Numerical Study On Combustion Process And Emission Characteristics Of A High-Pressure Direct-Injection Natural Gas Low-Speed Marine Engine

High-pressure direct-injection(HPDI)natual gas low-speed marine engines have drawn more and more attentions due to their high thermal efficiency and low emissions.Due to the interaction between the pilot spray and natural gas jet,the ignition kinetic process of CH4/NC7H16 under HPDI natural gas low-speed marine engine conditions were investigated.Then the combustion kinetic process of different combustion process under different stratification were studied.Based on the detailed kinetic model,a 35-step extremely simplified n-heptane mechanism and a 27-step high temperature n-heptane mechanism were constructed.Finally,the extremely simplified mechanism was applied to the computational fluid dynamics(CFD)simulation.The effects of exhaust gas recirculation(EGR)and and injection strategies on the combustion and emission characteristics of a HPDI natural gas low-speed marine engine were simulated,and a control strategy that can meet EPA Tier IV emission relulation was proposed.Firstly,the effect of high-activity intermediate products produced by n-heptane in different ignition stages on methane/n-heptane mixtures is simulated.The results showd that,even a small number of intermediate products can significantly promote the ignition delays of methane/n-heptane mixtures at low temperatures.As the concentration of intermediate products increases,the influence of intermediate products on the ignition delays of methane/n-heptane mixtures increases.However,some intermediate products(such as C2H4,C5H10-1,CH2 CO,etc.)have negative effect on the ignition process of methane/n-heptane mixtures at low temperatures,whereas they have positive effect on the ignition process at high temperatures.Temperature has significant effect on the ignition process of methane/n-heptane mixtures,while the intermediate products weaken the effect of equivalence ratio and methane mass fraction on the ignition process.When the concentrations of OH and other radicals in the intermediate products are relatively high,the equivalence ratio even has negative effect on the ignition processes of methane/n-heptane mixtures at high equivalence ratio.The the influence of intermediate products for n-heptane-related elementary reactions on the ignition processes of methane/n-heptane mixtures,and the intermediate products have little effect on the high temperature stage of igntion processes of the methane/n-heptane mxitures,mainly accelerate the low-temperature stage reaction to promote the ignition processes.The stratification has a significant impact on the ignition position and ignition timing of a HPDI natural gas low-speed marine engine,and has a significant impact on the representative exothermic reaction of the high heat release region in the ignition timing and initial combustion stage.However,the stratification has a negligible effect on the representative exothermic reaction of the high heat release region in the premixed and mixing-controlled combustion stage.At four combustion stage,the stratification has a significant effect on the representative creaction reactions of OH and CH2 O.When the mixtures get more homogenous,more CH4 is consumed through R177.In premixed combustion stage,the destruction of CH4 is mainly through R177,while in mixing-controlled combustion stage,the cunsumtion of CH4 by R176 increases.The reaction path analysis found that the reaction path from CH3 O to CO changes significantly with the advancement of natural gas injection timing.In the mixing-controlled combustion stage,the consumption path of CH2,CH2 O and HCO is significantly different from the premixed combustion stage.A 35-step n-heptane model was obtained through reaction path analysis and sensitivity analysis,then a 27-step high-temperautre oxidation model was obtained by Hychem method.Both the two extremely simplified models could reproduce the ignition delay of methane,n-heptane and methane/n-heptane mixtures.However,the main intermediate products and laminar flame speeds could not be well predicted by the two extremely simplified models.The application of two extremely simplified models and two semi-detailed mechanisms to 3D CFD simulation of a HPDI natural gas low-speed marine engine.The combustion and emission characteristics of a HPDI natural gas low-speed marine engine can be well predicted by the two extremely simplified models.Moreover,the two extremely simplified models can significantly shorten the CPU-hour of 3D CFD simulation.Finally,the 27-step model was applied to simulate the influence of control strategies on combustion and emission characteristics of a HPDI natural gas low-speed marine engine.When the EGR rate is less than 10%,the in-cylinder pressure and heat release rate(HRR)increase slightly.However,when the EGR rate is higher than 10%,as the EGR rate increases,the in-cylinder pressure,HRR and soot emissions increase;and indicating specific fuel consumption(ISFC)and NOx emissions decrease.70%EGR coupled the advance of absolute injection timing or natural gas injection timing can significantly improve the combustion performance of the HPDI natural gas low-speed marine engine.As the absolute injection timing or natural gas injection timing is advanced,the in-cylinder pressure and HRR increase,ISFC decreases.However,NOx emissions increase with the advance of absolute injectiong timing or natural gas injection timing.Overall,the NOx emissions are lower than the EPA Tier IV emission regulation,whereas the soot emissions are higher than the the emission regulation.70% EGR coupled pre-injection strategy can also improve engine performance.When the proportion of pre-injection natural gas was 30%,the increasement of injection interval has little effect on the in-cylinder pressure,HRR,ISFC,NOx and soot emissions.Increasing the proportion of the pre-injection fuel,the in-cylinder pressure and HRR increase significantly,and the ISFC decreases.However,the NOx emissions increase with the increasing proportion of pre-injection fuel.Considering the thermal efficiency,economy and emissions of the engine,when the EGR rate is 70%,the proportion of pre-injection fuel is 30%,and the injection interval is 6°CA,the emissions can meet EPA Tier IV emission regulation and ISFC can be improved.Present study is important for understanding the ignition and combustion process of low speed high pressure direct injection natural gas marine engine.The developed extremely simplified mechanism and combustion control strategy are important for the development of low speed high pressure direct injection natural gas marine engine.