Study On Methanol SICI Composite Combustion Mode Based On Multiple Control Factors

Internal combustion engines are currently the most widely used machinery for converting thermal energy into kinetic energy,and occupy a dominant position as power sources.However,due to the current energy and environmental crises limiting the development of the internal combustion engine industry,it is necessary to reduce the proportion of fossil fuels and vigorously develop renewable,alternative,and low-carbon energy.Methanol is considered to be the most suitable alternative fuel for China’s resource structure,and the promotion and application of coal to Methanol is a unique advantage of Methanol application in China.While vigorously developing alternative fuels,organizing and optimizing efficient and clean combustion of engines is also an effective way to save energy and reduce emissions.SICI（Spark Induced Compression Ignition）combustion is a composite combustion mode developed on the basis of HCCI（Homogeneous Charge Compression Ignition）combustion.Its advantages lie in its higher combustion efficiency and fuel economy compared to SI（Spark Ignition）combustion,and its controllable combustion characteristics compared to HCCI engines.It is a composite combustion mode between SI combustion and HCCI combustion.The author conducted research on the application of Methanol in SICI combustion mode.Firstly,an experimental platform was established based on a single cylinder engine,and the advantages and characteristics of composite combustion were identified by studying the specificity of Methanol SICI composite combustion;then,the control parameter range for achieving stable combustion under intake heating,internal EGR and external EGR（Exhaust Gas Circulation）strategies was studied;in order to optimize composite combustion,comprehensive weights and the influence of control parameters were studied to obtain the core control parameters of composite combustion;deeply analyzed the mechanism of composite combustion,studied the distribution of flow field and temperature field in the cylinder from a microscopic perspective,and described the microscopic evolution of the entire combustion process,including how to form spark ignition,flame propagation,and the process of unburned mixture self-ignition,providing reference for the industrial engineering problems of Methanol engine SICI technology.The author comprehensively analyzed the impact of multiple control factors on Methanol SICI composite combustion from the perspectives of basic bench test system and simulation.The main research content and conclusions are as follows:1.The author first achieved Methanol SICI composite combustion through experiments,and then compared and analyzed it with gasoline SICI mode.It was found that the ignition time for Methanol to achieve SICI combustion is earlier than gasoline,and the torque generated is smaller under higher fuel consumption.However,considering the production costs of the two fuels,Methanol has better fuel economy.Comparing the SI combustion mode,HCCI combustion mode,and SICI composite combustion mode of Methanol,it can be concluded that Methanol SICI composite combustion has a slight advantage in HC emissions,with high combustion efficiency.The disadvantage is that CO emissions are slightly higher,and other performance is between the two.Compared to SI combustion,it has a higher effective thermal efficiency,while compared to HCCI combustion,it has a lower maximum pressure rise rate.2.In order to clarify the mechanism of Methanol SICI composite combustion,the evolution of mass and heat transfer in the cylinder during the transition from composite combustion ignition to compression ignition mode was analyzed.The six stages of SICI composite combustion were summarized:mixture formation,spark ignition,flame propagation（ignition stage）,self-ignition point formation,compression ignition（coexistence of ignition and compression ignition）,and combustion annihilation stage.The composite combustion mode is the same as the ignition combustion mode before the occurrence of the self-ignition point,both of which are the continuous development of the flame surface to ignite the surrounding unburned mixture;after the appearance of the self-ignition point,the self-ignition combustion of the working fluid in the external area of the composite combustion flame surface rapidly develops,and the self-ignition combustion suppresses the development of the spark ignition flame surface.Different from ignition combustion,SICI composite combustion has a fast combustion speed and simultaneously involves flame propagation and spontaneous combustion,while ignition combustion only has one combustion mode of flame propagation.3.Based on the engine test platform,the author studies the control parameter range for achieving Methanol SICI composite combustion under the intake heating strategy.By analyzing the comprehensive weights of control parameters,it can be concluded that the core control parameter that affect the composite combustion P_me（Mean Effective Pressure）is intake temperature,and the core control parameters of CA10（combustion starting point）and CA50（combustion center of gravity）areλ（Excess air coefficient）,clarifying the correlation between control parameters and SICI composite combustion.4.The author defined the relevant parameters of Methanol SICI composite combustion segmentation and obtained the influence of control parameters on the composite combustion segmentation parameters.The core control parameters of composite combustion BP（Breakpoint）,SI duration,and SICI combustion duration areλ,The core control parameter of ignition and compression ignition ratio are BP.The influence of compression ignition ratio and BP point cylinder pressure and temperature on the duration of CI（Compression Ignition）is significant.The COV_imep（mean indicated pressure cyclic variation）and COV_pmax（peak cylinder pressure cyclic variation）of composite combustion have significant randomness,and the control parameters in the distribution areas with smaller values are similar.The increase in compression ratio can expand the lean burn upper limit of Methanol SICI composite combustion and reduce the lower limit of intake temperature.5.Using experimental and simulation methods to study the implementation of SICI composite combustion of Methanol under internal and external EGR strategies,the range of internal and external EGR for composite combustion was obtained.An increase in internal and external EGR rates will suppress combustion reactions,prolong the duration of SICI combustion,move the BP point backward,reduce NOx emissions,and unstable combustion in the cylinder.From the influence of different internal EGR rates on the micro field of SICI combustion,it can be seen that at the same time,as the internal EGR rate decreases,the spontaneous combustion area gradually increases,the area with higher unburned temperature increases,and the pressure difference increases.The ability to expand the spontaneous combustion point outward is enhanced,the combustion speed is accelerated,and the quality of carbon dioxide is reduced.The maximum EGR rate for achieving composite combustion under both internal and external EGR is around 10%.The P_mi（Mean Indicated Pressure）of external EGR is large,the combustion phase is advanced,the proportion of ignition phase is large,and the duration of SICI combustion is long.6.From the microscopic point of view,the correlation between in cylinder flow,temperature distribution and composite combustion characteristics is analyzed by studying the changes of combustion chamber shape and wall temperature.When studying the influence of combustion chamber shape,based on theω-type combustion chamber,the author designed five models with the same compression ratio.It was found that the cylinder flow of combustion chambers B,C,D,and E is conducive to organizational optimization of combustion.The BP’of combustion chamber B is the earliest,and the SI combustion ratio is the largest.Except for combustion chamber A,the combustion mode of other combustion chambers is ignition combustion.The average peak turbulent kinetic energy of the six combustion chambers during the combustion process after top dead center is 2.8 m²/s².The flow in the combustion chamber pit is mainly turbulent,and the flow velocity of the working fluid near the spark flame surface is higher in areas with higher unburned temperatures.In theωcombustion chambers,the C-type combustion chamber has the maximum P_mi,which can expand the composite combustion range of low load and lean burn conditions.The C-type combustion chamber is the optimal combustion chamber among the six types of combustion chambers.7.When studying the effect of wall temperature on SICI composite combustion,it was determined that the T_h range of wall temperature for achieving composite combustion was 360～440 K,and the P_mi value was the highest when T_h was 420 K.As the wall temperature increases,BP’moves forward,and there is a turning point in the combustion segmentation parameters at 420 K,with the entire combustion process mainly dominated by compression ignition;from the temperature distribution coefficient,it can be concluded that the number of high temperature discrete grids become more;from the temperature distribution pattern,it is known that the temperature of unburned area and burned area increases,and the distribution of high temperature area in the whole cylinder becomes wider.According to the analysis of flame propagation and spontaneous combustion characteristics under different temperature distribution patterns,with the increase of wall temperature,the time when the spontaneous ignition point appears becomes earlier,the propagation area and speed of spark and compression ignition flame become larger,the ignition and spontaneous combustion areas become larger,the pressure difference becomes larger,and the two flame surfaces interfere with each other.