| The energy crisis and environmental pollution caused by the automobile industry have been the focus of global attention,so the search for new clean alternative fuels and new efficient combustion methods has become a topic of research for scholars of internal combustion engines.Hydrogen-doped natural gas is an ideal combination of natural gas reserves,high volumetric calorific value,clean and renewable hydrogen,fast combustion rate,and wide ignition limits.The combustion method of homogeneous compression ignition combines the characteristics of homogeneous mixture of gasoline engine and high thermal efficiency of diesel engine,which is a new combustion technology with potential development.Therefore,the combination of hydrogen-doped natural gas fuel and the new combustion method of homogeneous compression ignition is highly advantageous.Since the addition of hydrogen improves in-cylinder combustion while also causing a significant increase in NOx emissions,and too fast a combustion rate leads to a high rate of pressure rise and is prone to detonation.The use of internal EGR strategy can effectively reduce NOx emissions while mitigating detonation in homogeneous compression-ignition engines,while changing the hydrogen doping ratio can help regulate the balance between engine power,economy and emissions.Therefore,the application of internal EGR coupled with hydrogen doping to a hydrogen-doped natural gas homogeneous compression-ignition engine is of high research value.In this paper,a simulation model of hydrogen-doped natural gas homogeneous compression-ignition engine was firstly built by GT-Power and CHEMKIN software,and the engine cylinder pressure and heat release rate curves were simulated based on the model,and the reliability of the model was verified by comparing with the test values.Then,the effects of internal EGR rate and hydrogen doping ratio on the engine power economy,combustion characteristics and emission characteristics were analyzed by the simulation model.Therefore,the engine performance under the synergistic effect of internal EGR rate and hydrogen doping ratio is simulated and analyzed,and the best solution of coupling internal EGR rate and hydrogen doping ratio is finally determined by considering the engine performance.Finally,the effects of speed,excess air coefficient and compression ratio on the combustion and emission of the hydrogen-doped natural gas homogeneous compression-ignition engine were simulated by changing the operating parameters under the selected coupling scheme.The results show that:with the increase of the internal EGR rate,the ignition start point is advanced and then delayed,and the maximum cylinder pressure,maximum temperature and peak exothermic rate of combustion all decrease to different degrees;the increase of the internal EGR rate has a positive effect on reducing NOx emission,and the engine economy can be slightly improved when the internal EGR rate is small,but when the internal EGR rate is large,it will have a negative effect on the engine power,economy and CO emission.economy and CO emissions.Increasing the fuel hydrogen doping ratio can effectively improve the engine power economy and reduce CO emissions caused by the internal EGR rate,but too large a hydrogen doping ratio will lead to an increase in negative compression work,which will affect the engine performance.Considering the performance of the engine under this condition,the best coupling scheme is to introduce an internal EGR rate of 17.5%and a fuel hydrogen doping volume fraction of 40%.Under the selected coupling scheme,increasing the excess air coefficient can significantly reduce the NOx emission,but the peak cylinder pressure,maximum temperature and heat release rate will also be significantly reduced;increasing the compression ratio will lead to early ignition,and the combustion and emission pattern is opposite to increasing the excess air coefficient;while increasing the speed does not have a significant effect on the in-cylinder combustion. |
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