Research On Reaction Mechanism And Knock Of Low-speed Dual-fuel Engine With Low Injection Pressure Of Natural Gas

The strict regulations on the marine emissions have been proposed by IMO.Using natural gas to replace diesel can significantly improve the SOx and NOx emissions of ships.Because of the pre-chamber system and premixed combustion,the dual-fuel engine with low injection pressure of natural gas has the advantages of low emissions and can satisfy IMO Tier III emission limits without any post-process device or EGR.However,the knock caused by premixed combustion prevents the improvement of engine power.The narrow combustion limit of natural gas increases the difficulty of air-fuel ratio control.Therefore,this thesis uses the method of numerical simulation to analysis the influence of working process on the knock in dual-fuel engine,such as the flame jet from pre-chamber and mixing process of natural gas and air,to find out the methods of reducing the engine knock.First of all,this thesis builds a three-dimensional model of dual-fuel engine with low injection pressure of natural gas by the CFD software CONVERGE.Embed grids to the important parts and set up the initial conditions and boundary conditions for the model.Then,the spray of diesel in the pre-chamber is validated by the spray experiments of similar diameters.This thesis develops a new diesel-NG dual-fuel reaction mechanism for dual-fuel engine with big bore.The n-heptane represents diesel and methane represents natural gas.The reaction mechanism GRI-Mech 3.0 of natural gas is optimized in the part of low temperature.The dual-fuel reaction mechanism is constructed by modifying GRI-Mech 3.0 and skeleton reaction mechanism of n-heptane.The new reaction mechanism is validated by the ignition delay and laminar flame velocity.Then the new reaction mechanism is applied to the dual-fuel engine model.Then,the analysis of the air flow in the pre-chamber,the mixing process of natural gas and air,the propagation process of flame jet and flame surface are carried out to find their relation to the knock.The effects of the swirl on propagation process of the flame and the mixing process of natural gas and air are studied.The combustion processes of lean combustion and high equivalence ratio combustion are compared.The three-stage heat release of dual-fuel engine in pre-chamber is proposed.Finally,the influence of the parameters such as swirl,mass of the diesel and EGR rate on the combustion process and knock of the dual-fuel engine with low injection pressure of natural gas is studied.It is found that the enhancement of the swirl in the cylinder will prevent the spontaneous combustion of the natural gas at the far end before the flame reach,and it can reduce the possibility of knock.Increasing the mass of the diesel will lead to multi-point ignition and multiple flame surfaces,and this can also increase the possibility of knock.EGR can reduce the temperature in the cylinder and this can prevent the spontaneous combustion of natural gas and reduce the possibility of knock.As described above,the knock of low-speed dual-fuel engine with pre-chamber can reduce by increasing the swirl and EGR rate or reducing mass of the diesel.