Simulation Of Working Process And Emission Optimization Of Marine Dual-Fuel Engine

Marine engines are the main power facility in the shipping field.It promoted shipping and economy,but caused serious environmental pollution.The International Maritime Organization has formulated strict regulations on pollutant discharge to control the pollution caused by ships.The reduction of pollutant emission can be achieved by controlling combustion in cylinder and using post-treatment technology.The natural gas is the most ideal alternative fuel for engine,which has the characteristics of rich gas reserves,high energy density,low price and good emission.In recent years,with the continuous development of the dual-fuel engine,more and more attention has been paid to the diesel natural gas dual-fuel engine.The aim of the research is to optimize the dual fuel engine to have higher combustion efficiency.In this paper,the MAN 8L 51/60 DF marine medium-speed four-stroke dual-fuel engine was taken as the research object.The Miller cycle and injection strategy were used to optimize the combustion and emission performance of dual fuel engine under low load operation.This paper creatively solved the problem that the one-dimensional software AVL-BOOST cannot mix two fuels at the same time,when studying the effect of the pre-injection strategy on engine combustion emissions under low-load conditions.On the basis of one-dimensional simulation,CONVERGE software is used to build a three-dimensional simulation model.The three-dimensional simulation model of the gas model combines the SAGE model based on the detailed chemical reaction mechanism and couples the detailed chemical reaction mechanism.The one-dimensional simulation model is used to provide boundary conditions,and the one-dimensional simulation results are corrected with the acceptance report.The cylinder pressure of the three-dimensional simulation model is in good agreement with the cylinder pressure of the one-dimensional simulation model.The NOx emission and peak pressure and the acceptance report error are within a reasonable range.The knock index was introduced into the model,and the pressure oscillations at 8 monitoring points in the cylinder were analyzed.In the three-dimensional simulation,the influence of fuel injection timing on the engine combustion mode and emissions were analyzed.The results show that the earlier injection timing changed the ignition mode of the dual-fuel engine,making the diesel compression ignition mode into a two-stage ignition combustion mode.The influence of start of injection on knock index shows a law of fluctuation when hen the injection timing is advanced to a certain extent.On this basis,the effects of pre-injection timing and pre-injection ratio on combustion and emission were studied.The results show that increasing the pre-injection timing and pre-injection ratio can effectively reduce NOx emission and reduce the knocking index within a certain range.Based on this theory,the response surface model of the pre-spraying strategy is established,through a large amount of pre-spraying timing and corresponding pre-spraying ratio data.The response surface design method is used to optimize the multi-objective optimization of the pre-injection strategy of the engine under low-load operating conditions.In multi-objective optimization,the lower NOx emission is the main goal without sacrificing fuel economy.One dimensional simulation results show that the power,fuel consumption,NOx emission and burst pressure change by+0.55%,-0.60%,-13.21%and-1.51%respectively,when the boost pressure,compression ratio and intake valve closing time change by 9.83%,+0.17%and-6.04%respectively.The three-dimensional simulation results show that the power,fuel consumption,NOx emission,explosion pressure and detonation index change by+1.34%,-1.32%,-79.82%,+2.52%and-54.85%,respectively,when the pre injection ratio is 0.7863 and the pre injection timing is 44.44° before the TDC.