| With the development of the marine transportation industry,the increase of the number of transportation ships,the increasing fuel consumption of ships,the air pollution caused by ships had been paid more and more attention.IMO had put strict limits on the pollutant emission of marine engines.In face of the energy,environment and emission regulations pressure,marine low-speed LNG/diesel dual fuel engines had developed rapidly.Although they meet the requirements of sulfur content and reduced NOx emissions,but most marine dual fuel engines still fail to meet the NOx emission limit of Tier III.At the same time,there were large MHC emissions problems in dual fuel engines.The scavenging method,injector position,and combustion chamber type of the marine low-speed two-stroke dual-fuel.engine were different from those of the small and medium-sized engines.The combustion process of marine dual.fuel engine had no accurate prediction method,and the emission rule could not completely copy the existing small and medium-sized engines’ rules.The combustion process in the cylinder,gas flow rule and the formation mechanism of emission products of HPDI low-speed two-stroke LNG/diesel dual-fuel engine were not clear.Therefore,it was great theoretical significance to research the combustion process rule and emission generation mechanism of marine low-speed two-stroke dual-fuel engines.The research methods of data collection by hundreds of tests,in marine large-scale low-speed dual-fuel engines,had limited by field and equipment.At the same time,the research on combustion process of the engine is rarely studied in the quantification of combustion process and the control of combustion stage.The systematic research of quantification method on the combustion process was especially lacked.Therefore,the research of quantitative method on combustion process of engine provided a practical reference for in-depth research of combustion characteristics!Based on graphics method,the combustion process of the engine was regarded as the reconstruction process of the instantaneous image in infinite cylinders.The logical mapping relationship between image features and combustion state in the frame of graphics was proposed.The three characteristic values of the high temperature volume,the high temperature volume rate and flame propagation speed were defined.Through the analysis of characteristic values,the instable phenomena and variation rules in combustion process of engine could be effectively analyzed.Based on the method of graphics quantification and numerical simulation,the HPDI low-speed two-stroke LNG/diesel dual fuel engine was taken as the research object.The combustion process of FD and DEDF and the influence rule of emission pollutants were explored.The difference of transient combustion and emission between DIDF and FD was further clarified.The mechanism of high temperature volume,flame propagation speed and CH4 combustion interruption in DIDF were revealed.In order to provide theoretical basis and research means for the optimization and application of marine low-speed two-stroke dual fuel engine,a more in-depth research on the effect of fuel injection timing and initial gas state on combustion process and emission of DIDF were carried out.The main research results include:(1)The flame propagation path under DIDF and FD is basically same,but the initial ignition position is different.The radial flame propagation speed under DIDF and FD show"single peak" fluctuation rule,and the axial flame propagation speed show "fluctuation" rule.The difference is that the radial flame propagation speed of FD shown two times of zero attenuation.The total combustion duration of DIDF is shorter than that of FD,and the flame propagation speed of HPDI natural gas combustion is faster than that of diesel combustion.There are two stages of combustion instability and one stage of combustion stability in DIDF.During the NG combustion process,there is combustion interruption.The combustion interruption factor shows the rule of first rising and then falling.The development speed of DIDF flame is less than that of NG premixed gas.MHC is concentrated on the upper and lower sides of the nozzle.With the development of NG flame,the MHC above the nozzle is burned off and the MHC below the nozzle moves towards the piston ring shore.(2)The average temperature of HPDI dual fuel engine is higher than that of traditional diesel engine,but the NOx emission is relatively less.From the perspective of combustion duration and high temperature distribution in the cylinder,the results show that the combustion duration of DIDF is shorter than that of FD,and the high temperature region is mainly concentrated in the ignition region of diesel fuel,and the high temperature region is significantly reduced compared with FD,resulting in the NOx emission of DIDF is significantly reduced compared with FD.(3)The dual fuel engine is difficult of meeting Tier Ⅲ of NOx emission and amount of MHC emission.The control strategy of combustion process and the optimal control strategy of NOx and CH4 emission reduction under the main/pilot timing were proposed.The control of ignition delay period and fast burning period shall realize mainly by changing the pilot timing,while the control of ignition delay period and after burning period shall realize mainly by changing the main timing.The NOx emission control strategy shall delay the main timing and pilot timing,and the CH4 emission control strategy shall advance the main timing and pilot timing.(4)Through the research of impact mechanism on the initial gas state(scavenging pressure,intake temperature and EGR)in DIDF,it was found that:the increase of scavenging pressure,the decrease of intake temperature and the increase of EGR result in the decrease of the temperature peak in the cyllinder,the decrease of high temperature volume ratio and the decrease of NOx emission with different gradients.However,for the effect of flame propagation speed,peak pressure and MHC emission,the scavenging pressure increases,while the intake air temperature decreases and EGR increases in a completely opposite way.With the increase of scavenging pressure and the decrease of intake air temperature,the flame propagation speed increases,the peak pressure in the cylinder increases,the MHC emission decreases,and the effect rules of EGR increases are in contrast.The innovations are following:(1)The numerical research method was applied to the quantitative research of the in-cylinder combustion process of the marine low-speed two-stroke LNG/diesel dual-fuel engine.The effects of the in-cylinder high-temperature volume,flame propagation speed and CH4 combustion state on the dual-fuel engine combustion process was discussed.The difference between the combustion process of natural gas ignited by pure diesel and diesel were analyzed.The unstable phenomena and change rules in the combustion process of natural gas ignited by diesel were revealed.(2)The effect of fuel injection timing and initial gas state in the combustion and emissions is studied for the optimized control of efficient and clean combustion of the marine dual fuel engine.The natural gas injection timing and diesel ignition timing were proposed to optimize control of each combustion stage in the combustion duration strategy.(3)The graphics technology was applied to the quantitative research of the combustion process in the low speed two-stroke marine dual fuel engine.The logical mapping relationship between image features and combustion status in the frame of graphics was proposed,which provided an intuitive and effective analysis method for the quantitative research of the instantaneous combustion state in the engine. |
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