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Environmental concerns related to intensive fossil fuel use have pursued the development of programs to promote the production and use of renewable fuels. This is specially true for diesel fuel used in transit bus fleets, in which the intensive and concentrated use in densely populated areas generates adverse environmental effects in cities all over the world. Although improvements in fossil fuel quality have been achieved in the last decades, most have to be done to make breath cleaner in world metropolis. One of the most promising options, known as Biodiesel, is an ester produced from vegetable oil or animal fat, in a chemical reaction with methanol or ethanol, through a process known as transesterification. It contains, on average, 8% and 12.5% less energy than diesel fuel in a volume and in a weight basis, respectively, which affects engine power, in accordance with the blend used.

Stricter environmental regulation and the increasing concern about fuel economy and emissions have driven transit agencies and operators toward environmental and economic concerns when selecting transit bus technology. In this scenario, hybrid bus, that combines two or more distinct propulsion systems (generally combustion engine and electric motor), has been seen as a choice that balances both the need for better environmental and efficiency performance and capital expenditures for introducing new technology-based transit bus fleets. The source of better performance of hybrid buses is the ability to i) optimize the operating point of combustion engine to achieve best fuel economy; ii) store energy generated during braking at storage devices (batteries, supercapacitors or flywheels), to be used to power the vehicle when needed, and, hence, iii) downsizing engine due to reduced average power requirements.