Research On Performance Optimization Strategy And Numerical Simulation Of High Compression Ratio Ignition Natural Gas Engine

With the development of the automobile industry and the large population growth,the energy crisis and environmental pollution have caused people to panic.The energy shortage crisis and the increasing environmental problems have forced people to invest a lot of manpower and financial resources to find an alternative clean energy as the fuel for internal combustion engines to get rid of dependence on crude oil.Natural gas is an alternative energy source for internal combustion engines with great development potential.Compared with gasoline and diesel,which are traditional internal combustion engine fuels,it has the advantages of high hydrogen-carbon ratio and high low heat value.However,due to its slow flame propagation and low thermal efficiency,it has not been favored by the market for many years.Most of research on natural gas engines is still staying in the modification and adjustment of diesel engines or gasoline engines.It was not considered the laws of various variables from their physical and chemical properties,internal combustion engine design parameters and operating parameters,so there are lot of digging space for energy-saving and emission-reduction of natural gas engines.In order to make up for the above deficiencies in the study of natural gas engines,this study used experiments combined with CFD multi-dimensional model simulation to improve the burning rate of natural gas in the cylinder from three different ways,and conducte a comprehensive study on the combustion and emissions performance of natural gas engines to improve thermal efficiency and increase the potential for energy-saving and emission-reduction.Firstly,a lean-burn natural gas engine was been transformed,an additional gas supply system was added on the basis of the original natural gas supply system.And according to the test requirements,the engine carried out steady-state tests with different speeds,loads and fuels.Combustion characteristic parameters and emission performance of various working conditions,the rules of combustion and emission performance of engines with different purity natural gas,different hydrogen fractions,and different carbon dioxide fractions were obtained,and the causes and effects of the differences were analyzed.Then,The effects of CO₂ dilution,H₂ blending,and different ratios of methane,ethane,propane,and n-butane in natural gas on the laminar flame velocity were investigated by using the simulation of the one-dimensional laminar flame of natural gas with different components,through the analysis of the laminar flame structure,the calculation of the laminar flame velocity,the sensitivity of the chemical reaction kinetics that affects the laminar flame velocity.In addition,a three-dimensional（3D）full-size combustion chamber including detailed intake and exhaust ports was established.And three-dimensional modeling technology was used to design and adjust the original combustion chamber.The CFD mode was simulated with different combustion chambers and double spark plugs.Finally,the effects of various combustion speed increasing strategies on engine combustion and emission performance were compared and analyzed.The combustion mode of dual spark plug ignition was selected,and for its NOx emissions,four different EGR ratios were simulated limit using CFD model.to the combustion heat release rate,and balance between NOx emissions and fuel economy.The research results of the paper is showed as followed,（1）The burning speed of LNG is faster than that of LMG,but the difference occurs in light load conditions.With the increase of speed and load,the difference between burning speed is getting smaller and smaller.Due to the LNG purity problem,the engine will delay the ignition advance angle due to knocking under heavy load,reducing thermal efficiency.In terms of emissions,due to the faster combustion speed of LNG and the higher combustion temperature,the NOx emission level is slightly higher than that of LMG,and increases with the increase of load.In terms of HC,there is no significant difference between LNG and LMG.Under certain operating conditions,LNG’s HC emissions are slightly greater than LMG.（2）Hydrogen doping can greatly increase the combustion speed of the gas mixture of natural gas engines.With the increase of the volume fraction of hydrogen,the 50%combustion point is advanced,the duration of 10%-90%is shortened,the effective thermal efficiency increases,NOx emissions increase rapidly,and HC The amount of emissions decreases as the volume fraction of hydrogen increases.Increasing the volume fraction of carbon dioxide in the combustion mixture slows the combustion rate and also greatly reduces engine NOx emissions.（3）Among methane,ethane,propane and n-butane,under the same external conditions,the laminar flame velocity of ethane is the largest,the laminar flame velocity of methane is the smallest,and the laminar flame velocity of propane and n-butane are different.Very small.When methane is blended with ethane,propane,and n-butane,the laminar flame velocity of the mixed gas increases most significantly with the decrease of the methane mole fraction.And in the methane-ethane mixed gas,when the ethane mole fraction is less than 40%,the laminar flame speed is the most significant increase,and its slope decreases with the increase of the ethane mole fraction,when it is greater than 40%,the laminar flame The speed increases nearly linearly with the mole fraction of ethane.（4）Mixing hydrogen in methane can increase the free radicals in the reaction and increase the reaction temperature to increase the laminar flame speed.Mixing carbon dioxide in methane reduces the speed of the reaction,inhibits the progress of the R31 reaction and reduces free radicals in the reaction The concentration,which reduces the velocity of the laminar flame.（5）Hydrogenation,modified combustion chamber and dual spark plug mode can effectively increase the combustion rate of natural gas mixture in the cylinder and improve the thermal efficiency.Among them,the hydrogenation method improves the ignition duration to10%and the position of 50%combustion point.The modification of the combustion chamber to increase the intensity of turbulence in the cylinder has a greater impact on the flame propagation speed in the medium term.The hydrogenation method has the highest indicated thermal efficiency,but also has the highest amount of NOx emissions.In general,the parameters of the dual spark plug ignition mode are easy to adjust and control.The optimized combustion strategy using dual spark plugs and EGR can effectively reduce NOx emissions while increasing the combustion rate.Through the research in this paper,we summarized various ways to solve the problem that natural gas improves the combustion rate,deeply studied the influence mechanism of different components in natural gas on its combustion rate,compared three methods,and proposed a modification strategy of dual spark plugs and EGR.It provides a good research method for in-depth understanding of engine thermal power conversion,provides an efficient and clean upgrade and optimization route for natural gas engines,and provides guidance for increasingly stringent emission regulations,as well as for the design and development of natural gas rapid combustion systems.Technical guidance and data support.