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 electric range. Travel beyond the 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, 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