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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.

Aviation industry currently accounts for almost 3% of worldwide greenhouse gas (GHG) emissions. Despite the continuous efforts to reduce this environmental footprint, with the use of technological efficiency driven solutions and operational changes to reduce climatic effects, such as engine improvements, fleet renewals and navigation operational improvements, the industry, which is permanently challenged by the continuously stringent standards, is aware of the need of additional measures to tackle, and even reduce, the GHG emissions, by decoupling the world's industry average growth (almost 4.1% annually) to the aviation's carbon emissions. Given its inherent operational features, the aviation sector requires fuels with high specific energy and energy density. This technical requirement makes the well known clean and efficient electrical propulsion technology to be limited to niche aviation segments (short range and low capacity airplanes) in the short and medium terms.