Numerical Simulation Of In-cylinder Combustion Of Diesel-methanol Dual Fuel

The application of diesel-methanol dual fuel (DMDF) on diesel engines cansignificantly reduce the reliance on diesel fuel. In a DMDF engine, the methanol isinjected into the intake manifolds and the diesel is direct-injected into the cylinder.Given the further investigation on DMDF combustion, a series of experiments wereconducted on a modified diesel engine and several DMDF combustion characteristicswere analyzed. A RANS (Reynolds-averaged Navier-Stokes) combustion model weredeveloped based onKIVA code and a LES (LargeEddySimulation) combustion modelwas also adopted in comparison study. The main work includes:Firstly, experiments including low and high methanol ratios with the same dieselinjection mass and timing, different diesel/methanol ratios with the same IMEP anddifferent intake temperatures with the same diesel/methanol ratio were conducted toobserve DMDF combustion phenomena. Experimental results show that methanol hasan inhibition effect on the ignition of diesel, and this effect is more obvious when themethanol ratio exceeds40%. However, in some high intake temperature conditionsmethanol auto-ignition before diesel injection occurs. As for emission, soot can bereduced up to96.49%by methanol addition, and large amounts of NO are convertedinto NO2bysmallamount ofmethanoladdition. InthesameIMEP cases, NOxemissionslightly decreases when diesel is replaced by methanol. Based on the combustioncharacteristics analysis, the in-cylinder combustion can be divided into three zones,which is the outline used in the numerical model development.Secondly, a RANS combustion model was developed to simulate the DMDFcombustion. The combustion model assumes that the reaction rate for each species isdetermined by the kinetic process as well as the relative magnitude of mixing andreaction effects, which are characterized as a turbulence time scale and a kinetic timescale. The turbulence time scale is formulated to be proportional to the eddy turnovertime. The kinetic time scale is interpreted as the time for the mixture to reach chemicalequilibrium based on the current reaction rate,orthe inverse ofreaction ratenormalizedby the specific enthalpy difference between the current state and equilibrium state.Further more kinetic model for soot, NO and NO2emission prediction was developedbased on a previous skeletal dual fuels mechanism. The simulation shows that thecombustion model could give good prediction for the low methanol ratio cases inrespect to engine pressure traces and heat release rates, but under predict the pressure traces when the methanol ratio exceeds60%. And also, ignition delay is over predictedfor the methanol auto-ignition operations.Thirdly, a LES combustion model was modified to be used in diesel-methanolcombustion simulation and the LES model and the RANS model were compared toinvestigatetheir applicationconditions. Theresults showthat theLES modelcould givegood prediction under most cases, but also gives deviated pressure traces for themethanolauto-ignitioncases. Finally, according to theneedsofintensivestudy, theheatrelease rates of diesel and methanol were separated by tracing the carbon atom frommethanol. This method provides a new approach to analyze the DMDF combustion.In conclusion, considering the precision and the computational consumption,RANS combustion model is the right solution when methanol ratio is less than60%.But LES can give better results when methanolratio exceeds60%.The diesel-methanoldual fuel combustion model could benefit the theoretical analysis and the engineeringapplication of diesel-methanol fuel. The application of methanol on diesel engine hassignificant implication to the socioeconomic benefits of China because methanol canbe derived from coal which is abundant in China.