Research On In-engine Purification Of Combined Injection Engine Based On Inner Selective Non-catalytic Reduction Technology

As a major automotive consumer,China has been facing the dual pressure of energy crisis and environmental pollution.In order to cope with increasingly stringent emission regulations,it is urgent to develop new energy-saving and emission-reduction technologies for engines.There is an obvious trade-off relationship between engine economy and emissions or between various emissions with traditional technical means.In this thesis,combining with water-blended combustion technology and selective non-catalytic reduction technology?SNCR?,a new type of in-engine purification method,namely inner SNCR with ammonia aqueous solution direct injection,has been developed.The technology can adapt to a wide use for engine requirements,weaken the game relationship between trade off indexes to a certain extent,and ensure the improvement of comprehensive emission reduction capability of NO_x and other emissions on the basis of satisfying power output.In view of the proposed SNCR scheme for ammonia direct injection in cylinder,extensive research by means of simulation and experiment is made,as well as a comprehensive evaluation.The main research work and conclusions in this paper are as follows:Firstly,based on CHEMKIN software,the laminar flame velocity,the chemical ignition delay and the variation of main radicals with primary reference fuel 90?PRF90?under homogeneous premixing conditions were calculated by using the existing literature mechanism,which were integrated to meet the requirements of the engine inner SNCR.The key factors affecting the reduction of NO_x by SNCR technology were explored.The results show that the laminar flame velocity,flame surface temperature and reaction rate of combustible mixture are reduced by the heat absorption of ammonia aqueous solution evaporation.Keeping the water/fuel ratio unchanged,the increase of NH₃ component means the decrease of water,and the cooling and dilution effect of water in the mixture is weakened.Compared with water,NH₃ promotes the flame development of homogeneous mixture to a certain extent,which leads to the increase of laminar flame velocity.The reduction of active substances,especially OH,delay the ignition time and prolong the chemical retardation period.In addition,the most influential external factors for SNCR are ordered by temperature>ammonia/nitrogen ratio>retention time>pressure.Secondly,a three-dimensional numerical model of a combined injection engine with inner SNCR process was established and calibrated accurately according to the experimental data of the prototype,by using CONVERGE code.The evolution of the spatial concentration gradient of combustion intermediates and NO_x under multi-physical field launching and different influencing factors of ammonia aqueous solution were studied by CFD simulation.The relationship between the key variation factors and the transformation of NO_x was analyzed as well.The results show that due to the high latent heat of vaporization of water,the high temperature zone in the cylinder decreases obviously,the average temperature decreases,the temperature distribution in the cylinder becomes more uniform,and the temperature gradient decreases.The suppression of the in-cylinder temperature and the temperature gradient is beneficial to inhibit NO_x formation by providing suitable SNCR temperature conditions.After water direct injection,the formation rate of combustion intermediates such as H₂O₂ and OH decreases synchronously,and the concentration gradient is the smallest when the water/fuel ratio is 10%.CO production increases obviously with the increase of water/fuel ratio,so the water/fuel ratio should be controlled within an appropriate range to avoid excessive incomplete combustion.The production of NO,NO₂ and N₂O decrease with the increase of ammonia concentration.Compared with NO,the concentration of NO₂ and N₂O is smaller,and the transformation of NO₂ and N₂O into harmless substances requires multi-step reactions,so the reduction rate of NO₂ and N₂O is less remarkable than that of NO.Thirdly,the existing engine bench was refitted into a dual injection system to realize the combined injection mode of gasoline injection in the intake port and ammonia injection in the cylinder.The effects of different direct injection strategies of ammonia solution on engine combustion and emissions under steady state conditions were studied experimentally.By using inner SNCR technology,the engine power performance and the variation of NO_x,HC,CO,NH₃and other gaseous emissions were studied quantitatively.The effects of different direct injection strategies of ammonia aqueous solution on solid phase emissions were studied quantitatively as well.The particle size distribution characteristics and influencing factors of particle number?PN?concentration and particle mass?PM?concentration were revealed.The results show that there is obvious synergistic effect between ignition timing and inner SNCR.SNCR technology with earlier ignition timing can ensure power output and synchronously reduce emissions of NO_x,as well as other substances.The direct injection timing and the direct injection mass of the ammonia aqueous solution have an important influence on engine combustion and emissions.Early direct injection timing and increasing ammonia injection mass can promote the conversion of NO_x.However,direct injection timing meeting with the formation of pyrotechnics and excessive direct injection of ammonia aqueous solution will lead to cyclic of variation deterioration.In addition,with the inner SNCR strategy,direct ammonia aqueous solution injection can inhibit particles of accumulation mode.The peak particle size shifts to a larger particle size range,meanwhile the peak mass concentration shows a downward trend.Finally,multi-objective optimization of a combined injection engine with inner SNCR technology under wide working conditions was carried out by using GT-POWER/CHEMKIN coupling model which involving the quick 1-D chemical response and Pareto front game principle.The optimal inner SNCR application strategy was obtained after a large-scale 1-D simulation calculation.