【作者】 于洪亮；

【导师】 孙培廷； 段树林；

【作者基本信息】 大连海事大学 ， 轮机工程， 2020， 博士

【摘要】 随着海上运输业的发展,运输船舶保有量逐年增加,船舶燃料消耗与日俱增,船舶引起的大气污染日益受到重视,IMO对船舶发动机的污染物排放提出了严格的限值,面对能源、环境和排放法规的压力,船用低速LNG/柴油双燃料发动机迅速发展,该发动机虽满足含硫量要求,降低了 NOx排放,但大部分船用双燃料发动机仍不能满足Tier Ⅲ阶段NOx排放限值,并普遍存在MHC排放大的问题。船用低速二冲程双燃料发动机的扫气方式、喷油器位置、燃烧室型式等与中小型机不同,燃烧过程没有准确的预测方式,排放规律不能完全照搬现有的中小型机规律。HPDI低速二冲程LNG/柴油双燃料发动机的燃烧过程、燃气流动规律以及排放产物生成机理不甚清楚。因此,研究船用低速二冲程双燃料发动机的燃烧过程规律及排放物生成机理有着重要的理论意义。船用大型低速双燃料发动机,通过成百上千次的试验进行数据采集的研究方法存在场地、设备的限制。同时发动机燃烧过程的研究中很少从燃烧过程量化和燃烧阶段的控制上进行研究,特别是缺少燃烧过程量化方法的系统研究。因此,开展发动机燃烧过程的量化方法研究,对深入研究燃烧特性提供了实用的方法参考。本文基于图像学方法,将发动机的燃烧过程看成是无限个缸内瞬时图像的重塑过程,提出了图像学框架下图像特征与燃烧状况的逻辑映射关系,定义了高温区体积、高温体积率和火焰传播速度三个特征值,通过特征值分析,能够对发动机燃烧过程中的不稳定现象及变动规律进行有效分析。基于图像量化研究方法及数值模拟技术,以HPDI低速二冲程LNG/柴油双燃料发动机为研究对象,探索了 FD和DIDF燃烧过程及排放污染物生成规律,进一步明晰了 DIDF与FD下瞬态燃烧及排放的差异,揭示了缸内高温体积、火焰传播速度及CH4燃烧中断对DIDF燃烧排放的作用机理。更深入研究了燃料喷射定时、初始气体状态对DIDF燃烧过程及排放产物生成的影响规律,为船用低速二冲程双燃料发动机的优化及应用提供理论基础与研究手段。主要研究结论如下:(1)DIDF和FD下火焰呈现的传播路径基本一致,但初始着火位置不同,DIDF和FD下的径向火焰传播速度均呈“单峰型”波动规律,轴向火焰传播速度均呈“波动型”规律,不同的是FD径向火焰传播速度出现了两次衰减为零的现象。DIDF下总燃烧持续期比FD下明显缩短,HPDI天然气燃烧火焰传播速度比柴油燃烧火焰传播速度快。DIDF下存在两段燃烧不稳定期和一段燃烧稳定期,燃烧过程中NG存在燃烧中断现象,燃烧中断系数呈现先升后降的规律,DIDF火焰发展速度小于NG预混合气形成速度,MHC集中于喷嘴上下两侧,随着NG火焰的发展,喷嘴上方的MHC被烧掉,喷嘴下侧MHC向活塞环岸运动。(2)从燃烧持续期和缸内高温区域分布角度,对HPDI双燃料发动机比传统柴油机缸内平均温度更高,NOx排放却相对较少的现象,分析了其产生机理。研究表明:与FD工况相比,DIDF下发动机的燃烧持续期更短,缸内高温区主要集中在引燃柴油的着火区域,且高温区域较FD下大幅减小,致使DIDF的NOx排放较FD大幅降低。(3)针对HPDI双燃料发动机普遍存在的NOx排放满足Tier Ⅲ阶段限值困难,和MHC排放大问题,提出了主/引定时下的燃烧阶段控制手段,并针对NOx和CH4减排提出了优化控制策略。滞燃期和速燃期的控制应主要通过改变引定时来实现,缓燃期和后燃期则主要通过改变主定时进行控制。NOx排放控制策略应延迟主定时和引定时,CH4排放控制策略应提前主定时、延迟引定时。(4)通过对DIDF下初始气体状态(扫气压力、进气温度和EGR)影响机理的研究发现:扫气压力提高,进气温度降低,EGR增大,均造成缸内温度峰值降低,高温体积率减小,NOx排放呈现不同梯度的下降规律。但对缸内火焰传播速度、缸内压力峰值和MHC排放的影响,扫气压力提高,进气温度降低和EGR增大却呈相反的规律。扫气压力提高和进气温度降低,均使缸内火焰传播速度加快,缸内压力峰值升高,MHC排放降低,与EGR增大的影响截然相反。本文可以提炼出以下创新点:(1)论文采用数值研究方法对船用低速二冲程LNG/柴油双燃料发动机的缸内燃烧过程进行量化研究,探讨了缸内高温体积、火焰传播速度与CH4燃烧状态对双燃料发动机燃烧过程影响的机理,分析了纯柴油和柴油引燃天然气燃烧过程及排放污染物生成规律的差异,揭示了柴油引燃天然气燃烧过程的不稳定现象及变化规律。(2)针对发动机高效清洁燃烧优化控制的需要,研究了燃料喷射定时及初始气体状态对发动机燃烧排放的影响规律,提出了天然气喷射定时协同柴油引燃定时对燃烧持续期中各燃烧阶段进行优化控制的策略。(3)论文将图像学用于船用低速二冲程LNG/柴油双燃料发动机的缸内燃烧过程的数值研究,提出了图像学框架下图像特征与燃烧状况的逻辑映射关系,为燃烧过程的量化研究提供了直观有效的分析手段。更多还原

【Abstract】 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.更多还原