Study On The Chemical Kinetics Of Diesel/Methanol Duel Fuel Combustion

Diesel/methanol dual fuel combustion is an economical, efficient and cleancombustion method, and it is helpful to solve the energy and environment problems.However the development of duel fuel combustion is hindered by the lack of the studyon chemical kinetics due to its complexity in duel fuel combustion. Modeling andexperimental research were carried out in present kinetics work to study the lowtemperature oxidation （auto ignition） and high temperature oxidation （combustion） ofthe diesel surrogate fuel/methanol blends. Based on the kinetics analysis, a skeletalmechanism used in dual fuel in-cylinder auto ignition and combustion was built andverified.N-heptane was selected as the diesel surrogate fuel in diesel/methanol lowtemperature oxidation. The ignition properties of methanol/n-heptane dual fuel havebeen studied via kinetics simulation. The resu lts show that the ignition of n-heptane isobviously delayed by methanol addition. The production and consumption of OH· andHO₂· radical in the radical pool are changed which is observed in the reaction pathand rate contribution analysis. OH· with high activity is converted into H₂O₂with lowactivity by the help of methanol below1000K. When the temperature exceeds1000K, the energy barrier of H₂O₂decomposition is broken, and then the inhibition effectof methanol on OH· concentration and ignition disappears consequently. Thus theignition inhibition observed in the engine in-cylinder combustion and in the constantvessel can be explained. Further research is expended to other common used highoctane number fuels. These fuels are classified according to their differentmechanisms of ignition inhibition effect on n-heptane. A universal rule of thechemical mechanism between high octane number fuel and high cetane number fuel isfinally obtained.N-heptane and n-heptane/toluene blends were selected as the diesel surrogatefuel in diesel/methanol high temperature oxidation under stoichiometric ratio and fuelrich conditions respectively. Low pressure premixed laminar flame were used in theexperimental and simulation works to seek the effect of methanol addition on dieselsurrogate combustion. The results show that methanol has little effect on thedegradation of hydrocarbon fuel under both stoichiometric ratio and fuel richconditions. However, in the fuel rich flames, the concentrations of polycyclic aromatic hydrocarbons （PAHs） decrease significantly after alcohol addition. The reason is: first,a part of hydrocarbon fuels is replaced by alcohol; second, toluene as the parent fuelof PAHs is consumed in intermediated temperature zone in advance by the help ofalcohol oxidation, and then toluene, which flows into PAHs formation zone with hightemperature, decreases. In addition, the polymerization reactions have anamplification effect which leads to the more significant decrease in the productionrates of PAHs than the decrease in the consumption rates of toluene.A methanol/n-heptane skeletal mechanism which is used for in-cylindercombustion simulation was built based on the kinetics analysis above and theproperties of dual fuel in-cylinder combustion. The mechanism contains41reactionsand30kinds of species. It has been validated by comparing with the results ofdetailed mechanism simulation and fundamental experiments, including auto ignition,HCCI engine combustion, premixed flame and methanol flame speed. Finally thepressure and heat release rate of diesel/methanol duel fuel in-cylinder combustionwere adopted to validate the reliability of skeletal mechanism coupling with CFDsimulation. Excellent consistencies between the numerical results and theexperimental results were obtained.