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Heavy duty vehicle (HDV) segment, as an important source of emissions that strongly impact air quality and human health - especially in urban centers - has been continuously challenged by the increasingly stringent emission limits. The adoption of emission standards for the heavy duty industry was initially launched by the United States, followed by the European Union and Japan, and, subsequently, by other countries, like Australia, Brazil, China and India, among others, generally with a time lag. This continuous “cleaning” effort has led to the current rigorous emission limits - materialized by the so called U.S. EPA 2010 and Euro VI and their foreign variants - which have provided huge emissions reductions (HC, CO, NOx, PM and smoke and, more recently, CO2).

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.