The Simulation Investigation About Effect Of Design Parameters Of Nozzle In A Dual Fuel Engine

Natural gas has many advantages,such as abundant reserves and less pollution for air.It has great significance on improving energy structure and reducing air pollution in China.Diesel/natural gas dual fuel engine is a form of natural gas utilization.There are some problems such as high hydrocarbon emissions and low thermal efficiency in low loads for Diesel/natural gas dual fuel engine.Hydrocarbon emission can be reduced and the thermal efficiency can be improved by changing the diesel injection pressure,injection timing,air-fuel ratio,natural gas energy substitution rate.The change of injection time and injection pressure affect the diesel distribution in cylinder before combustion and atomization effect.Design parameters of injector nozzle can also change the diesel distribution and atomization effect in the cylinder.This paper established simulation platform using CFD simulation software and studyed the influence of different injection timing,injection pressure and changing the nozzle design parameters for dual fuel engine in high natural gas replacement rate,low loads and low speed conditions.The design parameters of the nozzle include the spray cone angle,hole diameter,hole number.The investigation shows advancing injection within certain range,the peak pressure of the cylinder increased,and the shape of heat release rate changed from double peak to one peak.The unburned natural gas decreased at the end of combustion.The distribution of diesel from mass to annular,and the distribution position of diesel in the cylinder changed.A significant advance of diesel injection timing,increase the injection pressure in the cylinder will cause the distribution of diesel from mass to annular before combustion of diesel,diesel distribution position will be changed,the peak of heat release rate increased,the combustion rate and the peak cylinder pressure rise.When the diesel injection pressure is too high,the start of combustion retard,a lot of diesel fell to the top of the piston,the CA50 has a retarding trend,the maximum pressure droping.There are many conclusions for changing spray cone angle,hole diameter and hole number under the different diesel injection pressure:(1)Compared four different spray cone angle: when diesel injection pressure is 40 MPa,the heat release rate has only one peak,peak pressure of cylinder is higher for 152 and 160 degrees,CA50 is earlier and the combustion duration is shorter.When diesel injection pressure is 70 MPa,the shape of heat release rate changes from single peak to double following the decrease of spray cone angle,all the peak pressure of cylinder is high and the value is close except the 120 degrees,CA50 is earlier for 160 and 152 degrees,the combustion duration is shorter and the combustion rate of natural gas is fast.When diesel injection pressure is 100 MPa,the shape of heat release rate has one peak,the peak pressure of cylinder is highest while the spray cone angle is 120 degrees,CA50 is earlier and the combustion duration is shorter.Larger spray cone angle such as 152 and 160 degrees should be selected when the diesel injection pressure is 40 or 70 MPa,but smaller spray cone angle such as 120 degrees should be selected when the diesel injection pressure is 100 MPa.(2)Under three different diesel injection pressure: compared four different spray cone angle of lower layer,the heat release rate has one peak,peak pressure of cylinder?CA50 and the combustion duration are close.Compared three different hole diameters,the heat release rate has one peak and CA50 are close,the peak pressure of cylinder is higher and the combustion duration is shorter.There is no obvious influence for changing the lower layer spray cone angle under three different diesel injection pressure,smaller hole diameters should be selected.(3)Under 40 and 70 MPa diesel injection pressure,compared three different hole numbers,8 and 10 holes' heat release rates are single peak structure,the peak pressure of cylinder is higher and CA50 is earlier,while the heat release rate has double peak for 16 holes'.Under 100 MPa diesel injection pressure,all the heat release rates have only one peak,the peak pressure of cylinder is close.For the 8 holes',the peak pressure of cylinder is highest,CA50 is earliest and the combustion duration is longest.The 8 and 10 holes' have best combustion effects under three different diesel injection pressures.Besides the injection pressure of 100 MPa,the combustion rate of natural gas slows down by increasing the number of holes.(4)Most of the diesel is located near the junction of the combustion chamber and the top of piston before combustion,the peak value of the cylinder pressure is higher,the heat release rate is a single peak,the combustion duration is shorter and the CA50 is earlier,the combustion rate is faster of natural gas.On the one hand,the diesel oil in cylinder is circular distributed,the contact area of diesel and natural gas is large,the combustion overlap area of diesel and natural gas is wide.The ignition points of diesel and natural gas combustion is much more in the initial stage of combustion.On the other hand,the air flow is strong at the bulge in the cylinder,and the flame will spread easily to the adjacent unburned area at the early stage of combustion.The combustion rate of the fuel is fast and the high temperature zone expands rapidly after the stable fire source is formed in the cylinder.The diesel injection and the nozzle design parameters affect the distribution position of the diesel fuel in the cylinder and the atomization evaporation.Different injection parameters determine the optimal design parameters of the different nozzle design parameters,and the selection of the nozzle design parameters should be combined with the injection strategy.