Numerical Investigation On Combustion System Optimization Of High Efficiency Natural Gas Engine Base On Knock Control

In order to meet the Euro-Ⅵ emission regulations,lean burn natural gas engines need to be equipped with selective catalytic reduction(SCR),which increases engines cost.The stoichiometric operation natural gas engine can meet the regulatory requirements just requiring exhaust gas recycling(EGR)and three-way catalytic converter(TWC).However,the thermal efficiency of stoichiometric operation natural gas engine is low,and knocking is one of the important reasons for suppressing the increase of its thermal efficiency.Therefore,the present study investigated the effects of intake manifold,combustion chamber,Miller cycle and EGR on combustion and knock of stoichiometric operation natural gas engine by means of three-dimensional simulation.The major objectives of this study are to expand the knock boundary and explore the potential and technical measures to further improve the thermal efficiency of natural gas engine.The results of this study should have great theoretical significance and practical value to the development of combustion system of natural gas engine with high thermal efficiency.Firstly,the effects of different intake manifold and combustion chamber geometries on combustion and knock of natural gas engine were studied.The results show that both of the tangential and spiral intake manifold + K15 chamber and double tangential intake manifold + S5 chamber can generate strong turbulent kinetic energy and airflow speed in the near spark plug region,which increases the flame propagation speed,shortens the combustion duration,makes the combustion phase closer to the TDC and improves the antiknock and indicated mean pressure(IMEP).However,the asymmetry of flame surface development suppresses the further improvement of antiknock and IMEP.As the flame surface development of the double spiral intake manifold+S12 chamber is more symmetrical,which can improve the antiknock performance,the ignition timing can be advanced,thus the IMEP is improved.However,the low turbulence kinetic energy and the relatively weak airflow movement of this scheme restrain the further improvement of IMEP.The IMEP of double tangential intake manifold + S5 chamber and double spiral intake manifold + S12 chamber are similar and are both higher than that of the original intake manifold + S12 chamber,and the IMEP of the tangential and spiral intake manifold + K15 chamber is the highest.As a result,the combination of tangential and spiral intake manifold and K15 combustion chamber has the highest IMEP and was selected for further study.Then,the influence of Miller cycle on knocking and combustion of natural gas engine was studied on the basis of tangential and spiral intake manifold + K15 combustion chamber.The results show that Miller cycle can improve the antiknock performance of natural gas engine.However,considering the effects of intake boosting and effective compression ratio,the late closing angle of intake valve should be 20 (?)A(M20),which can obtain the best antiknock performance of and the highest IMEP.Therefore,the influence of EGR on the combustion and knock of natural gas engine is studied on the basis of M20.The results show that EGR can significantly reduce the incylinder temperature and improve the antiknock performance.Whereas,excessive EGR rate would deteriorate the combustion process.Therefore,with 30% EGR,the natural gas engine has better antiknock performance and the highest IMEP.All above,with tangential and spiral intake manifold + K15 combustion chamber,combined with late closing of intake valve(20(?)A)and EGR rate(30%),the IMEP of the new optimal combustion system within the knock boundary can be increased by5.21% compared with the original combustion system.Therefore,the present study can provide important reference for the development of stoichiometric operation natural gas engine.