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. Equations for specific heat at constant pressure varying with temperature, not available on literature, were developed for each fuel for temperatures over 4000 K.
The model output generates the cylinder gas pressure profiles as functions of crank angle for two different approaches. One, solving the differential equation system assuming a polytrophic process after the intake valves closure and before the combustion start and the other by solving the whole system since the intake valve closure. A Flexible fuel engine, 1.8 liter with NG kit, operating with gasohol, hydrated ethanol and NG on different conditions of speed and load, was used to validate the simulations. Results show that the general model is much more precise than the polytrophic simplified approach.