hybrid electric bus
A hybrid electric bus is a bus that combines a conventional internal combustion engine propulsion system with an electric propulsion system. These type of buses normally use a Diesel-electric powertrain and are also known as hybrid Diesel-electric buses.
The introduction of hybrid electric vehicles and other green vehicles for purposes of public transport forms a part of sustainable transport schemes.

Powertrain
Types of hybrid vehicle drivetrain
A hybrid electric bus may have either a parallel powertrain (e.g., Volvo B5LH) or a series powertrain (e.g., some versions of the Alexander Dennis Enviro400 MMC).

Plug-in hybrid
A plug-in hybrid school bus effort began in 2003 in Raleigh, NC, when Advanced Energy began working between districts across the country and manufacturers to understand the needs of both. The effort demonstrated both a technical and business feasibility and as a result was able to secure funding in 2005 from NASEO to purchase up to 20 buses. The resulting RFP from Advanced Energy was won by IC Bus using a product jointly produced with Enova for a 22-mile plug-in hybrid product with a $140k premium over existing buses. The buses performed well in testing with 70% reductions in fuel usage although only in specific conditions.
The United States Department of Energy (USDOE) announced the selection of Navistar Corporation for a cost-shared award of up to $10 million to develop, test, and deploy plug-in hybrid electric (PHEV) school buses. The project aims to deploy 60 vehicles for a three-year period in school bus fleets across the nation. The vehicles will be capable of running in either electric-only or hybrid modes and will be recharged from a standard electrical outlet. Because electricity will be their primary fuel, they will consume less petroleum than standard vehicles. To develop the PHEV school bus, Navistar will examine a range of hybrid architectures and evaluate advanced energy storage devices, with the goal of developing a vehicle with a 40-mile (64 km) electric range. Travel beyond the 40-mile (64 km) range will be facilitated by a clean Diesel engine capable of running on renewable fuels. The DOE funding will cover up to half of the project's cost and will be provided over three years, subject to annual appropriations.

Tribrid Bus
Tribrid buses have been developed by the University of Glamorgan in Wales. They are powered by hydrogen fuel or solar cells, batteries and ultracapacitors.

Air pollution and greenhouse gas emissions
A report prepared by Purdue University suggests introducing more hybrid Diesel-electric buses and a fuel containing 20% biodiesel (BD20) would further reduce greenhouse emissions and petroleum consumption.

Manufacturers
Current manufacturers of Diesel-electric hybrid buses include Alexander Dennis, Azure Dynamics Corporation, Ebus, Eletra (Brazil), New Flyer Industries, Tata (India), Gillig, Motor Coach Industries, Orion Bus Industries, North American Bus Industries, Daimler AG's Mitsubishi Fuso, MAN,  Designline, BAE Systems, Volvo Buses, VDL Bus & Coach,  Wrightbus, Castrosua, Tata Hispano and many more.
Toyota claims to have started with the Coaster Hybrid Bus in 1997 on the Japanese market. Since 1999, Hybrid electric buses with gas turbine generators have been developed by several manufacturers in the US and New Zealand, with the most successful design being the buses made by Designline of New Zealand. The first model went into commercial service in Christchurch since 1999, and later models were sold for daily service in Auckland, Hong Kong, Newcastle upon Tyne, and Tokyo. The Whispering Wheel bus is another HEV using in-wheel motors. It was tested in winter 2003–04 in Apeldoorn in the Netherlands.In Japan, Mitsubishi Fuso have developed a diesel engine hybrid bus using lithium batteries in 2002, and this model has since seen limited service in several Japanese cities. The Blue Ribbon City Hybrid bus was presented by Hino, a Toyota affiliate, in January 2005.
For the North American transit bus market, New Flyer Industries, Gillig, North American Bus Industries, and Nova Bus produce hybrid electric buses using components from either BAE Systems (series hybrid, initially branded HybriDrive and now branded Series-E), or Allison Transmission (parallel/series hybrid, branded Hybrid EP or H 40/50 EP). In May 2003 General Motors started to tour with hybrid electric buses developed together with Allison. General Electric introduced its hybrid electric gear shifters on the market in 2005. Several hundreds of those buses have entered into daily operation in the U.S. In 2006, Nova Bus, which had previously marketed the RTS hybrid before that model was discontinued, added a Diesel-electric hybrid option for its LFS series. 
In the United Kingdom, Wrightbus has introduced a development of the London "Double-Decker", a new interpretation of the traditional red buses that are a feature of the extreme traffic density in London. The Wright Pulsar Gemini HEV bus uses a small Diesel engine with electric storage through a lithium ion battery pack. The use of a 1.9-litre Diesel instead of the typical 7.0-litre engine in a traditional bus demonstrates the possible advantages of serial hybrids in extremely traffic-dense environments. Based on a London test cycle, a reduction in CO2 emissions of 31% and fuel savings in the range of 40% have been demonstrated, compared with a "Euro-4" compliant bus.

Former hybrid bus manufacturers
ISE Corporation ThunderVolt (filed for bankruptcy in 2010)
Azure Dynamics (filed for bankruptcy in 2012)

Conversions
Hybrid Electric Vehicle Technologies (HEVT) makes conversions of new and used vehicles  (aftermarket and retrofit conversions), from combustion buses  and conventional hybrid electric buses into plug-in buses.

List of transit authorities using hybrid electric buses
Transit authorities that use hybrid electric buses:

North America
United States
Federal funding generally comes from the federal Diesel Emissions Reduction Act.

ABQ RIDE (Albuquerque, New Mexico)
Ann Arbor Area Transportation Authority (AAATA) (Ann Arbor, Michigan)
Autoridad Metropolitana de Autobuses (San Juan, Puerto Rico)
Baltimore, Maryland
Bee-Line Bus System (Westchester County, New York)
Berks Area Reading Transportation Authority (Berks County, Pennsylvania)
Bloomington Transit (Bloomington, Indiana)
Broome County Transit (Broome County, New York)
Broward County Transit (Broward County, Florida)
Capital Area Transportation Authority (Lansing, Michigan)
Capital District Transportation Authority (Albany, New York)
Central New York Regional Transportation Authority (Syracuse, New York)
Charlotte Area Transit System (Charlotte, North Carolina)
Chatham Area Transit (Savannah, Georgia)
Chicago Transit Authority
Citibus (Lubbock, Texas)
Central Ohio Transit Authority (Columbus, Ohio)
Clarksville Transit System (CTS) (Clarksville, Tennessee)
Community Transit (Snohomish County, Washington)
C-Tran (Vancouver, Washington)
Citilink (Fort Wayne, Indiana)
Cache Valley Transit District (Logan, Utah)
DART First State (Delaware)
Durham Area Transit Authority (Durham, North Carolina)
Eureka Transit Service (Eureka, California)
GoRaleigh (formerly Capital Area Transit) (Raleigh, North Carolina)
Greater Lafayette Public Transportation Corporation (Lafayette, IN and West Lafayette, IN)
Greater Lynchburg Transit Company (Lynchburg, VA)
Greenville Area Transit (Greenville, North Carolina)
Hillsborough Area Regional Transit (Hillsborough County, Florida)
Howard Transit, (Howard County, Maryland)
IndyGo (Indianapolis, Indiana)
Jacksonville Transportation Authority
Kanawha Valley Regional Transportation Authority
Kansas City Area Transportation Authority
King County Metro Transit Authority (Seattle, Washington)
Lane Transit District (Lane County, Oregon)
Long Beach Transit (Long Beach, California)
LACMTA (Los Angeles, California)
LANta (Lehigh Valley, Pennsylvania)
Madison Metro Transit (Wisconsin)
Manatee County Area Transit (Manatee County, Florida)
Massachusetts Bay Transportation Authority (Boston, MA)
MATA (Memphis, Tennessee)
MATBUS – Metro Area Transit (Fargo, ND – Moorhead, MN)
Metropolitan Transit Authority of Harris County, Texas (Houston, Texas)
Minneapolis-Saint Paul Metro Transit
MTA Maryland (Baltimore, Maryland)
Nashville Metropolitan Transit Authority
New York City Transit Authority
Niagara Frontier Transportation Authority (Buffalo, New York)
North County Transit District (North San Diego County, California)
Orange County Transportation Authority (Orange County, California)
Pioneer Valley Transit Authority (Springfield, Massachusetts)
Port Authority of Allegheny County (Pittsburgh, Pennsylvania)
Regional Transportation Commission of Southern Nevada/Citizens Area Transit (Las Vegas, Nevada)
Rhode Island Public Transit Authority (Providence, Rhode Island). 1 gas and 1 diesel for testing use only; diesel was converted gas was hybrid from factory.
Roaring Fork Transportation Authority (Aspen, Colorado)
San Diego Metropolitan Transit System/San Diego Transit (San Diego, California)
San Francisco MUNI (San Francisco, California)
San Joaquin Regional Transit District (Stockton, California)
Santa Clara Valley Transportation Authority - VTA (Santa Clara County, California)
Santa Rosa CityBus (Santa Rosa, California)
Sarasota County Area Transit (Sarasota County, Florida)
Sound Transit (Puget Sound region, Washington)
Southeastern Pennsylvania Transportation Authority (Philadelphia, Pennsylvania)
Southwest Ohio Regional Transit Authority (Cincinnati, Ohio)
Spokane Transit Authority (Spokane, Washington)
TCAT (Ithaca, NY)
TheBus (Honolulu, Hawaii)
The Rapid The Interurban Transit Partnership Grand Rapids, Michigan *Has 5 vehicles used in fixed route service.
TriMet (Portland, Oregon): two vehicles
University of Michigan parking and transportation services (Ann Arbor, Michigan)
Utah Transit Authority (Salt Lake City, Utah)
Washington Metropolitan Area Transit Authority

Canada
Transit Windsor (Windsor, Ontario)
Edmonton Transit System (Edmonton, Alberta)
Hamilton Street Railway (Hamilton, Ontario)
OC Transpo (Ottawa, Ontario)
RTC (Quebec City, Quebec)
RTL (Longueuil, Quebec)
Saskatoon Transit, Saskatchewan
STL (Laval, Quebec)
STL (Lévis, Quebec)
STM (Montreal, Quebec)
STO (Gatineau, Quebec)
STS (Sherbrooke, Quebec)
St. Catharines Transit Commission (St. Catharines, Ontario)
Toronto Transit Commission buses [673 out of 2137 regular buses are hybrid as of 2020]
Coast Mountain Bus Company (Vancouver, British Columbia)
BC Transit  (Kelowna and Victoria).
GRT (Waterloo, Ontario) [currently 6 out of 218 buses in service are hybrid]
London Transit Commission (London, Ontario)
Strathcona County Transit (Strathcona County, Alberta) [as of 2014, 10 Nova Bus LFS HEV diesel-electric hybrid buses remain in service]
Halifax Transit, Halifax, Nova Scotia. Currently owns two hybrid-electric buses.
Lethbridge Transit, Lethbridge, Alberta. 11 out of 42 buses are hybrid.

Asia
China
Beijing Public Transport
Kunming Bus
Shenzhen Bus Group
Shenzhen Eastern Bus
Shenzhen Western Bus
Jinan Bus
Zhengzhou Bus Group

Hong Kong
Citybus
New World First Bus
Kowloon Motor Bus

India
Delhi Multi-Module Transit
Mumbai BEST CNG-Hybrid

Iran
Vehicle, Fuel and Environment Research Institute (VFERI)

Pakistan
TransP
esh
Greenline, Karachi
awar

Japan
Marunouchi Shuttleetc.

Philippines
Green Frog Hybrid Buses

Singapore
SBS Transit
SMRT Buses
Tower Transit Singapore

Thailand
BMTA

Europe
Belarus
Minsk
Slutsk

Germany
Dresden
Hagen
Lübeck
Munich
Nuremberg

Hungary
Budapest – The fleet consists of 28 Volvo 7900A Hybrid (articulated).
Kecskemét – The fleet consists of 20 Mercedes-Benz Citaro G BlueTec®-Hybrid (articulated).

Norway
Nettbuss, Hamar
Ruter, Oslo
Nettbuss, Trondheim
Nettbuss, Arendal
Nobina, Tromsø
Vestviken Kollektivtrafikk, Vestfold. Scania Citywide.

Romania
STB, Bucharest – The fleet consists of 130 Mercedes-Benz Citaro Hybrid.

UK
The Green Bus Fund is a fund which is supporting bus companies and local authorities in the UK to help them buy new electric buses.
London Buses, London. This is the largest fleet in the UK, with around 2,300 vehicles in use.
National Express West Midlands, Birmingham – 18 currently, 21 more planned
Stagecoach, Manchester, Oxford, Sheffield, Newcastle
Oxford Bus Company, Oxfordshire – 52 currently
FirstGroup, Bath, Somerset, Bristol, Manchester Metroshuttle, Leeds, Essex
Reading Buses
Lothian Buses
Cumfybus, Merseyside
Brighton & Hove
Stagecoach East Scotland, Aberdeen
Arriva Yorkshire, from April 2013

Spain
Barcelona (MAN Lion's City Hybrid)
Empresa Municipal de Transportes, Madrid
Figueres, within the electric bus Project, IDAE

Sweden
Jönköpings Länstrafik, Jönköping. MAN Lion's City Hybrid.
Göteborgs Spårvägar, Gothenburg. Volvo 7700 Hybrid.
Storstockholms Lokaltrafik, Stockholm. MAN Lion's City Hybrid.

Other European countries
Ljubljanski potniški promet (5 Kutsenits Hydra City II/III Hybrid's), Ljubljana, Slovenia
Paris: RATP is using a hybrid electric bus outfitted with ultracapacitors; the model used is the MAN Lion's City Hybrid.
Milan, Italy
Team Trafikk, Trondheim, Norway, with 10 Volvo B5L
Vienna, Austria
PostAuto, Switzerland: one vehicle is being tested since April 2010; the test will continue for three years.
Warsaw, Poland, 4 Solaris hybrid (combustion-electric) buses
Luxemburg (Sales-Lentz, Emile Weber and AVL)
Belgium / Flanders (De Lijn)
Belgium / Wallonia (TEC): 90 Volvo 7900H (plug-in hybrid) + 208 solaris (combustion-electric) ordered in 2016Q4

Other countries
Egypt IMUT: http://www.i-mut.net/en/about-us.

Buenos Aires, Argentina
Christchurch, New Zealand
Curitiba, Brazil
Mexico City, Mexico (Metrobús Line 4)
Bogotá, Colombia

See also
References
External links
The Plug-in Hybrid Electric School Bus Project Media related to Hybrid-powered buses at Wikimedia Commons