| In order to cope with the global energy shortage and environmental pollution,the scientific has started to construct feasible solutions from the aspects of renewable biological alternative fuels and new combustion methods.In terms of bio-alternative fuels,alcohol fuels such as ethanol and butanol can be made by fermentation from biomass raw materials.Currently,it has a relatively advanced production technology and a considerable production worldwide.According to researches carried out in compression ignition engines,bio-alcohol fuel is an ideal alternative fuel,which can reduce emissions while ensuring dynamic performance under operating conditions.At the same time,in-depth studies have found that the molecular structure characteristics of alcohols,including the hydroxyl position,carbon chain structure and carbon chain length,have a greater impact on the development of combustion flames and the combustion,performance and emissions of compression ignition engines.In terms of combustion methods,Reactivity Controlled Compression Ignition(RCCI)has proved to be a clean,efficient and controllable combustion mode,and has great fuel flexibility.The use of RCCI mode in combination with compression ignition engines and alcohol fuels has great prospects in dealing with energy shortage and environmental pollution.Therefore,in order to further combine the alcohol fuels and RCCI mode in the compression ignition engine,and to solve the defect of high HC and CO emissions of RCCI itself,the constant speed and total cycle energy in this paper were 1500 r/min and 2047 J respectively.By combining the boundary parameter oxygen ratio(Ro=8% ~ 14%)and start of injection(SOI=-8°~-25°CA ATDC),the effects of four alcohol fuels with different molecular structure characteristics,including ethanol,n-butanol,sec-butanol and iso-butanol,on the combustion,performance and emission characteristics of RCCI were studied.The main conclusions are as follows:(1)The position of the hydroxyl group and the structure of the carbon chain hardly affect the RCCI change rule caused by the Ro.With the increase of Ro from 8% to 14%,the ignition delay increases,the combustion duration gradually shortens,and the maximum heat release rate and cylinder pressure,maximum cylinder temperature and the pressure rise rate are continuously decreasing;the effective thermal efficiency,combustion efficiency and break mean effective pressure are decreasing,and the break specific fuel consumption is gradually increasing;HC and CO emissions are also increasing,and the difference between NOx and soot emissions is small.However,because the alcohol fuels with different carbon chain lengths resule in different oxygen contents,there is a certain difference in the injection amount under the same Ro.Therefore,under the influence of the Ro,the RCCI's heat release rate,pressure rise rate,combustion efficiency,and NOx and soot emissions show different or even opposite rules,while the other rules are similar.(2)Three different molecular structure characteristics,such as hydroxyl position,carbon chain structure and carbon chain length,have little influence on the change rule of RCCI caused by the boundary parameter SOI.Alcohol fuels with different molecular structures have similar rules with the change of SOI.As the SOI advances from-8 ° CA ATDC to-25 ° CA ATDC,the ignition delay shortens first and then increases,the combustion duration increases first and then shortens,the maximum heat release rate first decreases and then increases,the maximum cylinder pressure,temperature and pressure rise rate is increasing;the effective thermal efficiency and the break mean effective pressure show a slight increase and then decrease,the combustion efficiency increases,and the break specific fuel consumption decreases and then increases;HC and CO emissions decrease overall,and NOx emissions continuously increases,soot emissions showed a slight increase before decline.(3)Alcohol fuels with different hydroxyl positions,carbon chain structure and carbon chain length show certain differences in the combustion,performance and emission characteristics of RCCI.Compared with n-butanol,which has ?-hydroxyl groups and linear carbon chains,sec-butanol with ?-hydroxyl group and iso-butanol with branched carbon chains are used in RCCI,showing a longer ignition delay and shorter combustion duration,higher maximum heat release rate,cylinder pressure,pressure rise rate and lower maximum cylinder temperature under most operating conditions.In addition,it also has lower combustion efficiency,relatively higher HC emissions and slightly lower CO emissions.Compared with short-chain ethanol fuel,n-butanol fuel which with a longer carbon chain exhibits a longer ignition delay and a shorter combustion duration,lower maximum cylinder pressure,effective thermal efficiency,combustion efficiency,the break mean effective pressure and higher break specific fuel consumption.And,long-chain alcohols have lower HC and CO emissions and slightly higher NOx emissions under most operating conditions.When Ro is 8% and 10%,the RCCI combustion of long-chain n-butanol have a higher maximum heat release rate and a higher maximum pressure rise rate than that of short-chain ethanol,but the situation was reversed when Ro is increased to 12% and 14%.(4)Alcohol fuels with different molecular structures show certain differences in the combustion,performance and emission characteristics of RCCI,and the extent of the differences is affected by the boundary parameters Ro and SOI.In addition,among the three different molecular structure characteristics,the difference caused by the hydroxyl group position is relatively smallest,followed by the carbon chain structure,and the difference caused by the carbon chain length is the largest.(5)the injection of alcohol fuel into the RCCI in a high proportion is very unfavorable to the performance,HC and CO emissions of the engine,etc.,and a too low injection proportion will also lead to more soot and NOx emissions. |
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