Alternative fuel vehicles

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Public transit is often called upon as a measure to reduce environmental impacts of travel, both by consolidating travelers from single-occupant vehicles into one environmentally-efficient vehicle, and by using modern technology for cleaner propulsion. The American Public Transportation Association (APTA) estimated that by 2011, about 35% of the transit fleet in America was using alternative fuels or hybrid technologies [1]. Many technologies have been adapted for bus and rail transit, including electricity and battery, natural gas, and hydrogen.

Propulsion Technologies

Standard and Bio-fuels: Gasoline and Diesel

The most common fuels for all vehicles in the U.S. include unleaded gasoline, and diesel. Federal regulations attempting to reduce the impact of these fossil fuels on the environment have mandated supply of ultra-low sulfur diesel and the use of ethanol (also known as E85) in gasoline[2]. Biodiesel fuel blends can typically be used in any modern diesel engine, making an attractive opportunity for agencies to use alternative fuels while avoiding the high cost associated with other technologies such as hybrid-drive buses. However, in a 2011 report to Congress, the EPA warned that increased production of biomass, especially corn, to blend with fuel and decrease dependence on fossil fuels may not have overall positive effects on the environment [3]

Diesel Environmental Concerns

Although diesel engines are particularly efficient and one of the most common combustion-engine choices for buses and other commercial vehicles, they also cause significant harm to the environment in the form of particulate matter (PM) from engine exhaust. Research suggests that long-term exposure to diesel exhaust is linked to increases in asthma in children, exacerbation of allergies, and possibly premature death [4]. In response to research conducted by the California Air Resources Board (CARB) and others in the early 2000s, new regulations were placed into effect for diesel engines requiring fitting of diesel particulate filters (DPF). However, transit agencies are subject to different regulations than other buses and trucks[5], which went into effect earlier than the recent standards for retrofitting DPF to trucks operating in California [6].

Regulations pertaining to transit agencies (defined as "urban bus") are found in title 13 of the California Administrative Code (13 CCR § 2020 - 2023.4), provided here by CARB.

Engine Manufacturers

Practically all bus manufacturing firms offer diesel options, and cutaway buses are commonly available in either gasoline or diesel configurations. Cummins is an example of an engine manufacturer for transit buses that certifies their products for use with biodiesel fuel[7]

Natural Gases

Natural gas is used as a fuel in both liquid (LNG) and compressed-gas forms (CNG). (Explain engine differences). Santa Monica, California's Big Blue Bus includes a fleet of buses powered by LNG. Los Angeles County Metropolitan Transportation Authority (LACMTA, or Metro) operates the country's largest fleet of CNG buses.

Bus Manufacturers with Natural Gas Offerings

The Gillig Corporation introduced a CNG option for their buses in 2011. New Flyer and subsidiary NABI provide CNG vehicles.


Liquid Propane Gas (LPG) should not be confused with LNG, above.


Electric power for buses is one of the oldest propulsion technologies, adapted from electric streetcars. Buses powered by overhead wires are commonly called "trolley-buses" and still operate today in some cities such as Seattle, San Francisco, Dayton, Boston, and Philadelphia. Buses can also be powered by electric battery without external power such as overhead wires, but the range of these vehicles tends to be limited. San Francisco Municipal Transportation Agency (SFMTA)

The most common application of electric power for buses today is the hybrid-electric. SFMTA and Long Beach Transit operate fleets of hybrid-electric buses [8]. The Long Beach buses were purchased from New Flyer in 2005 for a published cost of $550,000 per vehicle[9][10]. Gillig and New Flyer both offer hybrid-electric bus options.

LACMTA, in partnership with the city and county of Los Angeles and the South Coast Air Quality Management District , form the Advanced Transit Vehicle Consortium (ATVC). Both LACMTA and Los Angeles Department of Transportation are testing all-electric buses supplied by manufacturer BYD.

Hydrogen Fuel Cell

Hydrogen fuel cells has been researched as a power source for buses using Federal funding [11]. AC Transit of California has participated in a hydrogen fuel cell bus testing program since 2000 using Van Hool buses and a power plant developed by UTC Power of Connecticut. In 2013, UTC Power was sold to ClearEdge Power, and the future of the fuel cell bus program is unknown[12]


  1. APTA. Press release, "More than 35% of U.S. Public Transit Buses Use Alternative Fuels or Hybrid Technology". 22 April 2013
  2. US EPA. Website: "Biofuels and the Environment: Basic Information". Accessed 15 April 2014.
  3. US EPA. "Biofuels and the Environment: First Triennial Report to Congress". December 2011.
  4. U.S. Environmental Protection Agency (EPA). (2002) Health assessment document for diesel engine exhaust. Prepared by the National Center for Environmental Assessment, Washington, DC, for the Office of Transportation and Air Quality; EPA/600/8-90/057F. Available from: National Technical Information Service, Springfield, VA; PB2002-107661, and
  5. CARB. "Fact Sheet: Fleet Rule for Transit Agencies Urban Bus Requirements".
  6. Transport Topics. "DPF Retrofits Growing Due to California Rule". 27 May 2013.
  7. Cummins. Website "Biodiesel Frequently Asked Questions". Accessed 15 April 2014.
  8. SFMTA. "MUNI Hybrid Buses". Accessed 12 May 2014.
  9. LBT. "EPower Fact Sheet". Accessed 12 May 2014.
  10. LBT. "Environmental Issues." Accessed 12 May 2014.
  12. Kane, Brad. Hartford Business Journal. "UTC Power sold to Oregon fuel cell firm". February 2013. Accessed 15 April 2014.