduction in PM
  • CRT effects: on the CBD cycle, reduction in average emissions compared to baseline fuel & catalyst muffler was:
    - 92% for THC,
    - 94% for CO, and
    - 88% for PM

(Emissions reductions on the NY Bus Cycle with the CRT filter were even higher than on CBD: 93 - 98% reduction in THC, CO, and PM.)
With regard to CO2 and NOx, neither the cleaner fuel nor the filter led to significant emission reductions, the values being in some cases slightly lower, in others slightly higher than for the baseline scenario.

Comparison between diesel technologies and CNG

The results for diesel buses with CRT particulate filters were also compared to the results for CNG buses equipped with oxidation catalysts:

  • PM emissions appear to be similar in both technologies;
  • CO and HC emissions from CRT-equipped buses are much lower than those from CNG buses;
  • Carbonyl emissions (aldehydes and ketones) from CRT-equipped buses are much lower than from CNG buses;
  • NOx emissions are generally higher in CRT-equipped buses than in CNG buses but show a wider range of variability.
    For more information see also the interim results of emission testing ( 2 ).
See Also
Programs and experiences - Cleaner Bus Fleets in New York City
General topics
Cleaner vehicles - Buses
Alternative fuels and energy sources - Compressed Natural Gas (CNG)
Alternative fuels and energy sources - Hydrogen and fuel cells

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