| Natural Gas is recognized as one of the most promising alternative fuels for internal combustion engines with its abundant reserves,low price and superior combustion performance.At home and abroad,it has begun to focus on the development of natural gas engines.Among them,diesel micro-ignition natural cylinder high-pressure direct injection engine has attracted more attention due to its superior dynamic performance,emission performance and high natural gas replacement rate.However,at this stage,all the CNG and LNG engines are essentially gaseous injections,which have problems such as low charging efficiency,high soot emission,and reduced power.Compared with gaseous injection,in-direct injection of LNG(liquid natural gas)for combustion can reduce emissions on the basis of ensuring the dynamic performance,and can also solve the problem of low inflation efficiency caused by gaseous injection,but there are still many technical difficulties for in-direct injection that need to be broken.At the same time,due to the ultra-low temperature characteristics of LNG,the ignition condition of liquid injection is more severe than that of gaseous injection.This paper focuses on the frontier research topic of diesel-ignited natural high-pressure direct injection engine.The numerical simulation method is used to simulate the ignition condition and pilot diesel injection of high-pressure direct injection diesel/natural gas engine with different state natural gas as fuel.An exploratory study was conducted on the effects of timingand ignition diesel on engine performance.Based on the L 21/31 marine medium speed diesel engine combustion chamber of MAN Company of Germany,this paper establishes a simulation model of direct injection natural gas in diesel ignition cylinder with AVL?FIRE software.Under the premise of verifying the reliability of the model,the natural gas fuel in different states is studied.The ignition condition of the engine and its influencing factors are analyzed.Then,based on this,the influence of ignition diesel injection timing and ignition diesel mass on the combustion,emission and dynamic performance of the direct injection dual-fuel engine is discussed.And comparative analysis of the impact of using different states of natural gas as fuel on the engine performance.Hope to provide theoretical guidance for the optimization of diesel-ignited natural cylinder direct injection engine.The conclusions under numerical simulation are as follows:1)Diesel ignition natural cylinder internal high pressure direct injection combustion has a wide range of normal operation.By gradually approaching,the ignition fuel quantity is gradually increased from 0%.It is found that for the high pressure direct injection combustion in gaseous and liquid natural cylinders,the ignition boundary is respectively To ignite diesel fuel by 0.4% and 0.5%.When the amount of ignition diesel is greater than the ignition boundary,the combustion in the cylinder is always normal combustion,and there is no knocking or partial combustion.2)For a diesel/natural gas dual-fuel HDPI engine,under the premise that the pilot diesel injection timing remains unchanged,there is a critical interval angle.When the injection interval angle is greater than the critical value,the ignition boundary(the lowest pilot diesel quantity)does not change when the injection interval angle increases.When less than the critical value,the minimum pilot diesel quantity increases as the interval angle decreases.When liquid natural gas is replaced by gaseous natural gas(NG),the ignition boundary is always unaffected by changes in the jet spacing angle.3)Under the premise that the pilot diesel and natural gas injection intervals remain unchanged,when different states(liquid,gaseous,gas-liquid two-phase)natural gas are used as fuel,the average pressure and average in the cylinder are delayed with the ignition timing of the pilot diesel.The peak temperature is continuously reduced,and the peak occurrence time is continuously delayed.As the range of the high temperature region is reduced,the amount of NOX generated is reduced,so that the NOx emission is continuously reduced,and the power of the modified dual fuel engine is delayed with the injection timing.The power consumption is reduced,but the power performance is reduced.4)Under the premise of igniting diesel and natural gas injection,when using different states(liquid,gaseous,gas-liquid two-phase)natural gas as fuel,as the amount of ignition diesel increases under 10%,the average pressure in the cylinder and the average temperature The peak value is slightly reduced,and the time is slightly delayed,but the change is not large,but the amount of NOx generated increases with the increase of the amount of diesel fuel,which leads to an increase in NOx emissions,but the emission is low.The power of the dual-fuel engine fluctuates with the increase of the amount of micro-ignition diesel,but the fluctuation range is small,which can meet the performance requirements.5)Under the same working conditions,as the proportion of natural gas fuel gas and liquid continues to increase,the average peak pressure in the cylinder increases continuously.From pure liquid natural gas to gas-liquid ratio of 1:1,the peak pressure in the cylinder is increased by 10%.It is more obvious,and when the gas-liquid ratio continues to increase,the peak pressure in the cylinder is only increased by 2.5% until the pure gas injection,and the influence is small.At the same time,the peak pressure in the cylinder is at 732 °CA.In the range of 1 °CA from left to right;the average temperature in the cylinder increases with the increase of the proportion of gas and liquid,and the occurrence time is always advanced,which is obvious.The NOx emission decreased first and then increased with the increase of the gas-liquid ratio.Under the same conditions,the 1:1 gas-liquid mixed natural gas direct injection reduced the NOx emissions of pure liquid and pure gaseous direct injection by 83.3%.And 71.4%,the emission reduction effect is better. |
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