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Environmental concerns and limited fossil fuels reserves have fostered an increased interest in alternative propulsion systems. In this scenario, electric traction, with its inherent zero local emissions, high efficiency and improved operational performance (acceleration and hill climbing potential), emerges as a desired option for public transport systems. Transit buses, the prevailing transport system in cities, and, hence, strong contributors to traffic environmental impact on urban areas, can reduce considerably their environment burden with the use of electric traction. This means less local pollutants, specially particulate matter - PM and nitrogen oxides - NOx, currently the “Achilles heel” of diesel engines, as well as CO2 greenhouse emissions - GHG.

Emissions from motor vehicles have been a subject of concern in urban areas, as great amounts of population have been permanently exposed to large amounts of pollutants, with intrinsic adverse health effects. In this context, in the last two decades, stringent emissions standards have been developed to control the maximum emission limits of the so called regulated pollutants. This continuous reduction of emission targets has imposed a great effort to engine and vehicle manufacturer in the development of technological solutions for emission limits compliance, which can be done by reducing engine-out emissions through improvements in combustion process and fuel management system, as well as by using aftertreatment devices in the exhaust system.

The increasing demand for urban mobility, combined with the constriction of investment capacity of transit authorities and private companies make bus based systems a great option for public transport systems, since they allow the provision of high quality services at a fraction of the costs of rail based systems. In this scenario, Bus Transit System - BTS and Bus Rapid Transit - BRT allow the implementation of transport networks at considerably lower costs than their rail system counterparts. This is specially true to developing-nation cities, that have infrastructure costs as a pre-eminent decision-making factor in technology selection. From an environmental perspective, traction technology and fuel option are decisive to define systems' performance.

Intensive use of fossil fuels in densely populated areas has caused adverse environmental effects in cities all over the world. This has fostered the evaluation of alternative technologies for transit applications, like hydrogen fuel cells - electrochemical energy conversion devices that operate with zero emission, quieter and with higher efficiencies than internal combustion engines, specially at part load regimes. Transit bus market is particularly well suited to technology innovations because they are i) centrally fueled and maintained, ii) professionally operated on fixed routes and schedules, iii) tolerate weight and volume requirement of new technologies and, finally, whenever necessary, iv) can be subsidized by government. In this scenario, considerable research, development and testing effort has been dedicated to hydrogen fuel cell bus technology, with the engagement of governments and transit authorities, bus industry and operators.

The growing concentration of population in world metropolis caused by increasing urbanization rates has pushed the demand for high capacity and efficient public transport systems. At the same time, environmental concerns have led to increasingly stricter emission standards. In this context, transit authorities have become strongly focused on making their bus fleets more efficient and cleaner, by incorporating new alternative fuels and clean propulsion technologies. This has led to increased interest in electric driven technologies, with their intrinsic efficient, quiet and environment friendly features. Trolleybuses, a well proven mature electric technology already adopted in some cities, although efficient and clean, are burdened by high infrastructure costs and operational inflexibility.

From the nineties there was a great interest in the use of compressed natural gas - CNG (predominantly composed of methane) on transit bus fleets around the globe. In a first moment, developed countries (US, EU and Japan) have focused their efforts to address serious urban air pollution problems caused by heavy duty diesel engines - since PM and NOx emissions were initially easier to control from natural gas engines than from conventional diesel engines - and also to offset growing oil imports. As such, for many years, dedicated methane fuelled city buses meeting emission requirements (Euro IV, V and EEV, US Federal and California, and Japan) either in a lean burn or stoichiometric technology, have been offered to the market.

Current massive urbanization process concentrates high amount of population and impose an increased demand on transport systems. In this context, transit bus system plays an important role, as the most dynamic and less capital intensive transit option available. At the same time, it is strongly dependant on fossil fuels, predominantly diesel fuel, with its intrinsic polluting and greenhouse (climate change) effects. This has boosted research and investments for alternative and renewable fuels. One solution currently receiving widespread recognition is biogas use in transit bus fleets, as it allows the use of a renewable fuel, made from substrates derived basically from waste and sewage that otherwise would produce methane released to the atmosphere.

Compression Ignition - CI or Diesel engines are currently considered the most fuel efficient combustion based drivetrain, and, for this reason, it has been historically used as the backbone for heavy duty markets, including transit bus fleets. At the same time, CI engines fueled by traditional crude oil based diesel fuel are facing the growing challenge of meeting the increasing stringent emission standards, specially on particulates matter, nitrogen oxides and greenhouse gases emissions limits. Moreover, petroleum based transport fuels are constantly faced by strategic and security concerns, due to the concentration of the main currently known reserves in political unstable regions. As such, it is both environmentally and economically important to find alternatives for crude oil based diesel fuel to be used in the transportation sector.