Experimental And Kinetic Modeling Study Of The Autoignition Characteristics Of Diesel And Large Molecular Weight Hydrocarbons Under Low-to-Intermediate Temperature Range

In the foreseeable future,fossil fuels will still play a dominant role in the transportation field.In addition,more stringent regulations,such as Stage 6 emission standard for light-duty vehicles and the Phase 4 average fuel consumption limit(5L/100 km)for passenger cars,will soon come into effect.This largely promotes the innovation of internal combustion engines in energy saving and emission reduction.HCCI-based(Homogeneous Charged Compression Ignition-based)new combustion technologies can simultaneously achieve high thermal efficiency and ultra-low emission,and are recognized as the direction of advanced internal combustion engines.Fuel ignition under these new combustion modes is dominated by reaction kinetics,and therefore the control of fuel autoignition is the core issue of the successful application of these new technologies to real engines.Hitherto,there are only few researches on the autoignition studies of real diesel or large molecule weight hydrocarbons in the low-to-intermediate temperature range.Considering this scarcity,this dissertation studied the low-to-intermediate temperature autoignition of three kinds of large molecule weight single-component hydrocarbons,two types of commercial diesels,and two types of diesel surrogates in a rapid compression machine(RCM).The study helps to deepen the understanding of autoignition of large hydrocarbons and diesel fuels,provide reliable data for surrogate development and model validation,and lay a fundamental theoretical foundation for fuel autoignition control in the advanced combustion modes.Based on the main hydrocarbon classes that constitutes diesel fuels,chapter 3-5 of this dissertation focuses on three types of typical large hydrocarbons: ncetane,iso-cetane,and decalin.Ignition delay times(IDTs)of the fuels in the low-to-intermediate temperature ranges were measured to study their autoignition characteristics,and effects of pressure,equivalence ratio,and oxygen mole fraction on IDT were systematically investigated.As for n-cetane,the representative of linear alkane class,the two-stage ignition behavior and NTC characteristics of IDT were experimentally confirmed.Besides,two mechanisms describing n-cetane oxidation(LLNL model and CRECK model)were verified,and suggestions of model modifications in the low-temperature sub-mechanism were also presented.As for iso-cetane,the representative of branched alkane class,the existence of low-temperature reactivity was experimentally confirmed.Results reveal that the first-stage ignition features “early and weak”,and the total IDT exists distinct “low-T NTC” behavior.This dissertation has tried best to explain the unique autoignition behavior of iso-cetane,and to validate and analyze an existing kinetic mechanism.As for decalin,the representative of naphthene class,the complete NTC phenomenon within 750-860 K was observed.More importantly,modifications were made to the existing decalin model so as to improve its predictability of decalin autoignition in the low temperature range.Based on the above research foundation of the autoignition of singlecomponent hydrocarbons,chapter 6 of this dissertation made a further step to investigate the autoignition characteristics of two Chinese commercial diesels(Stage-V and Stage-VI)under engine-relevant conditions.Experiments show that the two diesels also exhibit low-temperature reactivity and NTC characteristics,which are similar to the single-component hydrocarbons.The dissertation focuses on the composition effects on diesel autoignition by comparing the composition differences and IDT discrepancies of the two fuels.It is found that a higher naphthene content will prolong first-stage IDT and postpone the NTC behavior.Chapter 7 continues to explore the autoignition and oxidation characteristics of two diesel surrogates composed by typical components from chapter 3-5.Experiments show the IDT performance of the three-component surrogate matches well with that of Stage-V diesel,while the five-component surrogate better fits the Stage-VI diesel.The IDT difference between the two surrogates well corresponds with that of the two diesels,which further confirms the composition effects on autoignition.Modeling simulation indicates that the POLIMI mechanism can well reproduce the autoignition and oxidation characteristics of the three-component surrogates under wide operation ranges,whilst there still exists some discrepancy in the prediction of the first stage IDT.It is found that the addition of decalin into the five-component surrogate is primarily responsible for the lengthened first-stage ignition delay time.Furthermore,kinetic analysis was conducted to reveal the role of each hydrocarbon component of the surrogates during autoignition.