In the 80s, due to the oil crisis, natural gas (NG) appeared as a fuel with great potential for replacing diesel in heavy duty vehicles. At that time, Petrobras, together with other companies, had developed partial conversion kit technology for diesel to diesel plus NG, known as “dual fuel” technology. Engine dynamometer and vehicle bus tests have been developed to verify the technical and economic viability of the “dual fuel” kit. For several reasons, the dual fuel program did not advance and the trials were interrupted. At the same time, other trials, using NG Otto cycle bus engines, manufactured in Brazil, were being conducted, mainly in São Paulo, although unsuccessfully. Currently, factors such as, an increase of the NG converted light vehicle fleet; an excess of NG on the national market, expansion of the NG distribution network; tighter pollutant emission limits; government interest in increasing NG's energy matrix share and in reducing the consumption of diesel, together make a great scenario for expanding the use of NG as a substitute for diesel fuel in heavy duty vehicles.