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In the 80's, due to the oil crisis, natural gas (NG) began to be regarded as a fuel with great potential for replacing diesel in heavy duty vehicles. At that time, Petrobras, together with other companies, developed conversion kit technology for heavy duty diesel engines to run burning simultaneously diesel and NG, known as diesel gas. For several reasons, the trials were interrupted. In the late 90's and beginning of the 2000's, factors such as NG availability, expansion of NG distribution network; increase of NG converted light duty vehicle fleet, government interest in increasing NG's energy matrix share, all together made a scenario which stimulated new developments in diesel gas technology. The scenario changed in the last couple of years, with Brazilian NG demand reaching its offering, mainly due to thermoelectricity generation guarantee of supply. This makes the diesel gas technology more attractable than the NG dedicated technologies.

Over the last few years, a growth of the Brazilian light duty vehicle fleet converted to Natural Gas (NG) has been observed. This is mainly due to license tax reductions on NG vehicles; the increase of the NG distribution around the country and attractive price difference between NG and other fuels. The Brazilian CNG (Compressed Natural Gas) light-duty vehicle fleet has currently reached more than 1,4 million units, being the 2nd largest in the World. ANP (Brazilian National Petroleum Agency) published in 2002 Resolution no104, which defines the NG specification for automotive application. The IBAMA (Brazilian Institute for Environment and Natural Renewable Resources) published in 2002 Resolution no 291, which defines ways for the environmental certification of the NG conversion kits.

Brazil is known for its long experience on using alternative fuels, mainly ethanol for light duty vehicles. In 2002, it was released the Flexible fuel car that can run with gasohol (gasoline with 22% of ethanol), hydrated ethanol or any blend of these fuels. By the end of 2006, national production of these vehicles represented around 80% of the total. Brazil is also the second world fleet of Natural Gas Vehicles (NGV), with more than 1,4 million light duty converted vehicles. This paper describes the development of a computational thermodynamic model of compression, combustion and expansion processes of gasohol, ethanol and Natural Gas (NG) for the cylinder pressure curve prediction of a Flexible Fuel engine, working with a NG kit installed. The combustion process is modeled using a Wiebe function, which establishes the mass fraction of burned fuel. Convective heat transfer to cylinder walls is estimated with an empirical correlation for heat transfer coefficient determination.

The worldwide concerns and some countries stricter legislations regarding the CO2 emission of light duty vehicles are motivating new technologies adoption, such as hybrids and electric battery vehicles, and discussions about what fuel economy data comparison between different countries. International discussions were done about the need to reevaluate the existing standardized driving cycles due to large emission and fuel economy differences when compared to the real road values, leading to the creation of a new cycle called WLTC (Worldwide Harmonized Light Duty Vehicle Test Cycle). Light duty vehicle fuel economy tests are usually performed on a chassis dynamometer using standard driving cycles under controlled laboratory conditions. Each country regulation defines the standard cycles used for the fuel economy tests.

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.

This paper describes the development of a computational thermodynamic model of compression, combustion and expansion processes of gasohol, ethanol and Natural Gas (NG) for the cylinder pressure curve prediction of a Flexible Fuel engine, working with a NG kit installed. The combustion process is modeled using a Wiebe function. Equations for specific heat at constant pressure (Cp) were developed for each fuel for temperatures up to 4000 K. The model output generates the cylinder gas pressure and temperature, work output and heat release profiles as functions of crank angle, allowing studies of engine performance parameters in different working conditions for each fuel. The differences between the experimental and simulation results were lower than 4% for the maximum cylinder pressure value.