| In order to save oil resources, Meet the needs of automotive fuel diversification, And achieve more stringent vehicle emission standards in the future, Efficient and clean alternative fuel got more and more attention. Natural gas is considered to be the most promising alternative energy because of its abundant, high heat value, cheap price and clean combustion. Now, The ignition mode of natural gas engine include spark ignition and diesel ignition. Compare with the spark ignition, using diesel multi-point ignition can provide multiple ignition center and higher ignition energy, So the ignition effect is better, the power of engine is stronger. The emissions of HC, CO, smoke and SO2 of biodiesel is lower, Besides, Its cetane number is higher, So biodiesel can be used to ignite natural gas as well. Natural gas and biodiesel are complementary, Hence, They can achieve high efficiency and low carbon smoke emission during combustion process, But the NOx emission is higher, need to use EGR to control it, Then achieve efficient and low emission dual fuel engine lean-burn mode.In this paper, firstly, simplify the detail mechanism of natural gas GRI3.0, Then combine with the chemical kinetics mechanism of alternative mixtures of diesel and biodiesel, Build the natural gas/diesel and natural gas/biodiesel dual fuel chemical kinetics mechanism. Research the influence of natural gas replacement ratio, pilot fuel and EGR on combustion and emission characteristics of dual fuel engine by coupling the three-dimensional CFD model with chemical kinetics mechanism. Finally, from the perspective of burning mechanism, Compare the influence of low oxygen concentration, introduction of CO2 and H2O as a result of EGR on the combustion characteristic and products generation process of dual fuel engine.The results showed that, with the natural gas replacement ratio of gas/diesel dual fuel engine increase (0% to 60%), the combustion starting time (CA10) in cylinder delay, accumulated heat release decrease, the temperature distribution tend to be more uniform. At low replacement ratio (≤ 40%), Burning center (CA50), in-cylinder pressure and temperature have no obvious change. When the replacement ratio is 60%, the peak of pressure and temperature is reduced by 32% and 8.5% respectively, and there is no obvious ignition centre in cylinder. With the natural gas replacement ratio increase, the final emission of CH4 and CO increase, NO emission decrease. Especially at 60% replacement ratio, the temperature is lower because of inadequate burning, As a result, CH4 remains a lot at the bottom of the pit and near the cylinder wall, final emission of CO is 2.7 times of pure diesel combustion, NO emission decrease by 78%, the remaining amount of Soot increased slightly due to oxidation ability weaken. Gas replacement ratio within 40% can decrease carbon smoke emission, but without affecting combustion characteristic.Compare with diesel ignition, using biodiesel ignite natural gas, the ignition time is about 1.5℃A in advance, the peak of heat release and accumulated heat release is above 6.9% and 5.4% respectively, The peak of in-cylinder pressure and temperature is above 14% and 6% respectively, The highest ratio of pressure rise is about 30% higher than diesel ignition. At the beginning of combustion, when using biodiesel ignite natural gas, there is a large number of O radical generated in liquid fuel distribution area, in the spray, NO formed firstly, near the premixed flame, the formation of C2H2 and benzene (A1) is less. During combustion, the distribution of O radical concentration is uniform and broader, Soot distribution area is smaller, The generation of NO is higher than that of diesel igniting natural gas obviously. At the late combustion, the distribution of CO in cylinder is less.Compare with no EGR, When the EGR ratio is 32% and 50%, the peak of in-cylinder pressure of natural gas/biodiesel dual fuel engine is reduced by 4.3% and 12.5% respectively, the peak of temperature is reduced by 3.6% and 10.8% respectively, CH4 and CO emissions increase slightly, Soot and NOx emissions decrease. The oxygen concentration reduce caused by EGR is the main reason for ignition delay, It weaken the reaction h+o2<=>o+oh, As a result, Lower the content of O radical, Besides, the peak of C2H2 and Al generation increase obviously, The remainder of the CH4 is about 4.3 times of no EGR. The H2O of EGR can reduce the in-cylinder pressure and temperature effectively, promote H2O2 production, enhance the adverse reaction of 2oh (+m)<=> h2o2 (+m) and 2 oh<=> o+h2o, then make OH radical content increase, It also promote the CO oxidation, reduce its final mass fraction. The CO2 of EGR cause CH4 remaining amount increase by 4%, the peak of C2H2 and A1 generation decrease. The inhibitory effect of oxygen reduce and H2O on NO, NO2 generation is very obvious.Compare with biodiesel igniting natural gas, when using diesel ignite, The oxygen concentration reduce caused by EGR delay the ignition time more apparently, The drop of pressure and temperature peak increase, the peak of O radical generation decrease by 25%, So it is more sensitive on the dilution effect of EGR.
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