【作者】 梁晨；

【导师】 纪常伟；

【作者基本信息】 北京工业大学 ， 热能工程， 2013， 博士

【摘要】 随着交通运输行业中能源消耗量的持续增加，环境污染和能源紧缺等问题越来越成为了人们关注的焦点，也成为了社会发展亟待解决的问题之一，而汽车又是造成能源消耗和环境污染的主要源头之一。为了更好的保护环境并实现可持续发展，发掘新的可持续利用的燃料就成为了目前车用内燃机的主要研究方向。经过近些年的研究和发展，代用清洁燃料以其低排放、环境友好、可再生、可持续发展以及储量较大而成为了车用内燃机燃料的主要选择方向。为了降低使用成本并扩大内燃机转速及负荷的使用范围，目前车用内燃机普遍使用固定辛烷值的燃料，例如93#汽油等。然而，为了更加优化点燃式内燃机的运转状况，提高内燃机的热效率并降低排放，根据不同运转工况下的需求就需要对燃料的辛烷值进行调整。例如，在冷起动时就要适当降低燃料的辛烷值以便于内燃机起动，而在高负荷时就需要提高内燃机的辛烷值以避免爆震。综上，为了更好的满足内燃机在不同区间工况下的使用要求，本研究首次提出了利用高十六烷值燃料与高辛烷值燃料进行实时混合的燃料与工况协同控制策略。在本研究中首次将二甲醚（DME）作为高十六烷值燃料与汽油、醇类燃料等其它高辛烷值燃料进行不同比例的实时现场混合并在点燃式内燃机中燃烧，实现了内燃机的变辛烷值燃料，以满足车用内燃机在不同运转工况下的需求，提高热效率并拓宽内燃机的运转范围。相对其它车用内燃机代用燃料，DME的各方面理化特性都能更满足内燃机的运转需求。此外，DME有较宽的燃料来源，可以从天然气、煤、石油、废料及生物质能中获取。较高的十六烷值及气态燃料较低的沸点使DME可以缩短点火延迟并实现低温自燃，且在喷入缸内后快速混合雾化。此外，DME无毒对人体无害且对环境没有影响。DME较低的C/H比、较高的氧含量以及分子结构中没有C-C键等特性使其可以实现无碳烟排放和清洁燃烧。本试验在一台四缸点燃式内燃机上进行，对进气歧管进行改造并将DME喷嘴安装在靠近进气道处以满足DME可以和其它燃料在进气道进行混合。一台单独开发的电控单元HECU用于控制内燃机的点火时刻及DME和其它燃料的喷射时刻和脉宽。HECU通过原机ECU读取内燃机的信号数据并由工控机进行调整，实现对DME能量混合分数和体积混合分数进行控制。利用Horiba MEXA-7100DEGR尾气排放分析仪对NOx、HC和CO排放进行采集和分析。缸压和曲轴转角信号输入DEWETRON燃烧分析仪，并通过DEWE-CA燃烧分析软件进行后期处理，分析缸压、放热率、热效率、燃烧持续期、循环变动等燃烧参数。本文试验主要分为冷起动、怠速和1400r/min稳态三个工况以及DME与汽油、醇类等不同燃料进行混合试验，在试验过程中对DME能量混合分数及体积混合分数进行调整，每个工况下分为理论当量比混和气和稀燃工况两类试验。根据冷起动试验的相关结果，点燃式DME内燃机可以成功实现冷起动并稳定运转。相比较点燃式汽油原机，点燃式DME内燃机的运转特性和排放水平有了较大的提升，HC排放降幅达到75%，在冷起动20秒之后NOx和CO排放只有汽油内燃机的30%。此外，燃用DME时的能量流量只是传统汽油内燃机的50%。综上，DME作为点燃式内燃机冷起动工况下的燃料不仅是一种创新，更被证明是一种有效的可以改善内燃机效率和性能的方式。怠速工况的试验包括了纯DME、DME/汽油、DME/甲醇和DME/乙醇四种燃料。根据试验结果看出，DME可以单独用于点燃式内燃机，同时也可以与汽油或醇类燃料进行混合使用。试验证明， DME可以提高怠速工况下点燃式内燃机的热效率，缩短火焰发展期并实现内燃机的稳定运转，特别是DME单独作为燃料时的改善效果更明显。点燃式DME内燃机的指示热效率较汽油原机提高了30%，在理论当量比时，NOx排放只有20310-6，仅为原机的40%，而CO排放也在λ<1.20之后小于原机。综上，在怠速工况下，无论将DME单独使用还是与其它高辛烷值燃料混合使用均可以有效的改善内燃机的运转稳定性，降低燃料消耗并减少排放，实现对点燃式内燃机在怠速工况下整体性能的提升。稳态试验工况包括DME/汽油、DME/甲醇和DME/乙醇三种混合燃料，基于理论当量比混合下的DME/乙醇试验可以看出，随着内燃机负荷的增加，混合燃料内燃机的指示热效率呈现先上升后下降的变化趋势，当αDME=2%时平均提高幅度达到5.5%。此外，随着DME的混合还可以降低COVimep并缩短燃烧持续期。在排放方面，DME的添加可以有效降低HC排放，平均降幅达到40%。综上，基于不同燃料及不同工况的试验结果，在稳态情况下混合DME可以有效提高内燃机的性能并改善其存在的问题，特别是在中高负荷和转速情况下。更多还原

【Abstract】 With the tremendous increase of transportation energy consumption, theenvironmental pollution and energy shortage have emerged as a main societalproblem and a focus for more and more people. In order to protect the environmentand realize the sustainable development, finding better substitutes for fossil fuels hasbecome a major work for most of the researchers studying internal combustion (IC)engines. Due to the low emissions, environmental friendliness, alternative fuels havebeen considered as one of the best choices for future IC engines.Nowadays, SI engines use a fixed octane rating fuel such as#93gasoline under awide range of torques and speeds. However, to produce the possibly highest thermalefficiency and lowest emissions, SI engines should be fueled with the variable octanerating fuels for various engine operating conditions, e.g. at cold start, the low octanerating fuel is used to facilitate the engine start, and at high loads, the high octanerating fuel is utilized to avoid the engine knock. Thus, for the sake of improving theperformance of SI engines, the fuel and operating condition coordinative strategy isproposed in this paper.As a high cetane number fuel, DME is introduced into the SI engine to widen theoperational range and improve the performance of SI engines. DME can be producedfrom a variety of feedstocks such as natural gas, coal, crude oil, residual oil, wasteproducts and biomass. The high cetane number and low boiling point of DMEsymbolize the short ignition delay, low auto-ignition temperature and almostinstantaneous vaporization. Moreover, as DME is non-toxic and environmentallybenign, whenever at low or high mole fractions (percent by volume) in air, it hardlyhas any odor and causes no negative health effects. The DME has a lowcarbon-to-hydrogen ratio (C:H), a high oxygen content (around35%by mass) and noC-C bonds in its molecular structure, which help realize the smoke-free combustion.A four-cylinder SI engine is used in this experiment. The engine intake manifoldsis modified with four DME injectors mounted near the intake port of each cylinder, sothat DME and different high octane rating fuels can be injected and mixed in theintake ports online. A hybrid electronic control unit (HECU) is developed and adoptedto govern the spark timings, injection timings and durations of DME and differenthigh octane rating fuels. The HECU acquires the engine sensor data from the originalECU (OECU) and communicates with a calibration computer. The injection durationsof fuels can be adjusted freely through the commands from the calibration computer.The exhaust emissions of NOx, HC and CO from the test engine are measured andanalyzed by a Horiba MEXA-7100DEGR emissions analyzer. Cylinder pressure andcrank angle signals are sampled and treated via a DEWE-CA combustion analysissoftware embedded in the DEWETRON combustion analyzer to obtain profiles of cylinder pressure, heat release rate, indicated thermal efficiency, CA0-10, CA10-90,COVimep, COVnand so on.The tests are carried out at cold start, idle and1400r/min conditions. Differenthigh octane number fuels blended with DME under different addition levels are tested.The investigated mixture conditions include stoichiometric excess fuel air ratio andlean combustion. The main conclusions are as follows.At the cold start condition, the SI DME engine could be started successfully andrealize the stabile running. Meanwhile, compared with the original SI gasoline engine,the operating performance and emissions level of the SI DME engine are improvedunder the cold start condition. About the emissions performance, HC emissions of theSI DME engine are reduced by75%compared with the original SI gasoline engine,NOxand CO emissions of the SI DME engine obviously drop to only about30%ofthose of the SI gasoline engine after20s. Furthermore, the SI DME engine only needsabout50%fuel energy flow rate of the SI gasoline engine. Thus, based on the testresults, adding DME is found to be a quite innovative application way, as well as aneffective and economical measure for improving the cold start performance oftraditional gasoline engines.Under idle conditions, the performances of SI engines fueled with four fuels,including pure DME, DME/gasoline, DME/methanol and DME/ethanol mixtures, aretested. DME can be used independently as well as blended with other high octanerating fuels to enhance the idle performance for SI engines. DME addition to the SIengine could improve the indicated thermal efficiency, shorten the flame developmentperiod and meliorate the running stability, especially when it is used independently.The indicated thermal efficiency of the SI DME engine is increased by about30%compared with that of the SI gasoline engine. Under the stoichiometric condition, theNOxemissions of the SI DME engine are20310-6and only40%of those of thegasoline engine. The CO emission of the DME engine is also less than that of thegasoline engine when fuel/air ratio is less than1.2. In conclusion, whenever the SIengine is fueled with pure DME or DME-blended fuel, it is helpful for realizing thestable operation, decreasing the fuel consumption and reducing the emissions, whichcould effectively improvethe overall performance of SI engines at idle.Experiments on the SI engines fueled with DME/gasoline, DME/methanol, andDME/ethanol mixtures are performed to investigate the performance ofDME-enriched SI engines under steady operating conditions. The results of the enginefueled with DME/ethanol blends show that, under the stoichiometric condition, theindicated thermal efficiency is firstly increased and then decreased with the increaseof engine load. Meanwhile, DME addition obviously increases indicated thermalefficiency at all tested engine loads. The indicated thermal efficiency after DMEenrichment is averagely enhanced by about5.5%in comparison with that of the original engine at αDME=2%. Furthermore, the enrichment of DME also avails thereduction of COVimepand the shortened combustion duration. Moreover, DMEaddition could obviously decrease HC emissions at all test engine loads, which aredecreased by nearly40%on average. All in all, the test results indicate that DMEblending to high octane number fuels is a suitable way to overcome the shortcomingsof single fuel SI engines at low to mid engine speeds and loads.更多还原