Study On Lean Burn Characteristics Of Marine Dual-fuel Engine Based On Detailed Chemical Kinectics

Improving efficiency,reducing the pollutant emissions and utilizing clean alternative fuel have been the research hotspots of the marine engines.Liquefied natural gas(LNG)has the characteristics of abundant reserves,high heating value,clean and environmental protection,so the use of LNG instead of traditional diesel fuel has become the main technical way to improve energy consumption structure and reduce pollutant emissions of marine engines.The marine engines require high ignition energy and control precision due to its large power output,furthermore,natural gas(NG)is difficult to be ignited by compression and during its combustion process,abnormal combustion phenomena,such as deflagration,knocking and missing fire,can easily occur.Therefore,the marine engines usually adopt micro pilot ignition and lean burn technologies to achieve the purpose of high efficiency and clean combustion.However,the lean burn in the marine micro pilot ignition dual-fuel engine still has the following problems: first,the lack of detailed chemical reaction mechanism suitable for micro pilot ignition combustion mode make it difficult to carry out relatively accurate simulation analysis for lean burn characteristics;second,the microscopic characteristics of lean burn in dual-fuel engines have not yet been systematically recognized resulting in the difficulty to implement lean burn;third,the micro factors influencing the lean burn and their evolution law have not been clearly understood,making it extremely hard to achieve high efficiency and clean combustion of dual-fuel engine through control methods.Thus,the research work of this thesis focus on the following aspects:1.The three-dimensional combustion simulation model for dual-fuel engine based on detailed chemical kinetics was built.In order to study the combustion process and pilot igniton combustion mechanism in a marine mircro pilot igniton dual-fuel engine,KIVA-4 coupling CHEMKIN-II model was established.Through a variety of simplified methods,such as DRGEPSA,CSP and chemical reaction rate constant adjustmeng,a reaction mechanism which consists of 61 species and 199 reactions was developed to model the diesel and NG dual-fuel combustion in the marine micro pilot igniton dual-fuel engine.In this mechanism,n-heptane and methane are used to represent diesel and NG,respectively.To validate the mechanism,the predicted ignition delay and in-cylinder combustion process were compared with the experimental data obtained from a shock tube and engine setup,respectively.The results indicate the developed reaction mechanism is suitable for the modelling of diesel and NG fueled dual-fuel engine.Thus,this mechanism was further applied to study the marine micro pilot igniton dual-fuel engine combustion characteristics and emissions formation.2.The influence rule of control parameters of global equivalence ratio(ER)on the lean burn characteristics of micro pilot ignition dual-fuel engine was studied.To systematically investigate the effects of in-cylinder diesel/air and NG/air ratio on lean combustion characteristics and emissions formation in a dual-fuel engine form a macro point of view,parametric studies were conducted.The investigated significant parameters,such as global ER of diesel and NG,NG fuel percentage and EGR(exhaust gas recirculation)rate,were varied at various levels.Results on the combustion characteristics(i.e.,in-cylinder pressure,heat release rate(HRR),pressure rise rate(PRR),combustion phasing,ignition delay and combustion duration)and emissions(i.e.,NOx?CO?THC and soot)were compared to reveal the effects of global ER of diesel and NG and their interactions.In addition,the heat release processes under different global ERs were analyzed.The results show that: increasing the global equivalent ratio of diesel fuel can increase the combustion efficiency,but it will lead to the increase of NOx emission;when utilizing an earlier SOI timing,there is a NG global ER window,which can keep the emission of NOx,CO,THC and Soot at a lower level simultaneously;increasing the NG fuel percentage and the EGR rate in a certain range,the NOx emissions can be significantly reduced,while maintaining a relativelylower level of CO,THC and Soot emissions;through the optimization of the NG fuel percentage,diesel SOI timing and EGR rate,the combustion phase can be effectively controlled,and NOx and Soot emissions can be greatly reduced while maintaining a high dynamic perforamance.3.The influence rule of control parameters of local ER on the lean burn characteristics of micro pilot ignition dual-fuel engine was investigated.To study the effects of in-cylinder fuel-air mixing and fuel distribution on the lean combustion characteristics and emissions formation in a dual-fuel engine form microscopic point of view,the start of injection timing of diesel,injection pressure,injection rate and piston bowl geometries were varied at different levels.The combustion characteristics(i.e.,local ER distribution,in-cylinder pressure and combustion phasing etc.)and emissions(i.e.,NOx,CO,THC and soot)were compared with various injection parameters and bowl geometries.Furthermore,based on the dynamic?-T map,the common variation law of local ER with different diesel injection coordination parameters and pistion bowl geometries and the influence rule of local ERs on the NOx formation were found.The results indicate that: the combustion process is affected by the coupling effect of fuel injection coordination parameters,in which the effect of diesel SOI timing is the most significant;by optimizing the fuel injection coordination parameters,the local ER distribution in the cylinder can be improved and then low fuel consumption and emissions can be achieved;compared with the original diesel engine combustion chamber,the deep Omega combustion chamber with the contraction mouth is more suitable for the micro ignition dual-fuel engine;under different fuel injection coordination and combustion chamber geometry parameters,the local ER zones with the mass fraction less than 1.3 have the same varying tendency and obvious time-order characters;the generation of NOx depends not only on the temperature field,and the change of oxygen concentration caused by local ER distribution,but also on the change of combustion rate caused by the local ER distribution.4.The lean-burn optimization strategy for micro ignition dual-fuel engine based on the synergetic control of key influence parameters was studied.In order to optimize the combustion process of a dual-fuel engine under medium and high load,the main effects and interaction effects of the control parameters of the global and local ER on the main combustion and emission performance indexes were investigated under high NG fuel percentage,and the key parameters affecting the lear burn performance of the dual-fuel engine were found.Two lean burn optimization strategies based on synergetic control of key influence parameters were proposed.Strategy 1 utilized a quite high NG fuel percentage,an appropriate early SOI timing,a small EGR rate and without intake throttling;on the basis of Strategy 1,Strategy 2 adopted an earlier SOI timing,and moderately increased the diesel injection pressure and EGR rate.The results show that,compared with the original strategy,the optimized two strategies can significantly improve the indicated power,reduce the NOx?CO?THC and Soot emissions,decrease the consumption of diesel effectively;and to a certain extent,achieve high-efficiency low-emission combustion,and meanwhile,the Strategy 2 is better.Through the research of this thesis,the influence rules of control parameters of total ER and local ER on the lean burn characteristics and emission formation were revealed,which lay a good theoretical foundation for the organization and control of the lean burn in the marine micro pilot ignition dual-fuel engines.