| In recent years,the greenhouse effect and other environmental pollution caused by the burning of fossil fuels have become increasingly serious.The use of low-carbon and zerocarbon fuels has become the development trend of future Marine engines.Among them,biofuel can achieve zero carbon emissions in the whole life cycle,and is an ideal fuel to achieve the goal of "two-carbon" in the future.At present,biodiesel has been widely used in small vehicle engines,but in the field of ships,its application is still mainly concentrated on small inland ships,the research on large ocean transport ship engines is not enough,combustion and emission characteristics still need to be further explored.Therefore,based on a low speed twostroke engine,the combustion characteristics of biodiesel are systematically studied in this work,and appropriate optimization is carried out in order to provide guidance for the application of biodiesel in two-stroke Marine engines.Firstly,the CONVERGE 3D simulation software was used to model the two-stroke diesel engine in this work,and matched various module simulation sub-models to perform grid partitioning to ensure the accuracy of subsequent simulation predictions.For combustion simulation calculation,based on the existing simplified combustion mechanism models of soybean biodiesel and diesel,the combustion mechanism model of biodiesel-diesel hybrid fuel was fused to construct,including 188 species and 939 reactions.According to this model,the calibration of 75% load condition of diesel engine was carried out to ensure the accuracy of the simulation model.Secondly,according to the different mixing ratio of biodiesel and diesel,the combustion simulation calculation was carried out in the two-stroke diesel engine model to explore the feasibility of biodiesel in Marine diesel engine.The results showed that the blending of biodiesel could promote combustion,improve combustion pressure and advance combustion phase,and reduce the emission of CO and unburned HC.And as the proportion of biodiesel increased,these effects became more pronounced.In addition,pure biodiesel can replace fossil diesel combustion,but will increase fuel consumption and NOx emissions.Then,aiming at the problem of excessive fuel consumption when pure biodiesel was burned in diesel,the concept of steam reforming coupled combustion of biodiesel was proposed.The feasibility of the concept was verified by combustion path analysis and in-cylinder combustion state analysis.The analysis showed that adding hydrogen-rich reforming products could increase the concentration of OH group at the initial combustion,and with the increase of reforming proportion,the concentration of OH group would also increase.At the same time,coupled combustion in the engine cylinder could increase fuel combustion pressure and output power,and reduce fuel consumption.However,it should be noted that this will also increase the combustion temperature in the cylinder and increase NOx emission.Finally,aiming at the problem of overload operation of the diesel engine under the reforming coupled combustion condition,a scheme of adjusting injection time and reducing the amount of direct injection fuel in the cylinder was proposed,which could not only ensure the output power standard,but also achieve the reduction of fuel consumption.The results showed that the fuel injection time in advance could improve the output power and reduce the fuel consumption,but the injection time in advance would have adverse effects on the normal operation of the diesel engine.In addition,by adjusting the ratio of catalytic reforming and the ratio of direct injection reduction,a linear fitting formula was obtained,which could find the amount of circulating injection required to meet the rated power under the given ratio of reforming.The application of this formula was helpful to promote the use of biodiesel in Marine diesel engines,while reducing fuel consumption and cost.However,it was necessary to balance the pressure rise rate to avoid the impact on engine life. |
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