Optimization On The Combustion System Of Natural Gas Engine Based On Stoichiometric Operation Mode

The natural gas engines operated in stoichiometric mode,which only need to be equipped with a three-way catalyst(TWC),can effectively reduce emissions and keep low cost and has become the mainstream technology for meeting China VI emission standard.However,the knocking tendency,heat transfer loss and pumping loss of the stoichiometric operation mode are large,resulting in low thermal efficiency.In this work,for the development requirements of China VI natural gas engine operated in stoichiometric mode,the numerical and experimental investigations on the optimization of combustion system including the inlet and combustion chamber were carried out.The objectives of this study are to explore the ways to improve the combustion process and thermal efficiency of stoichiometric operation natural gas engines and to provide guidance for the development of efficient and clean natural gas engine combustion technologies.Firstly,the CFD model of natural gas engine was established by using CONVERGE software to simulate the effects of different inlet and combustion chamber structures on in-cylinder flow,combustion and performance of natural gas engine.The results show that reentrant combustion chamber can generate large region of high turbulent kinetic energy in cylinder and around the spark plug and high indicated mean effective pressure(IMEP),but the flame propagation has a large degree of asymmetry,which makes the combustion duration prolong.The reentrant combustion chamber with smaller compression ratio leads to stronger in-cylinder turbulent kinetic energy,but lower IMEP due to the reduced compression ratio.As the necking angle increased,the tumble ratio,turbulent kinetic energy,and IMEP of reentrant combustion chamber decrease.Nevertheless,the combustion chamber with smaller necking angle,double tangential inlet and the matching scheme of tangential & spiral inlet all increase the turbulent kinetic energy in cylinder and around the spark plug.Specially,the matching scheme of tangential & spiral inlet limits the flame development located in the intake valve side and prolongs the combustion duration due to high airflow speed and biased direction toward the exhaust valve side,which decreases the IMEP.However,the double tangential inlet matching with the S12-5 reentrant combustion chamber has the highest IMEP.Based on the above numerical simulation results,an experimental study was carried out to investigate the effects of preferred combustion chamber scheme,EGR and ignition control strategies on the performance and emissions of stoichiometric operation natural gas engine.The results show that as the EGR rate increases and the ignition timing advances,the indicated thermal efficiency(ITE)increases first and then decreases.Therefore,the EGR rate should be increased and the ignition time should be advanced appropriately to obtain better combustion phase under the condition that the engine does not knock or misfire,thereby reducing the exhaust temperature while obtaining high ITE.Compared with straight combustion chamber,the fuel/air mixture in reentrant combustion chamber can be burned steadily in a larger EGR rate range,by which the ITE is higher and the corresponding EGR rate is larger.Compared with the original reentrant combustion chamber,the compression ratio is appropriately reduced by deepening the exhaust valve yield pit and shortening the pit depth,it’s minimum ignition timing of maximum brake torque(MBT),CA10 and CA50 are advanced,the ITE is improved and the corresponding EGR rate is smaller.As a result,the S11.5combustion chamber has obvious advantages with the highest ITE 40.94% and corresponding specific gas consumption 195.20 g/(kW·h),respectively.