| Spark ignition-controlled auto-ignition(SI-CAI)hybrid combustion confronts problems in combustion phasing control and cyclic variation under high dilution conditions.In order to solve these problems,the concept of dimethyl ether(DME)micro flame ignition(MFI)was proposed,where small amount of DME is directly injected into the cylinder to form micro flame ignition source in place of SI to ignite incylinder mixture.The motivation of this study is to gain more comprehensive understandings in its influence mechanism on heat release control and suppressing cyclic variation of hybrid combustion under high dilution conditions.Based on the studied results,a predictive model has been established to calculate the start of main combustion(SOC_main)of DME MFI hybrid combustion.Firstly,the influence of DME MFI on combustion heat release has been studied in detail through combustion visualization experiments in an optical engine combined with CFD and chemical kinetics simulation.The results indicate that with DME start of injection(SOI)later than 40°CA BTDC,DME is more concentrated and easier to autoignite to form high energy ignition source.Then the mixture is ignited to form flame propagation until the auto-ignition of unburned gas.The whole combustion behaves as three stage sequential heat release characteristics: micro flame ignition-flame propagation-auto-ignition.The DME SOI,which determines the formation of ignition source,can be used to control combustion phasing.While in the cases with DME SOI earlier than 40°CA BTDC,DME is mch more disperse,the ignition delay of DME mixture is too long to form ignition source.In such cases,DME promotes the combustion by enhancing the overall mixture reactivity,where combustion is mainly governed by chemical reaction kinetics,and DME SOI can not be used to control combustion phasing.Secondly,the influence of MFI strategy on combustion cyclic variation in high dilution conditions has been studied.Through the comparison between SI-CAI and DME MFI hybrid combustion,it is realized that with higher ignition energy and larger ignition zone,MFI is beneficial in stabilizing SOC_main,accelerating the early combustion heat release and shortening flame propagation duration,which are helpful in suppressing the combustion cyclic variations.With EGR rate less than 18%,the standard deviation of SOC_main in DME MFI dropped by 1.1°CA compared to the SICAI case.In the cases with EGR rate up to 37.7%,the DME SOI can be used to linearly control SOC_main and cyclic variation of combustion phasing can be maintained under 1.5°CA.At last,a predictive model of the SOC_main for MFI hybrid combustion has been established with the influence of in-cylinder stratification taken into consideration.It is hypothesized that once the temperature of the top 5% mixture with shortest ignition delay reached a certain threshold,combustion was initiated.Then the effects of DME mole fraction and EGR ratio on the auto-ignition temperature have been studied.The results indicate that the auto-ignition temperature drops with higher DME mole fraction,and the auto-ignition temperature changes linearly with EGR rate.Based on the studied results and the existing framework of the auto-ignition temperature threshold judgment criterion,the predictive model of the SOC_main for DME MFI hybrid combustion is established and integrated in GT-Power platform to simulate the cyclic variation of SOC_main.Validation results show that the error of model calculated SOC_main is less than 2°CA compared with the experimental results within the studied operation range.And the model calculated cyclic variation of SOC_main increases from 0.28°CA to 1°CA with EGR rate from 27%~37.7%,which corresponds well with the experimental results.On the other hand,the error between calculated and experimental SOC_main in 50 continuous cycles is less than 1°CA under the condition of EGR rate 27% and DME SOI fixed at 30°CA BTDC.The results validate the accuracy and effectiveness of the model. |
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