Investigation On Controllable Ignition And Combustion Characteristics Of Mixture Formed By Two-direct Injection Dual Fuel In Constant Volume Chamber

In recent years,the dual fuel combustion mode has received widespread attention due to its flexibility and advantages in efficient combustion and less emission.However,most of the current studies on dual fuel combustion have focused on engine bench tests,during which low reactivity fuel is injected into the intake fold and the homogeneous mixture is ignited by the diesel directly injected.In order to meet the more stringent regulations,the two-direct injection method has attracted attention.This subject was based on two-direct injection constant volume combustion chamber(CVCC).Using pressure measurements,a high-speed camera and an intensified charge coupled device(ICCD)camera,we investigated the ignition and combustion characteristics of the mixture formed by two-direct injection.The effects of injection timing,boundary conditions,such as equivalent ratio and temperature(T),and fuel injection strategies,on dual fuel ignition and combustion characteristics were studied,which revealed the fuel concentration distribution in the chamber plays an important role.In different boundary conditions,combustion and emissions could be optimized by adjusting fuel injection strategy.At the same time,the similarities and differences of Diesel/Alcohol Fuel with different carbon number dual fuel combustion were researched.The main conclusions are as follows:1.Two different fuel injection sequences while using two-direct injection method had an important effect on the ignition and combustion process.Taking Diesel/Methanol Dual Fuel(DMDF)as an example,in the method of methanol injected before diesel,the interval between methanol and diesel(?tm)could change the concentration distribution of methanol in the CVCC.By optimizing?tm,efficient and clean combustion could be achieved,which had a longer ignition delay,higher AHRR,ultra-low or no soot generation,referred to as"optimal interval window"conditions.While methanol was injected later than diesel,because methanol missed the diesel combustion process,the ignition and combustion characteristics of diesel in DMDF were the same as those in diesel conditions(D).Methanol failed to be fully ignited by diesel and the oxidation of methanol was slow.2.Ambient conditions and fuel injection strategies had an important effect on the formation of the mixture and its ignition and combustion processes using two-direct injection.There was an“optimal interval window”under different boundary conditions.As the equivalent ratio of methanol(?_m)increases,the“optimum interval window”corresponding to?t_m advances.Under high T,the differences between DMDF and D disappeared gradually,and there was no"optimal interval window"condition.Under conditions of high?_m or T,methanol was liable to spontaneous ignition.The combustion of methanol promoted the ignition of diesel,and the soot increased drastically.The injection strategy of diesel had more significant effects on the ignition and combustion characteristics of Diesel/Ethanol Dual Fuel(DEDF)than that of ethanol.At low equivalent ratios of ethanol(?_e),large pilot fuel mass and moderate interval between ethanol and diesel(?t_e)were conducive to DEDF combustion,and at high?_e,less pilot fuel mass and shorter?t_e were more effective.As the mass of the pilot injection(m_dpi)or the interval between the pilot and the main injection(?t₁)increased,more ethanol would be ignited.The multi-peak combustion occurred and the combustion duration became longer.In order to avoid the deterioration of soot generation while taking into account the AHRR,shorter?t_e was more suitable.Double pilot injections were equivalent to increasing m_dpi,and there was no significant difference from the single pilot injection in reducing the AHRR peak,but the soot significantly increased.Compared with pilot injection,the post injection could better control the soot while decreasing the AHRR peak,but the combustion phase moved backward.For the triple injection strategy,comprehensively considering the AHRR and soot generation,diesel should be allocated to the three injections as much as possible,and?t_e should be shorter.For the"optimal interval window"in the single injection strategy,where ethanol was ignited dramatically in multiple injections,so the ignition delay was the shortest,and the natural flame luminosity was the strongest,and the soot increased significantly.Different ethanol injection strategies had little influence on the"optimal interval window",and double ethanol injection strategies had a slight advantage.3.The formation,ignition and combustion characteristics of the mixtures of different alcohol fuel and air had similar trends with the change of the interval between alcohol and diesel(?t)using two-direct injection.With the increase of?t,the ignition delay became longer and then shorter,and the natural flame luminosity became weaker and then stronger,and the shape of AHRR changed from m model to h or n model.Overall,ignition delay of DMDF was the longest,followed by DEDF,and that of Diesel/n-Butanol Dual Fuel(DBDF)was the shortest.The AHRR peak of DBDF was the highest,followed by that of DEDF,and that of DMDF was lowest.In terms of average SINL,DMDF was the lowest,DEDF was the second,and DBDF was the highest,which indicated methanol had the strongest inhibitory effect on soot generation.All of methanol,ethanol and n-butanol had an inhibitory effect on diesel ignition.From the analysis of oxidation reaction path of DMDF,DEDF and DBDF at the low temperature,alcohol fuel and n-heptane competed for OH·,and the OH·contribution of n-butanol was higher.Therefore,n-butanol had a weaker inhibitory effect on n-heptane ignition than methanol and ethanol.4.The ICCD camera could record the formation of OH·during the DEDF ignition process.Using an ICCD camera with a Semrock filter(centered at 315 nm with 15 nm FWHM)located before the lens,it was observed that ethanol inhibited OH·formation,and the inhibitory effect increased with the increase of ethanol concentration.And the high-speed camera could record the smokeless combustion process of alcohol ignited by trace diesel.Under super-exposure shooting conditions,the high-speed camera could record the flame of methanol,ethanol and n-butanol,which was ignited by trace diesel.Compared with complete overexposure of diesel diffusion combustion flame images,the flames of DMDF,DEDF and DBDF appeared slightly yellow or light blue,all of which achieved clean combustion with ultra-low or no soot.