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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.

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.

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.