Research On Combustion And Emission Characteristics Of LNG/diesel Engine Based On Chemical Reaction Kinetics

At present,marine dual-fuel engines have become one of the important ways to use natural gas because of their high efficiency and low emissions.However,the current diesel/natural gas dual-fuel engines are all supplied into the cylinders in the form of gaseous natural gas.It is a complicated LNG vaporization device,a large amount of waste of cold energy,and insufficient engine power caused by low engine charging efficiency.In order to solve these problems,the design concept of "LNG in-cylinder liquid injection" was proposed,that is,high-pressure LNG is injected directly into the cylinder,and the diesel is ignited in the cylinder by igniting the diesel.Dual-fuel engines involve a variety of different fuels,and the combustion process in the cylinder is very complicated.At the same time,there are two combustion methods: premixing and diffusion combustion.The fuel combustion in the cylinder is affected by many factors such as turbulence and chemical kinetics.If the chemical reaction mechanism model is introduced into CFD,it can not only better reflect the chemical reaction process during combustion,but also take into account the influence of turbulence in the actual combustion process,and can predict the main emissions and intermediate products.Therefore,the introduction of the chemical reaction mechanism including multi-step chemical reaction into the existing CFD for numerical simulation of the combustion process in the cylinder has become a hot research topic,but the current natural gas mechanism model is not applicable under low temperature conditions.Therefore,this article is devoted to studying a diesel/natural gas dual-fuel combustion mechanism suitable for low-temperature conditions.First,the new simplification strategy proposed in this paper is used in the CHEMKIN software to simplify the detailed diesel mechanism(LLNL3.1 mechanism).Simulate natural gas ignition and combustion emissions under low temperature conditions(T<1300K),and improve and optimize the detailed mechanism of natural gas(based on the GRI3.0mechanism).The direct relationship diagram(DRG)method,the direct relationship diagram(DRGEP)method considering error transmission,the sensitivity analysis(SA)method and other simplified strategies are used alternately,and the strategy of setting the threshold value of each method from small to large is simplified.Then,in the simplified natural gas mechanism,the intermediate products that play an important role under low temperature conditions-CH3O2,CH3O2 H,and nitrogen oxide mechanism models are added.Based on the sensitivity analysis results,the key kinetic parameters of the simplified natural gas mechanism model are optimized and optimized.Adjustment.After the flame retardation period,laminar flame speed,and in-cylinder combustion verify the accuracy of the mechanism.Then the two simplified mechanisms are combined to establish a diesel/natural gas dual-fuel dynamic mechanism model,which contains 172 components and 818 elementary reactions.Coupled with AVL FIRE software,analyze and study the influence of different LNG replacement rate and LNG pre-main injection interval angle on engine performance.The main conclusions are as follows:(1)Simplify the detailed diesel mechanism(LLNL3.1 mechanism)to form a simplified mechanism containing 161 substances and 682 reactions.The final simplified diesel mechanism was verified by the ignition delay experiment and flame speed experiment.The results show that the simplified diesel mechanism has a good predictive effect.(2)The research results show that the simplified low temperature natural gas(LTNG)mechanism established in this paper can accurately predict the ignition delay period in the CHEMKIN homogeneous constant volume reactor model.The research results also show that in the selected zero-dimensional engine simulator,the low-temperature natural gas mechanism can also accurately predict the key free radicals(O,OH)that affect the formation of carbon-containing gases and the formation of gas emissions during the combustion process.(3)Due to the lack of experimental data for dual-fuel engines,it can only be verified against original diesel engines.Import the n-heptane simplification mechanism and nitrogen oxide mechanism into the established engine numerical model,and verify the initial conditions and boundaries of the established numerical model based on the actual measured cylinder pressure curve and nitrogen oxide curve under the actual operating conditions of the model The accuracy of the conditions and the effectiveness of the chemical reaction mechanism coupled with the CFD software.On this basis,FIRE software was used to couple the simplified natural gas mechanism and natural gas simplified mechanism 5 after calibration in this article(this mechanism has not been calibrated at low temperature),and calculate the combustion temperature of natural gas in the cylinder with the crankshaft angle change,which is judged to be calibrated The latter simplified natural gas mechanism can indeed predict the stagnation period more accurately,and provide a basis for the establishment of the following dual-fuel engine model.(4)As the LNG replacement rate increases,the average temperature peak in the cylinder tends to decrease,and the appearance of the peak tends to lag behind.The average pressure peak in the cylinder tends to decrease.In terms of emission performance,with the increase in LNG substitution rate,NOx emissions have shown a decreasing trend,and are significantly lower than the pure diesel model;CO emissions have shown an increasing trend;HC emissions have shown an overall increase Trend;SOOT emissions show a decreasing trend and are significantly lower than the pure diesel model.(5)The pre-main injection of LNG in the cylinder can improve the formation quality of the mixed gas in the cylinder.As 20% of the pre-injection of LNG is injected into the cylinder,it will be ignited after rapid evaporation and the temperature in the cylinder will continue to rise.,The pre-mixed combustion formed can quickly ignite the LNG injected by the main injection,which is conducive to the combustion of the fuel in the cylinder.According to simulation calculation and analysis,when the pre-main-injection interval angle is 4°CA,the average temperature peak in the cylinder is the highest and the high temperature range is the widest.At this time,the LNG is burned more thoroughly.(6)With the increase of the pre-main injection interval angle,the NOX emissions tend to increase first and then decrease.When the interval angle is 4°CA,the NOX emission reaches the maximum,4°CA~10°CA,the NOX emission gradually decreases,mainly distributed in the high temperature area.The emission of SOOT and CH4 follow the same trend,and the emission value is the lowest when the separation angle is 6°CA.With the increase of the pre-main injection interval angle,the CO emissions have a tendency to first decrease and then increase,and CO2 emissions are the opposite.Combined with the analysis of in-cylinder combustion emissions,for this model,the most suitable pre-main separation angle is 4°CA~6°CA.