Influence Of Injection Strategy On Combustion And Emissions Of Natural Gas/Diesel Dual Fuel Engine

As a low-carbon fuel,natural gas is regarded as a high-quality alternative fuel that can achieve efficient and clean engine combustion and reduce CO₂.It has been widely used in the transportation field,but natural gas engines have poor combustion stability under small load conditions and the bottleneck limit of the replacement rate cannot be improved.In order to promote its further application.In this paper,numerical simulation methods are mainly used to study the effects of different injection strategies on dual-fuel engines in light of low load and high substitution rate conditions,and to explore the possibility of efficient and clean combustion.Firstly,a natural gas port injection model was constructed based on the test data to study the effects of diesel pre-injection strategy and pre-injection coupled EGR strategy on the combustion and emissions of dual-fuel engines.Secondly,a direct injection model of natural gas was constructed and the jet model was verified by free jet and wall-impinging jet,and studied the angle between the axis of the natural gas jet and the horizontal direction,the relative angle between the two jet axis directions,the timing of the natural gas jet,and the direct injection/airway.The effect of the compound injection ratio on the mixing process of natural gas,the concentration distribution in the cylinder and the ignition and combustion process.On this basis,several injection strategies are designed and compared.Studies have shown that the use of appropriate pre-injection strategies in the natural gas port injection mode can optimize the cylinder temperature,pressure and combustion phase,and can keep emissions other than NOx at a low level.If the pre-injection timing is unchanged,only the pre-injection amount is increased,or the pre-injection amount is unchanged and only the pre-injection timing is advanced,which can accelerate the combustion rate,increase the temperature and pressure,and advance the combustion center of gravity.The strategy of pre-injection combined with EGR slows down the combustion rate in the cylinder,the overall combustion is postponed,pressure and temperature are reduced,and NOx emissions are reduced,but Soot,CO,CH₄,and HC emissions will increase with the increase of EGR rate.Under this working condition,the coupling strategy of 15%EGR rate,-20℃A ATDC pre-injection timing,and 30%pre-injection proportion is adopted,and the main emissions are lower than the original single injection.Compared with the port injection of natural gas,direct injection of natural gas in the cylinder is unevenly distributed in the cylinder during fuel injection,and more natural gas-diesel-air mixture is formed,which makes it easier to reach the ignition limit,which improves the premixed gas to a certain extent.In the case of uniform over-lean,the overall heat release is more,the cylinder pressure and heat release rate are both on the rise,and the HC and CO in the emission decrease,and the NOx rises accordingly.Different horizontal anglesαmake different concentration distributions in the cylinder.Whenα=50 and 60,a large-scale stable vortex structure can be formed in the combustion chamber,which entrains a large amount of natural gas,and forms a local rich mixture area.After fuel injection,it is affected by the interaction of the fuel jet and the original vortex in the cylinder,and the fuel jet wraps around.With the influence of the suction effect and the flow of the Coanda jet,the fuel mixes more fully.Therefore,with the increase ofα,the cylinder pressure gradually increases;the double peak of heat release rate gradually becomes single peak and the peak value gradually increases;the combustion center of gravity is closer to the top dead center;NOx emissions rise,and HC,CO,and CH4 emissions decrease.α=50 is higher than30,cylinder pressure is 31.2%higher,CA50 is advanced by 4 degrees,NOx increases by 2.48 times,HC,CO,and CH4 decrease by 36.1%,68.9%,and 39.0%,respectively.The relative intersection angleβwill affect the mixing process in the cylinder after fuel injection.Whenβ=0 and 10,a layered distribution centered on the lip position and a large concentration difference will be formed;whenβ=20,a concentration difference centered on the pit will be formed.Smaller hierarchical distribution;whenβ=30 and 40,a large amount of diesel-natural gas-air mixture is formed by flow and distributed in a wide range in the cylinder.Therefore,asβincreases,the cylinder pressure gradually increases;the double peaks of the heat release rate move closer to a single peak and the peak value gradually increases,and the heat release center of gravity is advanced;NOx emissions rise,and HC and CO emissions decrease.The peak pressure ofβ=40 is about 24.8%higher than that ofβ=0,and the peak heat release rate is increased by 1.92 times;NO emissionβ=30 is 3.45times that ofβ=0;β=40 HC and CO emissions are the lowest,respectively lower thanβ=0 85.4%,80%.The timing of natural gas injection is delayed from-50℃A ATDC to-30℃A ATDC.Natural gas accumulation can be formed near the pit in the combustion chamber.During the subsequent injection,the mixture is better under the action of the flow in the cylinder,and more diesel is produced-The natural gas-air mixture speeds up the combustion rate,the heat release rate and the cylinder pressure are higher;when it is postponed to-20℃A ATDC,the mixing is insufficient,the diesel distribution range is small,and the combustion is mainly carried out near the compression clearance of low temperature and weak flow.Cylinder pressure and heat release rate are 10.4%and 22.3%lower than the former;the mixing time is the shortest when it is postponed to-10℃A ATDC,natural gas accumulates on the bottom and central area of the combustion chamber,and the distribution range of pilot diesel is smaller.The first three jet timing emissions have little difference,SOI=-20℃A ATDC reduces NOx emissions by 44.4%compared with the first three,and HC,CO,and CH4 emissions increase by 151.2%,71.6%,and 150.1%respectively.When natural gas uses direct injection/portal compound injection,as the proportion of direct injection increases,more natural gas participates in combustion,speeds up,and releases more heat.Therefore,cylinder pressure,heat release rate and cylinder temperature gradually increase,eliminating NOx emissions Gradually increase,the remaining emissions gradually decrease,and the change range of HC and CH₄ is based on the direct injection ratio of 40%.When it is lower than 40%,the decrease will be slower,and the follow-up will accelerate.The in-cylinder mixing and combustion processes are different under different injection strategies.Option 2 Natural gas inlet port injection,diesel fuel is injected twice in advance,the first diesel injection is earlier and of higher quality,there is sufficient time to form a uniform natural gas-diesel-air mixture,and a large-area simultaneous fire starts before the top dead center is reached,The combustion presents a single peak heat release,the heat release rate is fast,the combustion center of gravity is at the top dead center,the cylinder temperature,cylinder pressure and the pressure rise rate are relatively large;Scheme 4 natural gas in-cylinder direct injection,diesel two injections,the first time is more Early fuel injection,but late jet timing and short effective mixing time.In addition,the amount of fuel injected for the second time is small,and the lower equivalent ratio delays the ignition start point,and the overall heat release shifts back.With natural gas port injection,diesel fuel is injected multiple times in advance than the single injection near top dead center,NOx in emissions increases by 134.9%,HC decreases by 95.2%,CO and CH₄ are almost zero;single injection near diesel top dead center,natural gas cylinder Compared with the port injection,the NOx increases by 388.5%,but the HC,CO,and CH₄ are reduced by90.7%,80.2%,91.2%respectively;the direct injection of natural gas and the two injections of diesel are compared with the gas port injection,HC,CO,CH₄ emissions were reduced by 43.5%,15.5%,and 65.1%respectively;natural gas uses compound injection,compared with the direct injection mode of natural gas,the amount of natural gas involved in combustion is reduced,and the main emissions can be balanced.