Experimental Research And Simulation Optimization On Working Process And Performance Of High Compression Ratio Engine Fueled With Liquefied Methane Gas

The promotion and application of natural gas engine is an important way to optimize China’s energy structure,alleviate energy crisis,and achieve energy conservation and emission reduction.It is also an important measure to control fog.At present,the research on the performance of natural gas engines does not combine well with the fuel characteristics of natural gas and the engine operating characteristics.It does not fundamentally solve the problem of low thermal efficiency caused by low compression ratio and slow combustion.However,increasing the compression ratio will lead to an increase in knocking tendency and NOx emissions,and this problem is also difficult to solve.The performance of natural gas engines and the potential for energy saving and emission reduction are far from what we expected.In order to solve the above problems,this thesis proposes a new combustion technology scheme for natural gas engines,and carries out a series of of experimental and simulation optimization studies to explore the energy saving potential of the new combustion technology scheme.Based on the engine control and operation line detection method proposed by our research group,a heavy-duty truck LNG engine is used as a research prototype.Through the collaborative optimization of “fuel design(purification of natural gas,hydrogenation)” and “design/operation parameters(compression ratio,ignition advance angle,etc.)”,the law of variation between compression ratio,ignition advance angle and combustion process,performance and emissions is explored and summarized.According to the basic principles of thermodynamics,the thermal balance model of liquid methane engine is established to reveal the heat-work conversion process and the influence mechanism of thermal balance in the cylinder.Based on the experimental data and the quantitative relationship between engine design,operation,and control parameters,a onedimensional performance simulation model of the liquid methane engine is established to simulate and optimize the working process and performance of the liquid methane engine.By coupling the chemical reaction kinetics mechanism,a three-dimensional CFD simulation model of the liquid methane engine for hydrogenation combustion is established,and detailed simulations of in-cylinder combustion and thermodynamic processes are conducted to further explore the influence mechanism and improvement methods of the combustion and emission performance of the liquid methane engine.The research results of this thesis show that:(1)The compression ratio has little effect on the instantaneous heat release rate of the liquid methane engine and the 10-90% combustion duration,but has a significant effect on the ignition delay.As the compression ratio increases,the ignition delay is shortened,the combustion timing is advanced,and the combustion phase is advanced.As the compression ratio increases,the torque of the liquid methane engine can be increased by 9.5%,and the BSFC can be reduced by up to 10.9%.When the compression ratio increases too much,the BSFC of the liquid methane engine is only slightly reduced.The reduction of the liquid methane engine BSFC is the result of an effective expansion ratio increase and a combustion phase advance.(2)The compression ratio has little effect on the NOx emissions of liquid methane engines under low speed and low load conditions.As speed and load increase,NOx emissions of liquid methane engine increase dramatically.It is only necessary to adjust the ignition advance angle in a small range to achieve the NOx emission level of the original LNG engine,and has little effect on the BSFC.The retarded ignition timing can effectively solve excessive NOx emission in the liquid methane engine.When the engine is burned with liquid methane,the knocking tendency is reduced,the effective compression ratio can be expanded,and the compression ratio of the liquid methane engine can be increased to about 14.6 with other parameters unchanged.(3)It is important to select the best SOC(or ignition timing)and adjust the HRR according to the change of EER for the improvement of high pressure cycle efficiency.The combustion stability can be improved and the heat transfer loss of the liquid methane engine can be reduced by selecting suitable operating conditions.Engine speed has a greater impact on the high pressure cycle efficiency of liquid methane engines than load.For low speed conditions,the effective thermal efficiency of the liquid methane engine can be further improved by optimizing the thermodynamic process.For high speed conditions,reducing friction losses and pumping losses is more effective.(4)Compared with the intake timing,the effect of exhaust timing on the gas exchange process,the energy distribution of the in-cylinder process,and the power and economy of the liquid methane engine are more obvious.In a certain range,the exhaust timing can be properly advanced to improve the thermal efficiency of the liquid methane engine,thereby achieving the purpose of simultaneously improving the power and economy.Appropriate adjustment of the combustion phase has little effect on the power and economy of the liquid methane engine,so the emission index can be optimized by appropriately adjusting the ignition timing.(5)Hydrogenation has a significant effect on the in-cylinder combustion rate,mainly affecting the combustion phase and 10-90% combustion duration to affect the in-cylinder circulation efficiency.When the hydrogen energy fraction is between 8% and 12%,the indicated thermal efficiency reaches a maximum.The effect of hydrogenation on specific heat ratio and heat transfer loss is relatively small.Reasonable optimization of combustion phase is an effective way to further improve the indicated thermal efficiency of the liquid methane hydrogenation engine.The above research results show that the technical scheme of “purifying natural gas-increasing compression ratio-hydrogenation” proposed in this thesis can further improve the power and economy of natural gas engines.These provide technical guidance and data support for the design and development of a highly efficient clean combustion system of liquid methane engine,thereby enhancing the energy-saving and emission reduction potential of natural gas engines.