Research On Combustion Optimization And Emission Control Of Technology Engine Based On Coal-to-liquid Diesel And Fuel Design

Energy shortage and environmental pollution issues have attracted extensive attention worldwide in recent years.Strict fuel consumption and emission regulations have put forward higher requirements for the technological progress of internal combustion engine.So,the development and application of clean alternative fuels and new combustion modes have become critical technical means for engine to achieve efficient and clean combustion.As China’s energy structure characterized by coal-rich,oil-poor,and gas-lack,promoting coal-to-liquid fuels(CTL)is vital for domestic energy security.Meanwhile,the combustion process of the new combustion mode for next-generation internal combustion engine is mainly controlled by the fuel chemical kinetic of the reaction,so the fuel design coupling of the combustion boundary conditions to actively regulate in-cylinder combustion process has received much attention.In this study,we explore the technical ways to achieve efficient and clean combustion of CTL in compression-ignition engine based on the fuel design and combustion boundary condition control with the National Natural Science Foundation of China.The paper applies thermodynamic experimental research,optical visualisation tests,chemical kinetic analysis and CFD numerical simulation to optimise the physicochemical properties of the fuel mixture by adding oxygenated fuel and gasoline to the CTL.The fuel properties and combustion boundary conditions are systematically investigated in terms of the influence and the intrinsic mechanism of actio on the in-cylinder mixture preparation,combustion process,flame development and pollutant generation history.In this study,an electronically controlled common rail four-valve diesel engine is used as a prototype to establish a thermodynamic engine test and control platform,a simulated boost system and a cooled exhaust gas recirculation system are designed to achieve flexible adjustment of intake parameters,an open electronic control unit is matched to achieve flexible and accurate control of injection parameters,the engine combustion and emission test system is built to achieve accurate collection and analysis of engine combustion and emission parameters.An optical visualization test platform for basic combustion research has also been designed and developed based on an optical engine and a fixed volume combustion bomb:a high-speed photography and optical test system has been built based on a four-stroke single-cylinder optical visualisation engine,achieving accurate measurement of the flame development and evolution of in-cylinder combustion;a schlieren method test system has been built based on a visualised fixed volume combustion bomb to collect and analyse spray morphological characteristics and parameters.In addition,a simulation platform coupling chemical kinetics and CFD has been built to create the conditions for in-depth analysis of the fuel combustion reaction mechanism and the pollutant generation process.The main research contents and conclusions are as follows:1.The effects of CTL and National Ⅵ fossil diesel on the combustion process and pollutant emission characteristics of CI engine were investigated,and the differences in combustion and emissions characteristics between the two fuels were compared and analyzed to provide data to support further optimization of the combustion process of CTL by means of fuel design.The results show that the combustion process of CTL with a high cetane number is dominated by diffusive combustion,and the premixed combustion rate is reduced by more than 39%and the peak exothermic rate of combustion is significantly lower under the test conditions compared to petrochemical diesel.In terms of emission performance,the combustion of CTL contributed to lower emissions of NO_x,CO and THC and resulted in relatively low total particulate emissions,but the lack of premixed combustion led to an increase in the number of large particle size particles in the accumulated mode compared to the lower aromatic content of National Ⅵ petrodiesel,resulting in increasing particulate mass emissions.2.To address the problem of increased particulate mass emissions due to insufficient premixed combustion of CTL,two oxygenated fuels,n-butanol and dimethyl carbonate(DMC),were added to CTL to improve the chemical reactivity of CTL and alleviate the in-cylinder local oxygen deficiency during combustion.The kinetics of the combustion chemical reactions between n-butanol and DMC and the mechanisms underlying the reduction of carbon soot emissions were investigated in depth by means of experimental studies and simulations,and the potential of different oxygen-bonded forms of oxygenated fuels for optimizing the combustion and emissions characteristics of CTL was clarified.The results show that the addition of n-butanol slow down the OH radical accumulation process resulting in a longer ignition delay compared to DMC.At the same time,n-butanol has a greater potential to reduce soot emissions due to its outstanding ability to consume PAHs,and the combustion of CTL/n-butanol blends is significantly more effective than CTL/DMC blends in reducing large particle size emissions.Improving the physicochemical properties of CTL with oxygenated fuels can increase premixed combustion rates while reducing particulate emissions,with an increase in premixed combustion rate of 55.48%and a 68.71%reduction in soot mass emissions at 30%butanol in the fuel.Although the addition of oxygenated fuel results in a small increase in NO_x,a good compromise between NO_xand particulate emissions can be achieved by combined EGR regulation.Results of in-cylinder combustion flame characteristics tests show that the CTL oxygenated fuel blend has a more uniform combustion flame temperature distribution cloud,which helps to reduce the combustion temperature gradient and promotes in-cylinder mass homogenization.The proportion and amount of high KL factors produced by the CTL/n-butanol blend is significantly lower than that of the CTL/DMC blend when the fuel oxygen content is the same,further demonstrating that n-butanol has a more significant effect on carbon soot emissions.3.To further optimize the combustion process of CTL and explore the technical ways to realize the premixed compression ignition mode,a wide distillation range fuel(WDF)with similar calorific value of low activity fuel gasoline is blended with CTL balancing volatility and ignition properties.This study investigates the fuel characteristics and combustion boundary conditions synergistic control strategies for CTL combustion’s premixed compression ignition mode.The results show that the CTL/gasoline blend has a lower reactivity and better spray characteristics,which effectively increased the premixed combustion rate of the combustion process.When the proportion of gasoline in the fuel is greater than 40%,the premixed combustion rate increased by more than 90%compared to CTL,while effectively mitigating the tendency for EGR-induced combustion and emission deterioration.The full premixed combustion mode is basically achieved by delaying the injection combining with EGR at 60%gasoline.The in-cylinder combustion flame characteristics show that the combustion of WDF produces a large blue flame at the beginning of combustion,representing premixed combustion,which not only reduces the amount of strong soot radiation but also reduces the area and the maintenance time covered by the high KL factor,effectively suppressing the generation of soot in the combustion flame.4.CTL/gasoline blend was used in conjunction with a two-stage injection strategy to adjust the spatial and temporal distribution of the in-cylinder fuel,creating a stratified mixture reactivity and modulating the in-cylinder activation/thermal atmosphere further.The results show that the combination of a two-stage injection strategy with WDF can effectively improve the quality of the pre-ignition combustible mixture and optimise in-cylinder ignition conditions,with a reduction in particulate mass emissions of up to 50%compared to a single injection for CG40fuel with 40%gasoline.The choice of 15°CA to 25°CA pre-injection moment with 45%to 55%gasoline ratio help reduce pressure rise rate peaks and maintain low NO_xand soot emissions while balancing high indicated thermal efficiency.5.In order to investigate the microscopic mechanism of CTL/gasoline blends on the combustion process and the pollutant generation process,a simplified chemical reaction kinetics mechanism of CTL/gasoline was developed,and the evolution of intermediate products,reactive radicals and soot in the combustion process of wide-range fuels was investigated.This mechanism was coupled with CFD simulations to investigate the influence of WDF on the microscopic evolution of in-cylinder mixture preparation,combustion field activity distribution and pollutant generation.The results show that the prolonged ignition delay of CTL/gasoline blend is due to the introduction of the gasoline component,which reduceed the concentration of OH reactive radicals from the decomposition of long-chain alkanes in the low-temperature reaction stage,increasing the proportion of gasoline in the fuel can significantly suppress the production of PAHs and thus reduce soot emissions.Analysis of the in-cylinder combustion process shows that the combustion of WDFs results in a more uniform distribution of the in-cylinder temperature and concentration fields,which help to optimize the in-cylinder fuel-gas mixing conditions and reduce soot emissions.Under the two-stage injection strategy,the pre-injected fuel reacted at low temperature,accumulating a large number of reactive radicals before the main injection fuel ignite.The extended main pre-injection interval essentially eliminates the high equivalent ratio region within the main injection jet,which help to suppress the generation of soot,while the average in-cylinder temperature is rapidly reduced to suppress NO_x generation.