Study On Influence Of Diesel And Natural Gas Injection On Reactivity Controlled Compression Ignition Combustion Characteristics Of A Dual Fuel Engine

Low temperature combustion is an effective way to achieve high efficiency and low emission of engine,and it is a hot topic in the world.Among them,the Reactivity Controlled Compression Igniton combustion technology adopts the mixture organization mode of injecting low reactivity fuel in the intake port and injecting high reactivity fuel in the cylinder and the ignition mode of compression combustion,which can not only ensure the high efficiency of the engine and low NOx emission in a large operating range,but also make the start point phase and rate of fuel combustion can be flexibly controlled through adjusting the mixture concentration stratification in the cylinder.However,the problem of high HC and CO emissions at low load limits the operation range of the combustion mode.In order to provide a theoretical way to solve this problem,this paper,based on a turbocharged high-pressure common rail natural gas/diesel dual fuel engine,used the method of combining test and simulation,under the condition of 1000r/min speed and 25%load,to explore the influence of gas injection parameters and fuel injection parameters on the combustion characteristics of RCCI and investigated fully the combustion process of fuel in cylinder.The study found that proper retarded natural gas timing could form a concentration stratification for CH₄ in the cylinder to improve the combustion efficiency of CH₄.Based on this,when the diesel first injection timing was earlier than-50°CA ATDC,and the diesel first injection quantity was in the range of 40%-60%,the starting point of the combustion of diesel and natural gas could be moved to the edge area of the combustion chamber,so that CH₄ in this area be fully burned.This situation not only reduced CO emissions,but also reduced NO production,which due to shortening combustion duration and reducing high-temperature areas in the cylinder.It also do not lead to rough engine operation caused by high cylinder pressure rise rate.In addition,the unburned amount of CH₄ and the production of CO could also be reduced by advancing diesel second injection timing,but this lead to the early combustion reaction which can produce negative work,so that the power performance of the engine was reduced.Based on the above research,the response surface design method was used to analyze the significance of four parameters,including natural gas injection timing,diesel first injection timing,diesel first injection quantity and diesel second injection timing.The analysis results showed that the weak stratification of natural gas concentration had no significant effect on engine performance compared with diesel concentration stratification in cylinder.Therefore,the fuel injection parameters are selected to analyze the interaction between the two parameters on engine performance.The results show that advancing diesel first injection timing and diesel second injection timing can help to accelerate the fuel combustion rate,which makes CA50 close to the compression TDC,and can not lead to the maximum pressure rise rate is too large.So the CH₄ combustion efficiency was improve and the amount of CO production was reduced.But at the same time,the increase of the high temperature region in the cylinder also increased the NOx production.The results showed that the advance of the pre-injection timing and the main injection timing can help accelerate the fuel combustion rate,make the CA50 close to the top dead center of compression,and not cause the maximum pressure increase rate to be too large,which improves the combustion efficiency of CH₄ and reduces The amount of CO generated.However,at the same time,the increase in the high temperature area in the cylinder will also increase the amount of NOx.In the aspect of the coupling effect of the diesel first injection quantity and the diesel second injection timing on the engine performance,the effect of the diesel second injection timing on the performance of the dual-fuel engine gradually became significant with the decrease of diesel first injection quantity.When the diesel first injection quantity increased and the diesel second injection timing advanced,the phase of the starting point of the mixture combustion was advanced and the combustion rate of the mixture was accelerated.Although it made the emissions of unburned CH₄ and CO was decreased,the NOx emissions was increased.