Comparison of NO_x emissions from hydrous ethanol and n-butanol predicted by an Otto cycle two-zone model using the Zeldovich reactions mechanism

摘要

Among the gases usually emitted by internal combustion engines, NO_x chemical species appear as some of the causes of great environmental impact and damage to human health, which shows the relevance of its study and quantification, as well as the constant search for the reduction of these emissions. The use of biofuels such as hydrous ethanol and n-butanol has the goal of reducing CO_2 emissions in comparison to fossil fuels. However, it has to be accomplished without increasing NO_x emissions. Analyzing combustion of these two fuels through a two-zone model for an Otto cycle engine, this work compared quantitatively the NO_x emissions with the Zeldovich reaction mechanism, which can predict the formation and consumption of these chemical species during the engine's combustion cycle, being thus known as thermal NO. Unlike what is common for the two-zone models whose chemical species concentration are calculated based on chemical equilibrium, the Zeldovich mechanism implies the nitrogen monoxide concentration to be calculated through chemical kinetics, ensuring results more consistent with the real phenomena. The controlled variables whose influence were investigated were the compression ratio, engine speed and fuel-air equivalence ratio for each test. Results showed that the optimum operating condition in terms of emission for both renewable fuels was a slightly rich mixture with a greater amount of residual gases from combustion. The compression ratio variation was considered not relevant to impact the level of emissions. The engine speed variation showed significant influence in nitrogen oxides emission, with higher emission rates at higher engine speeds. In addition, the results predicted that, in all cases considered, n-butanol NO_x emissions were lower than the ones from hydrous ethanol, which contributes to reinforce it as a renewable fuel option to be used.