| From the first industrial revolution in 18th century,human has been witnessing a rapid growth and development of science and technology to bring the infinite benefit into our lives,but we gradually fall into the over-depending on fossil fuels,now the extreme consumption of fossil fuels are pushing us to the dangerous brink.This is because the crude oil as a non-renewable fossil fuel is closely related to the modern industrial,military,life and so on.Until now,there were several large-scale wars were caused by crude oil problems.In order to prevent the tragedy replaying in the energy depletion future,scholars have committed to explore suitable alternative fuels over the years.This makes biodiesel present in people's vision,and it get the favor of many scholars,due to the advantages of simple production process,low cost,abundant feed stock and used directly in diesel engine without any modification,etc.In recent years,though many experiments were conducted on biodiesel in diesel engine,there are still some vague concepts existing in the knowledge field of biodiesel combustion,this is mainly due to the lack of suitable combustion mechanisms so that the relevant numerical simulations are difficult to launch.With the development of the detailed biodiesel combustion mechanisms,some skeletal combustion mechanisms that can be used in engine combustion simulation are gradually presented,but these mechanisms cannot depth investigate the effects of FAMEs(fatty acid methyl esters)proportion on biodiesel combustion and emission characteristics in diesel engines.For three reasons,the first one is a part of skeletal combustion mechanisms use a fixed percentage or a single component to instead of all biodiesels.The next one is almost all skeletal combustion mechanisms lack an important C=C double?bond component named methyl linoleate(ML).The last on is the lack of soot prediction function.Therefore,the three points became the breakthrough of this thesis.The mainline and the main innovations are listed in following.Firstly,a skeletal quadra-component combustion biodiesel combustion mechanism comprising methyl decenoate,methyl-5-decenoate,n-decane and methyl linoleate is proposed in the present study.The motivation of this work is to develop a mechanism for more accurate researching in the effect of varying fatty acid methyl ester proportion on biodiesel ignition and combustion.The skeletal methyl linoleate combustion mechanism consists of 92 species and 249 reactions,and the coupled biodiesel blend surrogates mechanism consists of 106 species and 263 reactions.In order to validate the applicability of new developed skeletal combustion mechanism,zero-dimension ignition delay testing,reflected shock tube experimental results and tri-dimensional engine testing were employed in different conditions.The results indicate that the new developed skeletal combustion mechanism is suitable to predict ignition behavior and combustion characteristics of biodiesel with varying fatty acid methyl ester.Secondly,four different typical biodiesels were employed to investigate the effects of fatty acid methyl esters proportion on combustion and emission characteristics of a biodiesel fueled diesel engine in terms of heat release rate,cylinder pressure,indicated power and formation of NOx emission.The simulation results indicated that chemical ignition delay time and kinetic viscosity of biodiesel played very important roles in combustion process.Higher saturation level could shorten chemical ignition delay time,but the higher saturation contents like C16:0 and C18:0 together with C18:1(a single double bond methyl ester)would increase the kinetic viscosity,resulting in poor fuel-air mixing and evaporation process.Lower kinetic viscosity methyl esters like C18:2 and C18:3 was favorable for better fuel-air mixing and subsequent combustion,however,a higher NOx emission was discovered.Therefore,the relationship between saturation levels and combustion and emission characteristics of biodiesels is not simple and straightforward,the balance of five majority components is very important.Then,taking into account that the soot is one of main substances in the combustion exhaust gas,reaction pathways about soot from the detailed n-heptane combustion mechanism and the H abstraction&C2H2 addition mechanism were added into the aforementioned four component skeletal combustion mechanism.The rebuild skeletal combustion mechanism was developed and it consists of 134 species and 475 reactions.The 0-D ignition delay prediction,constant volume combustion chamber experiment and diesel engine experiment were conducted to validate its applicability.Results showed that the rebuild skeletal combustion mechanism is suitable to predict the 0-D ignition behavior of each component,the fire lift-off length and soot distribution trends in constant volume combustion chamber and the combustion characteristics in diesel engine.After this,the rebuild skeletal combustion mechanism was employed to predict the soot emission trends of the four typical biodiesels.It was found that,under 10%load condition,the soot formation is mainly related to the ignition characteristics and the physical properties of fuel itself.Under the 50%load condition,the soot formation and oxidation is closely related to the combustion chamber geometry.By using the combustion chamber geometry of this thesis,if the ambient temperature was low,the soot would be accumulated near the inner war surface of combustion chamber,so they were difficult to be oxidized by air.Under 100%load condition,the effects of fuel physical properties on soot formation reduced due to the higher injection pressure,turbulence intensity and initial temperature.However,the fuel with higher ambient temperatures had better soot oxidation process.Lastly,the rebuild skeletal combustion mechanism was employed in parametric studies to analysis the effects of compress ratio,start of injection,EGR(exhaust gas recirculation)ratio and combustion chamber geometry on the engine performance under 2400rpm and 10%,50%100%load.Results showed that there are significant impacts on the cylinder pressure,the heat release rate,the indicated power,and the NOx/soot formation trends,especially NOx and soot formation had the opposite trends,so it is difficult to simultaneously reduce their emission.On the one hand,the improving of compress ration or injection advance would cause the higher cylinder pressure,the earlier ignition,but the NOx emission would be raised,and the indicate power and soot emission would be different with the varying loads.The improving of EGR ratio would cause the reduction of cylinder pressure,indicate power and NOx emission,but the ignition delay time would not be changed,and the soot emission would be raised.On the other hand,it should be avoild the existing of the structure that could be stacked soot in the design of combustion chamber,the soot emission under 50%and 100%loads could be reduced significantly in this way.What's more,the effects of throat length on engine performance could not be ignored.The longer throat length could cause a higher indicated power under 100%load,but the accompanying longer combustion duration also increased the NOx emission,and it could lead to a very poor fuel atomization characteristics under the 10%load due to the weak turbulence.Two new development skeletal combustion mechanism also could be used in the prediction of combustion and emission characteristcis of biodiesels with varying FAMEs proportion,but the later one has a longer compution time due to the coupling of soot reactions.The analysis results on four typical biodiesls could provide a powerful reference on the choice of biodiesels,and the parametric studies also could provide a power reference on the optimization of biodiesel engine. |
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