Simulation Study On The Effect Of Oxygenated Fuel On Combustion And Emissions Of Compression Ignition Engines

In order to alleviate the dependence on petroleum fuels and solve the trade-off problem of traditional diesel engines in reducing NOx and soot emission at the same time,the researches for new clean alternative fuels have attracted the attention of researc hers.Oxygenated fuels have become one of the most promising alternative fuels.Based on the National Natural Science Foundation,this research aims to achieve the goal of efficient and clean combustion of internal combustion engines by coordinating the f uel characteristics and the control boundaries.Several oxygen-containing fuels with different oxygen bonding forms such as alcohols,esters and ethers are used as diesel additives to adjust the physicochemical properties and molecular configuration of the fuels.Through the combination of chemical reaction mechanism and three-dimensional computational fluid dynamics,the effects of different blending ratios and different fuel oxygen bonding forms on engine combustion and emissions are investigated.Moreover,this work also covers chemical mechanism analysis for the effects of oxygen on combustion and pollutant formation,and the mechanism and influence of different oxygen bonding forms on soot formation and oxidation.By changing the control boundaries such as intake and injection conditions,the control methods for improving combustion and emission performances of the engine is explored based on the coordinated control of oxygen-containing fuel and boundary conditions;and the difference in the usage characteristics between oxygenated fuels and pure diesel is analyzed.Results of the present research indicates that:The addition of oxygenated fuels has significant impacts on combustion and emissions.After adding oxygenated fuel to the diesel,the fuel-air mixture become more uniform.With the increase of n-butanol blending ratio,the ignition delay period is gradually extended and the proportion of premixed combustion is increased;the soot emission is reduced while the NOx emissions stay still.From the view of chemical kinetics,the ignition of the fuel in diesel engine is mainly triggered by the low-temperature active diesel component n-heptane.For the n-butanol/diesel mixed fuels,during the low-temperature reaction before ignition,the OH radical produced by n-heptane is consumed by n-butanol.The low-temperature dehydrogenation reaction of n-heptane is suppressed and leading to delayed ignition.Meanwhile the addition of n-butanol provides more oxidative radicals such as OH and HO₂which enhance the oxidation activity of the combustion atmosphere and promote the oxidation of soot and PAH.Furthermore,the combination of oxygen atom in n-butanol with carbon atom forms stable CO,which reduces the amount of carbon atoms as main element that forms precursors of PAH;and therefore the soot emissions gradually decrease as the n-butanol blending ratio increases.Under the same oxygen content,oxy-fuel's structure mainly affects the low-temperature oxidation stage of combustion.It finds that the DME can promote the ignition while DMC and n-butanol inhibit it.As a result,the durations of ignition delay for different fuels is the following order:diesel/n-butanol>diesel/DMC>diesel>diesel/DME.N-butanol enhances the oxidation activity within the combustion space,while DMC weakens the oxidation activity.Though in different ways,adding n-butanol,DMC,and DME to the diesel can reduce the soot emissions.Combined with carbon atom,the oxygen atom in the DMC forms CO₂ while the oxygen atom in n-butanol and DME molecules eventually forms CO.Unequal utilization ratio of O of fuels and oxidation activity performance different inhibition ability on PAH and soot.So the PAH/soot inhibition ability of the tested fuels should be in the fowllowing order:n-butanol>DME>DMC.Finally,DMC can reduce NOx emissions,and n-butanol and DME have less impact on NOx emissions.The optimization results of combustion boundary conditions for B30 fuel show that:Increasing the injection pressure can promote fuel atomization and improve the homogeneity of mixture so that the combustion is shifted twoards the premixed mode,with the soot emission being reduced consequently.Because n-butanol itself has good volatility,the need for injection pressure of n-butanol/diesel mixed fuel are effectively decreased.Although high injection pressure leads to more NOx emissions,B30 has a lower NOx increment than pure diesel.The injection timing affects the thermal atmosphere prior to combustion in the cylinder directly.Premature or late injection will both lead to an increase in the ignition delay period and harm to the thermal efficiency.Delayed injection reduces NOx emissions while increases soot emissions significantly.However,the soot emissions of B30 remain lower at different injection timings.Increasing the intake pressure contributes to the homogenization and thinning of the mixture,thus it improves the combustion thermal efficiency.In condition of constant fuel supply,the high temperature region is greatly reduced.Therefore,t he emissions of NOx and soot can both be reduced with the increasing intake pressure under the condition that the n-butanol is used as addictive to the disel.By adding CO₂ to the intake air,the oxygen concentration and temperature in the cylinder decrease gradually with the increasing properation of the CO₂,which is beneficial for transferring the combustion mode to the low-temperature combustion.The chemical reaction rate of combustion decreases as the ignition timing is delayed.With the decrease of the intake oxygen concentration,the NOx emissions are significantly reduced,and the soot emissions are gradually increased.The burning of n-butanol/diesel blended fuel can reduce the NOx while keeping the soot emissions at a lower level,which provides a good conditions for applying a larger EGR rate.In the end,it can be concluded that the n-butanol/diesel blended fuel and the pure diesel have different characteristics due to differences in physical and chemical properties between them,especially in terms of emission characteristics.The"trade-off"relationship between NOx and soot can be effectively attenuated by carrying out a coupling control to the fuel characteristics and the boundary conditions.