The Effects Of Twice Diesel Injection On The Combustion Process Of Diesel/Natural Gas Dual Fuel Engine

As an important form of natural gas engine,diesel/natural gas dual fuel engine has been widely studied at present.The diesel/natural gas engine uses natura l gas as the main fuel,and a small amount of diesel fuel is injected into the cylinder to ignite natural gas.However,there are still some shortcomings in diesel/natural gas dual fuel engine,including low thermal efficiency and a large number of unburned HC and CO under middle and low loads.In order to find a method to solve the above problem with the fuel injection strategy,the effects of different fuel injection strategies on critical combustion parameters,combustion and emission evaluation indexes were studied on the CA6DL2-35E5 RB diesel/natural gas dual fuel engine based on the typical working conditions.Different injection strategies include many parameters,such as single injection timing(SOI)and fuel injection pressure(P),the first injection timing(SOI1)and the second fuel injection timing(SOI2)of the twice injection,the first injection mass ratio(m1)and the injection pressure(P)of twice injection.This paper will analyze the influences law of different strategies under middle and low loads based on A25(1335r/min,375 N·m)and A50(1335r/min,750 N·m)working conditions.First of all,SOI decides the ignition mode of pilot diesel when using the single injection of diesel oil.The combustion mode of the pilot diesel changes once SOI exceeds a certain time engine at low loads.In this combustion mode,the trade-off relationship between THC and NO x emissions can be solve and higher thermal efficiency can be achieved.The changes of combustion and emissions in cylinder with SOI are similar to that of low loads under middle loads.The difference is that the cylinder pressure and pressure rise rate of medium loads are too high to achieve the transformation of diesel combustion mode,but the earliest SOI can achieve the effective thermal efficiency level approaching the original diesel engine.Secondly,by defining SOI2-CA5,comparing the heat release curves of single injection and twice injection,combustion order of two diesel injections under different two fuel injection timing combinations can be predicted.For an early SOI2,there are three kinds of combustion sequence of two diesel injections.1.The first injection of diesel fuel burns first then the second injection of diesel fuel burns,and there is a certain interval between the two exothermic peaks.2.The second injection of diesel begins to burn quickly after the first injection of diesel fuel burning with only one exothermic peak.3.The second injection of diesel fuel burns first,then the first injection of diesel fuel burns,and there is a certain interval between the two exothermic peaks.And there are only first two circumstances for late SOI2.Third,in order to achieve higher thermal efficiency with twice diesel injections,SOI1 should be more advanced and the SOI2 should be made to ensure that the first injection of diesel burns first,and then second injection of diesel fuel,and ensure that the combustion of two diesel fuels has a certain interval,but the interval should not be too large,that is,the interval between the two exothermic peaks should not be too large.The effects of mass ratio and the injection pressure may be very different under different twice injection timing combinations.But,in general,the combustion and emissions performances are better when the two parameters are large in the middle.Finally,the single injection and twice injection are optimized for the A25 and A50 working conditions.In A25 working condition with 85% PES(percentage energy substitution),compared with the traditional injection strategy,the effective thermal efficiency of single injection increased by 7.72%(from 27.44% to 35.16%),that of twice injection increased by 6.92%(from 27.44% to 34.36%),and the effective efficiency of single injection is higher than twice injection.The THC emissions reduced by 81.4%(from 7275 ppm to 1353ppm),NO x emissions reduced by 50.3%(from 559 ppm to 278ppm)of optimized single injection;the THC emissions reduced by 56.9%(from 7275 ppm to 3139ppm)of twice injection but NOx emissions is almost a constant.In A50 working condition with 85% PES,compared with traditional injection strategy,the effective thermal efficiency of optimized single injection increased by 6.36%(from 31.64% to 38%),and the optimized twice injection increased by 6.72%(from 31.64% to 38.36%),the twice injection can achieve higher effective thermal efficiency.After optimization,the THC emissions of single injection decreased by 44.9%(from 8245 ppm to 4544ppm),and the THC emissions of the twice injection decreased by 61.5%(from 8245 ppm to 3174ppm).