Numerical Simulation Study On The Combustion Process Fuelled With N-butanol/biodiesel Dual Fuel

Application of new combustion technologies and alternative fuels are effective means of development of clean and efficient engines. In this study, the n-butanol/biodiesel dual fuel combustion mode was investigated. The n-butanol was port-injection, the biodiesel was directly injected into the cylinder. To understand the combustion mechanism of the n-butanol/biodiesel dual fuel, the reduced mechanism of the dual fuel was developed. The A KIVA code, coupled with the reduced mechanism, was used to investigate the combustion process and the formation mechanism of harmful emissions.An experimental study has been carried out in a single-cylinder common-rail CI engine with soybean biodiesel and two biodiesel surrogates containing neat methyl decanoate and methyl decanoate/n-heptane blends. Tests has been conducted with various intake oxygen concentrations ranging from 21% to approximately 9% at intake temperatures of 25 °C and 50 °C. The results showed that the ignition delay and smoke emissions of neat methyl decanoate were closer to that of soybean biodiesel as compared with methyl decanoate/n-heptane blends.Between the two surrogates, the neat methyl decanoate is better suited one for CI engines.A skeletal combustion mechanism with 146 species and 652 reactions of methyl decanoate(MD) as a surrogate for biodiesel fuels was developed for compression ignition engine simulations. The skeletal mechanism of MD was derived by reducing the detailed mechanism based on an integrated reduction method that contains a directed relation graph, sensitivity analysis, and reaction path analysis. A reduced polycyclic aromatic hydrocarbon mechanism was merged into the skeletal combustion mechanism of MD to predict the soot emission. The skeletal mechanism was validated against the experimental data of ignition delays in a shock tube, as well as the mole fractions of the reactants and the intermediate species in a jet-stirred reactor. The skeletal mechanism maintains accuracy with its dramatically reduced size, compared with the detailed mechanism. The skeletal mechanism was coupled with the KIVA code for 3-D biodiesel combustion simulation. Compared with the soot measurements in an optical constant volume combustion chamber, the simulation results showed similar soot location and occurrence during the combustion. Engine simulations were conducted with various conditions. The predictions profiles of the pressure and the heat release rate for various conditions agreed well with the experimental data. The skeletal mechanism predicted the emissions, including CO, HC, NOx, and soot accurately.A reduced chemical kinetic mechanism of then-butanol/biodiesel blend was developed for dual fuel engine simulations. The reaction flow analysisreduction methodwas adopted to lump and remove the unimportant species and the related reactions. The reduced mechanism of n-butanol contains 71 species and 349 reactions. The reduced mechanism of n-butanol was merged into a reduced mechanism of biodiesel to construct a combined mechanism ofn-butanol/biodiesel with 171 species and 765 reactions. The combined mechanism was validated against the n-butanol experimental data including ignition delays in shock tubes and the mole fractions of species in a jet-stirred reactor. The n-butanol/biodiesel mechanism was further validated against the engine experiments fuelled with the n-butanol/biodiesel dual fuel under multiple operating conditions. The predicted pressure and the heat release rate profiles, as well as CO, HC, NOx, and soot emissions under various conditions agreed well with the experimental data.Numerical studies were conducted to understand the performance of an engine fuelled with n-butanol/biodiesel dual fuel. The ignition mechanism, combustion process, and the formation mechanism of harmful emissions under various biodiesel injection timings, blend ratios, and EGR rates, were investaged. The results showed that, the various parameters for combustion controlhad effect on the low temperature reaction process of the n-butanol/biodiesel dual fuel. Along with the delay of the biodiesel injection timing, the proportion of low temperature reaction of fuel was reduced, the proportion of mid- and high temperature pyrolysis reaction of fuel was increased, which made greatimprovement of the activity of reaction system. At early biodiesel injection timing, low proportion of n-butanol, and low EGR rate, the low-temperature reaction intermediate product of biodiesel(MDXO2 isomer) occurred early, the low-temperature reaction intermediate product of n-butanol(nC3H7CHO) had the same distribution zone of MDXO2. With the increase of the biodiesel injection timings and proportion, the mixture of n-butanol and biodiesel tended to become homogeneous. The interaction of the free radicalpromoted theoxidation of the two fuel, thereby cutting the CO emissions. With the decrease of the biodiesel injection proportion, the delay of biodiesel injection timing, and the increase of the EGR rate, the local high temperature area of in-cylinder reduced, which made the reduction of NOx emission. The soot formation regions located primarily at biodiesel distribution area. The reduction of the proportion of biodiesel and the increase of EGR rate facilitated the reduction of the soot emission.