Optimization Of Fuel Injection Strategy And Combustion Chamber For A Pilot Ignited Natural Gas Marine Engine

The traditional diesel engine has high emission and terrible pollution,which can not meet the increasingly strict emission regulations.Because of clean combustion and abundant reserves,natural gas is suitable for the alternative fuel of traditional diesel engines.Based on the research of L23 / 30 marine diesel engine,this paper transforms it into two kinds of natural gas dual fuel engines with different natural gas intake mode,and redesign the combustion chamber structure and injector arrangement of the original engine.For the pilot ignited intake injection natural gas engine,the combustion chamber were changed into different shapes.For the direct injection dual fuel engine,different injectior layouts were taken into account.gas intake port injection diesel direct injection dual fuel engine and the dual fuel cylinder direct injection engine respectively Based on the simulation of combustion and emission of the engine with injector arrangement,the optimization scheme of combustion chamber shape and injector arrangement for dual fuel engine is proposed.Based on the simulation results,the optimization of the combustion chamber shape and the injector arrangement of the dual fuel engine is put forward.The dual fuel combustion mechanism was constructed based on GRI3.0 mechanism of CH4 and Valeri mechanism of NC7H16.A three-dimensional CFD simulation model of coupled chemical reaction dynamics was established.Verify the validity of the spray model,combustion model and emission model based on test statistics.For the natural gas inlet injection diesel engine,the physical and chemical properties of diesel and natural gas are quite different.The combustion chamber structure of the original diesel engine is not fully suitable for natural gas combustion.Based on the structure of the original combustion chamber,this paper analyzes and compares the change of combustion performance of dual fuel by changing the throat diameter,depth ratio and extrusion shape.It is found that the turbulent kinetic energy in the cylinder of large bore engine is low and the air flow is weak,which is not conducive to the flame propagation of natural gas.The structure of reentrant shape is beneficial to form small-scale vortex.Increased squish area and higher squish flow intensity can be obtained by reducing throat diameter.Increased turbulent kinetic energy in the area of squish and near the wall,faster flame propagation speed and improved the combustion efficiency can be achieved,as well.However,when the throat diameter is reduced to a certain extent,deflagration will occur.Due to the low turbulent kinetic energy in the cylinder,the guide effect of the boss on the air flow is not obvious,but the existence of the boss can occupy the dead zone of the central air flow and improve the combustion efficiency of natural gas.The smaller the depth ratio is,the faster the turbulent energy dissipation is,and the combustion center of CA50 delays.The concave convex structure of the wall is conducive to the formation of vortex motion at the corner of the wall.The cruciform structure has the largest corner.So,the vortex duration in this chamber is the longest.Also,the turbulent kinetic energy in the cylinder is always significantly higher than the other two chambers.The octagonal structure has the smoothest wall and the weakest air flow in cylinder.For the direct injection dual fuel engine,the distance between the injector affects the relative position of the pilot flame core and the natural gas jet,and then affects the ignition effect of the natural gas.When the jet passes through the inner part of the diesel flame,the natural gas is ignited from the circumferential direction.The ignition efficiency is higher and the ignition delay period is shortened.Due to the difference of fuel distribution,the influences of injector distance on combustion under two arrangements are different.Under the bilateral symmetry arrangement,fuels are concentrated in the half of the cylinder.The flame propagation speed is slower.The afterburning phenomenon is serious.The soot emissions are higher.When the distance of injectors is increased,the condition of rich combustion can be relieved.Under the centrosymmetry arrangement,the jet flames propagate clockwise after impinging the wall and fuels are distributed in the whole cylinder.When the distance of injectors is increased,the flame propagation distance increases and the combustion duration prolongs.Comparing the two schemes,it is found that the centrosymmetry arrangement can improve the mixture distribution and flame propagation in the cylinder.Higher IMEP can be obtained.The combustion center CA50 is closer to the top dead center.The combustion process has high isochority,which is helpful to improve the engine power performance and combustion thermal efficiency.However,the NOx emissions are increased.For the engine type studied in this paper,the combustion and emission performance of the engine is better when the relative distance between the oil and gas injectors is 90 mm with centrosymmetry layout.Both too long and too short injector distances lead to higher maximal rate of pressure rise and higher ringing intensity.