| As the stringent fuel consumption and vehicle emission regulations,many advanece combustion modes and alternative fuels are adopted to achive high efficiency and clean comsbution.The traditional fuel design process initiates from available alternative fuels to its combustion/emission performance,and finally assesses if specific fuel is suitable for typical combustion mode and its advantages and drawbacks.However,proceeding this way for every emerging molecular structure of fuel is time-consuming and may not lead to a systematic improvement of the fuel design process.In this research a target-oriented fuel design concept is proposed that uses the combustion and emission requirements as design criteria to identify suitable molecular structures for fuel candidates as synthetic targets.Typically,a specific functional configuration of the fuel components corresponds to specific combustion mode.The required functional configuration of the fuel components for homogeneous charge autoigntion?HCAI?combustion mode is identified in this research.The study has four primary goals:The first target is to select the potential fuel candidates for HCAI combustion mode based on the fuel autoigntion?lower flammability limits,autoignition temperature?and oxidation?temperature and species profiles,ignition indicator,rate of production,reaction pathway?characteristics.First,the flammability limits of various fuel types are evaluated using the extended adiabatic flame temperature method.The autoigntion temperatures?AITs?of n-heptane,methanol,ethanol and butanol are measured and the primary oxidation reaction pathways of n-hpetane,methanol and ethanol are analyzed.The results indicate that the AITs of n-heptane range locate within 258304oC at the pressure of 0.7723.861MPa and at the equivalence ratio of 0.42.0 while the those of ethanol locate within 318442 oC.The large difference of the fuel reactivity between n-heptane and ethanol make them possible to establish the reactivity gradient by using the blending fuels.The second target is to investigate the effect of fuel reactivity gradient?constructed by blending high cetane number fuels and high octane number fuels?on the ignition,flame maintaining and extinction.From the perspective of the ignition,it contains the AITs,ignition delay and corresponding dominant reactions.The AIT range of the pure n-heptane is within 258<sup>304oC at the pressure of 0.7723.861MPa,at the equivalence ratio of 0.42.0,in other words,the maximum AIT difference is 46oC.While the maximum AIT differences are 56oC,62oC,84oC and 124oC as the ethanol blending ratios are 25%,50%,75%and 100%respectively.The essence of constracing fuel reactivity gradient is to establish the ignition delay gradient.The sensitive factors affecting the ignition delay time in descending order are initial mixture temperature,ethanol blending ratio,equivalence ratio and initial mixture pressure.The low temperature?550750K?ignition of n-heptane/ethanol mixture depends on the reaction rate of the H-atom abstraction from n-heptane;while the reaction type of the QOOH+O2=O2QOOH inhibits within the temperature range of750955K is the primary cause of the NTC region of n-heptane/ethanol oxidation.Therefore,the NTC behavior inhibited by adding ethanol is caused by the replacement of n-heptane by ethanol while the ethanol does not change the reaction pathway;the unimolecular fuel decomposition grows in importance at high temperature regime?9551250K?.From the perspective of the flame mainting,it contains the heat release process,combustion efficiency and dominant reactions.The60%n-heptane-20%PODE3-20%ethanol exhibits three stages of heat release including LTHR,1stt HTHR and 2nd HTHR.The LTHR is mainly caused by the PODE3 and the n-heptane is the second.The 1st HTHR attributes to the n-heptane and ethanol while the 2nd HTHR is derived from ethanol oxidation.The mixture temperature increase during the LTHR is 142.44K and the occurrence of CH2O is an indicator.The ethanol constituent inhibits the LTHR by providing the CH2O which consumes the OH radical by CH2O+OH=HCO+H2O.The mixture temperature at the 1st HTHR exceeds 1000K so that the H2O2 molecules decomposes to produce OH radicals which trigger the high temperature heat release process.During the 2nd HTHR,three major reactions including H+O2?+M?=HO2?+M?,HO2+OH=H2O+O2 and CO+OH=CO2+H are responsible for the main heat release process.At low load??=0.4?,the key to improve the combustion efficiency is to enhance the fuel-air mixture ignitability,therefore,the proportion of high reactivity fuel?like n-heptane?should be increased,therefore,the pure n-heptane acquires the highest combustion efficiency up to 81.06%at?=0.4.At mediate and high load??=0.61.0?,the ignitability is no longer an issue while controlling the heat release rate and improving the combustion efficiency become the top priorities.Hence,the proportion of high reactivity fuel?like n-heptane?should be reduced to cut down the amount of chemical ignition source at high load.At?=0.6,0.8 and 1.0,the combustion efficiency of n-heptane-ethanol mixture reach 95.56%,97.76%and 98.98%as the ethanol blending ratio reach 60%,40%and 20%.From the perspective of extinction,it contains the gas phase/particulate matter emissions and soot precursors formation.The CO/HC can be inhibited below the level of 1000ppm given that the combustion temperature exceeds 1400K while the NOx emission can be limited below 20ppm as the combustion temperature is lower 2200K.Therefore,the combustion temperature window for the HCAI combustion mode ranges from 1400K to 2200K which corresponds to the CO/HC oxidation limit and the NOx production limit.The PODE3 and ethanol are free of n-C4H3 emission and they can substantially reduce the C2H2 and C3H3 emission.Then-C4H3originatesfromthen-heptanethroughn-C7H16?C7H15-2?pC4H9??C2H5??C2H4?C2H3??C2H2??n-C4H3.Introducing 20%,40%,60%,80%and 100%ethanol into n-heptane-ethanol mixture,the acetylene emission decreases by 0.24%,4.76%,13.15%,31.19%and 68.98%compared to pure n-heptane.Similarly,the declines for C3H3 are up to 2.85%,8.22%,18.74%,40.52%and 99.09%respectively.The C2H2 and C3H3 emissions for pure PODE3 are 81.35%and 99.61%lower than the pure n-heptane.Furthermore,the PODE3 has a better effect on C2H2 reduction than ethanol.The third target is to identify the ideal functional configuration of fuel components for HCAI combustion mode.The combusiton and emission characterisctics of n-heptane-PODE3-ethanol are assessed using constant volume vessel and the fuel interaction for the heat release process is elucidated by the chemical kinetic simulation.The results indicate that the ideal functional configuration of fuel components for HCAI combustion mode is“chemical ignition source-PM inhibitor-homogeneous charge”.The n-heptane can act as the chemical ignition source to improve the combustion efficiency,the PODE3 can substantially reduce the PM mass and number emissions.The ethanol has high volatility so that it is suitable to form homogeneous charge.The 40%n-heptane-60%ethanol and 60%n-heptane-20%PODE3-20%ethanol can break the NOx-PM and NOx-HC/CO trade-off.Meanwhile,they break the?c-NOx and?c-PPRR direct proportional relationship at?=0.6.Generally,the specific combustion mode corresponds to an ideal functional configuration of fuel components,so that it provides a great freedom for fuel design since the fuel components selection is“function orientation”.Any substance that can fulfill the role without serious side-effects can be introduced to verify the combustion and emission performance.The fourth target is to propose a generalized fuel design concept that can be applied to various combustors and combustion modes.The target-oriented fuel design integrates the fields of fuel combustion/emission and the fuel synthetic process in a reverse direction First,the combustion and emission requirements are evaluated to identify the physicochemical properties demands?also known as functional configuration of fuel components?of the fuel.Then the combustion performance and fuel physicochemical properties are applied as the design criteria to identify the potential fuel candidates.Afterward,a retrosynthetic analysis from the fuel candidates to the feedstock is conducted to establish the new or adapt existing transformation pathway.Therein,the biochemical synthesis can fulfill the mission of CO2 footprint reduction and economic value generation simultaneously. |
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