Combustion Process Analysis And Optimization Of Dual Fuel Low Speed Engine With High-pressure Direct Injection

With the serious situation of greenhouse gas emissions and the increased demand of fossil fuel,researching alternative fuel of fossil energy is an effective method to improve these problems.Natural gas as an alternative fuel of fossil energy in marine power machine,which is mainly used in dual fuel low speed engines.There are only two types of diesel-natural gas dual fuel low speed engines,namely low pressure premixed and high-pressure direct injection.The high power output of high-pressure direct injection dual fuel low speed engines is obtained by gas jet diffusion combustion.However,compared with low pressure premixed,the NO_x emissions of high-pressure direct injection dual fuel low speed engine is relatively higher,which is usually need post-treatment technology to meet the IMO Tier III emission regulations.In order to make optimization strategy to reduce NO_xemission in low speed engines,it is necessary to analyze the combustion and emission process of low speed engines.However,the combustion principle of diesel pilot-ignited natural gas jet is not particularly clear at present,and there is no ideal research method to study the combustion and emission characteristics of dual fuel.Therefore,this thesis mainly used CONVERGE simulation software,according to the experiment data to establish and validate dual fuel injection model and jet combustion model,further predict the working process of high-pressure direct injection dual fuel low speed engines and analyze the NO_x generation principle.According to the relevant experiment parameters,the diesel spray model,the high-pressure natural gas jet model and diesel pilot-ignited natural gas jet model were established and validated,respectively.Through the validated gas jet model,the flow and mixing characteristics of gas jet under different nozzle diameters,ambient temperature,ambient pressure and swirl ratio were further studied.The results showed that the swirl ratio condition had a significant effect on the flow and mixing characteristics of high-pressure natural gas jet.Then,based on the validated diesel pilot-ignited natural gas jet combustion model,the effects of dual fuel injection angle variation and dual fuel injector distance variation on the combustion and emission process were analyzed.The results showed that the combustible mixture variation of high-pressure natural gas jet was an important factor of NO_xemissions.Combined with the numerical simulation parameters of dual fuel injection model and jet combustion model,the three-dimensional simulation model of high-pressure direct injection dual fuel low speed engine was established,which is used to predict the actual working process of low speed engine and analyze the dual fuel combustion and emissions process,especially understand the NO_x generation principle.In order to optimize the NO_x emissions condition of high-pressure direct injection dual fuel low speed engine,according to the NO_x generation principle,the combustion and emission process were mainly analyzed from the variation of natural gas injection timing,swirl ratio,and EGR rate.The simulation results showed that with the delayed of natural gas injection timing,it can decrease the NO_x emissions,low speed engine power and thermal efficiency.Also,with the reduction of swirl ratio,it can decrease the NO_x emissions,affect the power and thermal efficiency.Only the increased of EGR rate can reduce NO_x emissions,but it is not conducive to maintaining the power and thermal efficiency of low speed engine.Therefore,it is necessary to carry out an optimization research of different parameters coupled with EGR rate.The results showed that when the natural gas injection timing variable parameter is-1°CA and the swirl ratio is 2 coupled with 5%EGR rate,respectively.There are both can reduce NO_x emissions to meet the Tier III emission regulations requirements,and has little effect on the power and thermal efficiency of low speed engine.