【作者】 耿超；

【导师】 尧命发；

【作者基本信息】 天津大学 ， 动力机械及工程， 2019， 博士

【摘要】 为了实现内燃机高效清洁燃烧,均质压燃（HCCI）、反应活性控制压燃（RCCI）等多种燃烧模式相继提出,重整制氢技术在内燃机中也得到了广泛应用。氢气活性单一,这限制了内燃机缸内充量活性随工况变化的灵活控制,而燃料低温氧化产物的多样性为内燃机缸内活性的灵活变化提供了可能,且低温氧化可以根据边界条件变化具有灵活的产物组合,同时不受催化剂选择和活性优化的限制,低的重整温度更容易实现;因此,低温重整在实现内燃机全负荷高效清洁燃烧方面具有很大潜力。为此,本文提出了内燃机低温重整燃烧新模式,同时针对燃料低温重整对发动机燃烧影响机理进行探究。本文搭建了一套发动机低温重整系统,该系统可以实现重整条件的灵活控制;本研究还建立了在线定量气相色谱（GC）检测系统,对低温重整产物进行检测,运用光学诊断方法（高速摄影成像法和平面激光诱导荧光法）对缸内燃烧火焰发展进行深入分析,同时还利用CHEMKIN软件结合详细的化学反应机理对重整产物进行活性评价,以及对重整燃烧进行化学动力学分析。首先,本文对HCCI低温重整燃烧机理进行了研究,研究结果发现,重整温度较低（423 K）时,PRF0（正庚烷）和PRF50都不会发生低温氧化;低温重整产物主要包括氢气、一氧化碳、烯烃、醛类、烷烃、炔烃、醇类和酮类;相比于PRF50,PRF0能产生更多的重整产物。低温重整能推迟HCCI着火,减缓HCCI燃烧速率,促进更多燃料的完全燃烧,进一步抑制缸内燃烧过程中的碳烟生成。每一种低温重整产物的活性不同,大部分低温重整产物都会降低缸内充量活性,这应该最终导致了HCCI发动机测试中着火时刻的推迟。相比于PRF0,促进PRF50着火的重整产物明显增多,而大部分重整产物对PRF50燃料着火时刻的影响能力减弱。低温重整可以作为解决HCCI燃烧相位控制困难、燃烧速率过快等问题的有效途径。将燃料低温重整产物通过气道进入发动机缸内,通过预混燃料的低温重整产物,结合缸内喷射相同的燃料,本文提出了基于低温重整的单燃料RCCI概念,并针对该概念开展了研究。结果表明,重整温度提高（523K→623K）,正庚烷重整转化率增大。低温重整能推迟单燃料RCCI着火,且重整温度越高,滞燃期越长。当重整没有发生时,单燃料RCCI出现两阶段低温放热,第一阶段的低温放热是由气道进入缸内的未发生重整的正庚烷引起的;当重整发生时,由于低温重整产物较低的活性,两阶段低温放热消失,同时,一些来自于低温重整产物中的醛类和酮类产生的PLIF信号出现在低温放热之前;低温重整减缓了低温放热阶段的甲醛演变过程,使单燃料RCCI燃烧更加平缓,使碳烟生成量降低。低温重整产物的热稀释影响和化学影响共同决定着着火时刻的变化,每一种低温重整产物的热稀释影响均会导致着火的推迟,而在各重整产物的化学影响下,既有能推迟着火的物质,也有能促进着火的物质;对于综合影响下可以促进着火的物质,其化学影响占据主导地位,而对于综合影响下可以推迟着火的物质,是热稀释影响还是化学影响占据主导,要根据推迟程度进行判断。低温重整能降低低温放热阶段的主反应速率,这是导致着火推迟的主要原因,此时,低温重整产物的热稀释影响在影响着火的因素中占据主导。正丁醇作为柴油燃料的氧添加剂,因其良好的发动机性能表现得到了广泛关注,为此,本文研究了正庚烷/正丁醇掺混燃料的低温重整对发动机燃烧的影响。研究结果表明,随着丁醇掺混比例（30v%<B30>、50v%<B50>和70v%<B70>）的增加,重整产物中CO和醛类的摩尔分数升高,而烃类、H₂、酮类和醇类的量降低。无论缸内早喷还是晚喷,低温重整均会推迟B30和B50燃料的着火,但对B50燃料的影响更小些,却能促进B70燃料的着火,低温重整引起的活性变化趋势并不受缸内混合气浓度分层度的影响。相比于早喷,低温重整对晚喷工况的影响较小。低温重整可以导致B30和B50燃料的火焰发展模式更趋向于火焰传播,火焰发展变缓;尽管对于晚喷的B70燃料来说,无论是否发生重整,火焰传播在火焰发展中均占据主导,但低温重整仍使得早喷和晚喷的B70燃料的火焰发展都有顺序自然的趋势,火焰发展速度加快。然而在晚喷情况下,对B30和B50燃料来说,低温重整会导致燃烧室出现无焰区,这或许会引起未燃碳氢排放的增多。相对于早喷,晚喷会有较多的碳烟生成;低温重整可以导致B30和B50燃料的碳烟生成减少,而B70燃料的碳烟生成会因重整而增多。超过半数的重整产物均会推迟三种掺混燃料的着火,3-C₇H₁₄、1,3-C₅H₈、C₃H₈和C₂H₆具有较强的推迟着火的能力,而C₂H₃CHO、C₂H₂和CH₃CHO具有更强的促进着火的能力。除了CO、1-C₅H₁₀、1-C₆H₁₂和1-C₇H₁₄外,随着当量比的增大,其余物质对三种掺混燃料着火的影响均是减弱的;无论混合气浓度大小,主要重整产物对三种燃料着火的影响趋势是一致的。重整产物中的醛类是改变缸内充量活性的关键物质。低温重整产物对发动机着火时刻的影响同时取决于重整产物中的物质种类和浓度,而每一种重整产物的浓度甚至种类,通常会受重整条件（燃料种类、重整当量比、重整温度、滞留时间等）的影响而发生改变,因此可以推断出,低温重整是灵活控制发动机缸内充量活性的一种方法,具有实现内燃机全负荷高效清洁燃烧的潜力。更多还原

【Abstract】 In order to achieve efficient and clean combustion of ICE（Internal Combustion Engine）,HCCI（Homogeneous Charge Compression Ignition）,RCCI（Reactivity Controlled Compression Ignition）and other combustion modes have been proposed,and fuel reforming technology for hydrogen has also been widely used in ICE.The single reactivity of hydrogen limits the flexible control of the in-cylinder reactivity in ICE.The diversity of low temperature oxidation products of fuels makes it possible to change the in-cylinder reactivity flexibly.Low temperature oxidation can cause the flexible product combination according to the change of reforming conditions,and is not restricted by the selection and the reactivity optimization of catalysts.And the low reforming temperature is easier to achieve.Therefore,low temperature reforming（LTR）has great potential in realizing efficient and clean combustion of ICE at full load.In this work,a new model of low temperature reforming combustion for ICE is proposed,and the influence mechanism of low temperature reforming of fuel on engine combustion is explored.In this work,an LTR system for the engine was established,which could realize the flexible control of reforming conditions.In addition,an on-line quantitative GC detection system was established to detect reforming products.Optical diagnostic methods（high-speed imaging and PLIF）were used to analyze the development of combustion flame in cylinder.Meanwhile,CHEMKIN software coupled with detailed chemical reactions mechanism,was used to evaluate the activity of reforming products and analyze the reforming combustion.Firstly,the HCCI combustion mechanism with LTR was studied.Results show that at the lower reforming temperature（423 K）,PRF0（n-heptane）and PRF50 will not undergo low-temperature oxidation.The reforming products mainly include hydrogen,carbon monoxide,olefins,aldehydes,alkanes,alkynes,alcohols and ketones.Compared with PRF50,PRF0 can produce more reforming products.Low temperature reforming can delay the ignition of HCCI,slow down the combustion rate of HCCI,promote the complete combustion of more fuel,and further inhibit the soot formation during the combustion.The reactivity of each low temperature reforming product is different.Most of LTR products decrease the in-cylinder charge reactivity,which should ultimately lead to the delay of ignition timing in HCCI combustion.Compared with PRF0,the reforming products promoting the ignition of PRF50 increase significantly,while for PRF50 the influences of most reforming products on ignition timing are weakened.LTR can be used as an effective way to solve the problems of difficulty of combustion phase control and high combustion rate in HCCI combustion.By premixing the LTR products entering into the engine cylinder through the inlet and in-cylinder direct injection,the concept of single-fuel RCCI through LTR was proposed and studied in this paper.Results show that with increasing reforming temperature（523K→623K）,the reforming conversion of n-heptane increases.LTR can delay the ignition of single-fuel RCCI,and the higher the reforming temperature,the longer the ignition delay.When reforming does not occur,two stages of low temperature heat release（LTHR）appear in the single-fuel RCCI.The first stage of LTHR is caused by the no-reformed n-heptane entering into the cylinder through the intake port.When reforming occurs,the two stages of LTHR due to the low reactivity of reforming products.The PLIF signals produced by aldehydes and ketones from LTR products,appear before LTHR.And LTR slows down the evolution of formaldehyde during LTHR,makes the single-fuel RCCI combustion more smoothly and reduces soot formation.The ignition timing depends on both the thermal dilution effect and chemical effect of reforming products.The thermal dilution effect of each product will lead to the delay of ignition.Under the chemical influence of the reforming products,LTR products can delay ignition or promote ignition.For the products promoting the ignition under the comprehensive influence,their chemical influences are dominant.For the products which can delay the ignition under the comprehensive influence,whether the thermal dilution effect or the chemical influence is dominant,it should be judged according to the degree of delay.LTR can reduce the main reaction rate during LTHR,which is the main reason for the delayed ignition.Then the thermal dilution effect of reforming products dominates the factors affecting ignition.As an oxygen additive for diesel fuel,n-butanol has attracted wide attention because of its excellent engine performance.Therefore,the effect of LTR of n-heptane/n-butanol on engine combustion was studied.Results reveal that the mole fractions of CO and aldehydes in reforming products increase with the increasing volume fraction of butanol（30v%<B30>,50v%<B50>and 70v%<B70>）in the blended fuel,while the amounts of hydrocarbons,H₂,ketones and alcohols decrease.Regardless of in-cylinder early or late injection,LTR delays the ignitions of B30 and B50,but has less effect on B50.However,LTR promotes the ignition of B70.The change trend of reactivity caused by LTR is not affected by the concentration stratification of in-cylinder mixture.Compared with early injection,LTR has less influence on late injection.LTR can lead to flame development of B30 and B50 to tend to the flame propagation,and the flame development slows down.Although flame propagation dominates the flame development of late-injected B70 whether reforming occurs or not,LTR still makes the flame development of early-injected and late-injected B70have a trend of sequence autoignition,and accelerates the flame development.However,for late injections of B30 and B50,LTR leads to flameless zone in the combustion chamber,which may lead to an increase in unburned hydrocarbon emissions.Compared with early injection,late injection will cause more soot formation.However,LTR can reduce soot formation of B30 and B50,while soot formation of B70 will increase due to LTR.More than half of the reformed products can delay the ignitions of three blended fuels.3-C₇H₁₄,1,3-C₅H₈,C₃H₈ and C₂H₆ have strong ability to delay ignition,while C₂H₃CHO,C₂H₂ and CH₃CHO have stronger ability to promote ignition.Except for CO,1-C₅H₁₀,1-C₆H₁₂ and 1-C₇H₁₄,with the increase of equivalence ratio,the influences of reforming products on the ignition of three blended fuels are weakened.The influence trend of main reforming products on the ignition of the three fuels is consistent regardless of the mixture concentration.Aldehydes in reforming products are the key substances to change the in-cylinder charge reactivity.Effect of LTR product on ignition timing depend on both the type and the species concentration in the reforming products.The concentration,even the type of reforming product usually changes along with the change of reforming conditions（fuel type,reforming equivalent ratio,reforming temperature,residence time,etc.）.Therefore,it can be inferred that LTR is a method of controlling in-cylinder charge reactivity flexibly,and has the potential of realizing efficient and clean combustion of ICE at full load.更多还原