| Utilizing advanced low-temperature combustion(LTC)modes combined with alternative fuels is an important approach to improving the combustion and emission performance of an engine.Meanwhile,using renewable carbon-neutral alternative fuels is a critical way to achieve zero carbon emissions for an engine.As a carbon-neutral fuel,polyoxymethylene dimethyl ethers(PODEn)exhibits a great potential for improving engine performance.Moreover,compared with conventional diesel combustion,many LTC modes,such as homogeneous charge compression ignition(HCCI),partial fuel stratification(PFS),partially premixed combustion(PPC),and reactivity-controlled compression ignition(RCCI),offer considerable advantages in reducing both nitrogen oxides(NOx)and soot emissions.However,at present,how to effectively combine PODEn and these advanced LTC modes and then fully take advantage of PODEn to improve the engine performance has not been clear.Therefore,the combustion and emission performances of LTC modes were comprehensively investigated in this study based on a single-cylinder light-duty diesel engine fueled with gasoline and PODEn.To achieve this goal,a complete experimental test system for the engine was firstly built based on the CompactRIO and CompactDAQ platforms.Then,the combustion stability and dominant factors of HCCI,PFS,and RCCI were investigated in detail using this experimental test system.Besides,to overcome the limitations of the current LTC modes in improving the performance of the engine used in the study,the concept of RCCI with reverse reactivity stratification(R-RCCI)was proposed based on PODEn,and the effects of the premixed ratio(PR)and start of injection(SOI)timing on the performance of R-RCCI were investigated simultaneously.In addition,the performance of R-RCCI was compared with that of PPC and RCCI,and the differences in the mixture organization and combustion processes for the three combustion modes were also explored.(1)The combustion stability of HCCI,PFS,and RCCI was investigated based on PODEn.The results indicated that the cyclic variations of RCCI can be considerably reduced with PODEn as the direct-injection(DI)high-reactivity fuel of RCCI owing to its higher reactivity and better volatility compared with diesel.For RCCI,the trends of cyclic variations of burning duration(BD),peak pressure(PP),and indicated mean effective pressure(IMEP)were nearly identical,and the cyclic variations of PP and IMEP are significantly influenced by BD.Among the three combustion modes,PFS shows the best combustion stability.This can be explained by two main factors.First,there is no significant difference in intake temperature(Tin)for PFS and HCCI,but it is considerably higher than that of RCCI.The higher Tin effectively reduces the sensitivity of the ignition and combustion process to the fluctuations of intake conditions and thereby makes the combustion process more stable.Second,compared with HCCI,the in-cylinder higher local fuel concentration of PFS can effectively improve its ignition stability,and thereby reducing the cyclic variations.(2)The dominant factors of HCCI,PFS,and RCCI were explored at low-load conditions based on gasoline and PODEn.The results showed that for gasoline and P20G80(PODEn/gasoline blends with the volume fraction of 20%/80%),compared with fuel reactivity,Tin plays a dominant role in the combustion process of HCCI at the current test conditions.Therefore,the lower Tin of P20G80 results in lower combustion rate and NOx emissions,but it also leads to lower combustion efficiency and thermal efficiency.Different from HCCI,the combustion process of PFS is dominated by fuel reactivity rather than Tin at the same test conditions,and thereby the combustion rate of P20G80 is higher than that of gasoline.However,the higher combustion rate of P20G80 does not lead to a rapid rise in combustion temperature,so its combustion efficiency and NOx emissions are lower than that of gasoline.This is because the higher ?sensitivity of P20G80 effectively reduces the combustion rate,thereby limiting a rapid increase in combustion temperature.Different from gasoline PFS and gasoline/P20G80 RCCI,for gasoline/PODEn RCCI,the combined effects of the higher?sensitivity of PODEn and lower Tin play an important role in reducing the combustion rate.Therefore,gasoline/PODEn RCCI can be operated at a considerably earlier CA50(50%burn point)without knock compared with gasoline PFS and gasoline/P20G80 RCCI,so its thermal efficiency is greatly improved.Meanwhile,at the same CA50,the SOI timing of gasoline/PODEn RCCI is the earliest among the three combustion modes,thereby avoiding the formation of over-rich regions and reducing NOx and soot emissions.(3)Although PODEn exhibits a great potential for improving the engine performance in gasoline/PODEn RCCI,only 37%indicated thermal efficiency is obtained for the engine used in this study.Therefore,the current engine configurations may not be appropriate for RCCI.For this reason,the concept of R-RCCI was proposed based on PODEn,and the performance of R-RCCI was simultaneously investigated at the same test conditions as that for RCCI.The results indicated that R-RCCI can effectively break the trade-off between combustion efficiency and NOx emissions when adjusting the SOI timing.This is because the combustion efficiency of R-RCCI is mainly dominated by the spray location rather than CA50.By comparing the performance of PODEn/gasoline and PODEn/P20G80 R-RCCI,it was found that the maximum thermal efficiency of the latter is significantly higher than that of the former.This is mainly because the higher reactivity of P20G80 effectively reduces the sensitivity of CA50 to SOI timing,so its SOI timing can be further retarded to increase combustion efficiency and thermal efficiency.At the current test conditions,43%indicated thermal efficiency can be obtained with R-RCCI,which is significantly higher than that of RCCl.(4)To further explore the potential of R-RCCI,the performance of R-RCCI was compared with PPC and RCCI at low-to-medium loads.The results showed that from PPC to RCCI via R-RCCI,the decreased local fuel concentration in the cylinder results in a lower combustion rate,which leads to worse combustion efficiency and combustion stability.With the increase in intake pressure(Pin),the reduced combustion temperature results in worse combustion stability and combustion efficiency for all the three combustion modes.However,the magnitude of the effect of Pin on the combustion efficiency and combustion stability of the three combustion modes is different.With the increased degree of premixed combustion from PPC to RCCI via R-RCCI,the magnitude of the effect of Pin on combustion efficiency and combustion stability becomes larger.Among the three combustion modes,R-RCCI has the best fuel economy because of its optimal CA50 and combustion rate,and simultaneously its NOx and soot emissions are lower than that of PPC.RCCI has the lowest soot emissions at low loads,but because its soot emissions are very sensitive to the volatility and fuel masses of the DI fuel,its soot emissions significantly increase at medium loads and exceed the Euro VI emission regulations.Compared with RCCI,R-RCCI has a better balance between the degree of premixed combustion and combustion temperature,so its soot emissions are very low at both low and medium loads. |
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