船舶柴油机第4章第6节.ppt

第四章 燃油喷射与燃烧,第四章 燃油喷射与燃烧,第一节 燃油与燃油添加剂 第二节 燃油的喷射 第三节 喷油泵及喷油器 第四节 混合气的形成和燃烧 第五节 故障与维护管理 第六节 提高柴油机功率和经济性的措施 第七节 柴油机排气污染与净化 第八节 电控喷射,第六节 提高柴油机功率和经济性的措施,关键：功率不变，则减少损失； 损失不变，则提高功率。 一、提高强载度 1、采用增压技术和对增压空气进行中冷 2、改善燃油与空气的混合 3、改善换气过程，提高v 二、减少损失,第七节 柴油机排气污染与净化,一、柴油机排气污染物及其危害 二、柴油机排气污染物生成机理及控制措施,一、柴油机排气污染物及其危害,1排放物种类 有害：一氧化碳（CO）、氮氧化物（NOX、碳氢化物（HC）、二氧化硫（SO2）、臭氧和微粒物质（如碳烟、油雾等） 无害：燃料的完全燃烧产物，如二氧化碳（CO2）、水蒸汽（H2O）、过量空气以及残余氮（N2）等 2有害排放物的危害,二、柴油机排气污染物生成机理 及控制措施,1排气污染物生成机理 燃烧室温度升高、氧气浓度增加、燃气在高温区滞留时间增长均使NOX 增加 HC、CO排放物是由燃油不完全燃烧形成的 黑烟、蓝烟（0.5微米以下）和白烟（1微米以上） 。 2排气污染物控制措施,2排气污染物控制措施,1）前处理 对燃油或空气在进入气缸前进行预先处理 .如脱硫、EGR、乳化 2）后处理（除尘、SCR等） 3）机内处理(改进柴油机的缸内过程) （1）推迟喷油并适当提高喷油速率。 （2）提高喷射质量，保证良好的混合。 （3）增压空气中冷。 （4）提高压缩比。 （5）利用有关可变控制机构和微机进行最佳控制。 （6）研制使用低污染的燃烧室。,第八节 电控喷射柴油机,一、发展概况 车用四冲程柴油机的电控喷射，在上个世纪末已经趋于成熟，大量装备在重型车辆上。船用中低速机上使用是本世纪初的最新技术。 二、两种主要的电控喷油系统 RT-flex的共轨控制 ME的液压喷射控制 智能柴油机的优点 P194 六条,RTflex共轨系统,S70ME,Combustion in CI Engine,In a CI engine the fuel is sprayed directly into the cylinder and the fuel-air mixture ignites spontaneously. These photos are taken in a RCM under CI engine conditions with swirl air flow,0.4 ms after ignition,3.2 ms after ignition,3.2 ms after ignition,Late in combustion process,1 cm,In Cylinder Measurements,This graph shows the fuel injection flow rate, net heat release rate and cylinder pressure for a direct injection CI engine.,Start of injection,Start of combustion,End of injection,Combustion in CI Engine,The combustion process proceeds by the following stages: Ignition delay (ab) - fuel is injected directly into the cylinder towards the end of the compression stroke. The liquid fuel atomizes into small drops and penetrates into the combustion chamber. The fuel vaporizes and mixes with the high-temperature high-pressure air. Premixed combustion phase (bc) combustion of the fuel which has mixed with the air to within the flammability limits (air at high-temperature and high- pressure) during the ignition delay period occurs rapidly in a few crank angles. Mixing controlled combustion phase (cd) after premixed gas consumed, the burning rate is controlled by the rate at which mixture becomes available for burning. The rate of burning is controlled in this phase primarily by the fuel-air mixing process. Late combustion phase (de) heat release may proceed at a lower rate well into the expansion stroke (no additional fuel injected during this phase). Combustion of any unburned liquid fuel and soot is responsible for this.,Four Stages of Combustion in CI Engines,Start of injection,End of injecction,-10,TC,-20,10,20,30,CI Engine Types,Two basic categories of CI engines: Direct-injection have a single open combustion chamber into which fuel is injected directly Indirect-injection chamber is divided into two regions and the fuel is injected into the “prechamber” which is connected to the main chamber via a nozzle, or one or more orifices. For very-large engines (stationary power generation) which operate at low engine speeds the time available for mixing is long so a direct injection quiescent chamber type is used (open or shallow bowl in piston). As engine size decreases and engine speed increases, increasing amounts of swirl are used to achieve fuel-air mixing (deep bowl in piston) For small high-speed engines used in automobiles chamber swirl is not sufficient, indirect injection is used where high swirl or turbulence is generated in the pre-chamber during compression and products/fuel blowdown and mix with main chamber air.,Combustion Characteristic,Combustion occurs throughout the chamber over a range of equivalence ratios dictated by the fuel-air mixing before and during the combustion phase. In general most of the combustion occurs under very rich conditions within the head of the jet, this produces a considerable amount of solid carbon (soot).,Ignition Delay,Ignition delay is defined as the time (or crank angle interval) from when the fuel injection starts to the onset of combustion. Both physical and chemical processes must take place before a significant fraction of the chemical energy of the injected liquid is released. Physical processes are fuel spray atomization, evaporation and mixing of fuel vapour with cylinder air. Good atomization requires high fuel-injection pressure, small injector hole diam., optimum fuel viscosity, high cylinder pressure (large divergence angle). Rate of vaporization of the fuel droplets depends on droplet diameter, velocity, fuel volatility, pressure and temperature of the air. Chemical processes similar to that described for autoignition phenomenon in premixed fuel-air, only more complex since heterogeneous reactions (reactions occurring on the liquid fuel drop surface) also occur.,Fuel Ignition Quality,The ignition characteristics of the fuel affect the ignition delay. The ignition quality of a fuel is defined by its cetane number CN. For low cetane fuels the ignition delay is long and most of the fuel is injected before autoignition and rapidly burns, under extreme cases this produces an audible knocking sound referred to as “diesel knock”. For high cetane fuels the ignition delay is short and very little fuel is injected before autoignition, the heat release rate is controlled by the rate of fuel injection and fuel-air mixing smoother engine operation.,Cetane Number,The method used to determine the ignition quality in terms of CN is analogous to that used for determining the antiknock quality using the ON. The cetane number scale is defined by blends of two pure hydrocarbon reference fuels. By definition, isocetane (heptamethylnonane, HMN) has a cetane number of 15 and cetane (n-hexadecane, C16H34) has a value of 100. In the original procedures a-methylnaphtalene (C11H10) with a cetane number of zero represented the bottom of the scale. This has since been replaced by HMN which is a more stable compound. The higher the CN the better the ignition quality, i.e., shorter ignition delay. The cetane number is given by: CN = (% hexadecane) + 0.15 (% HMN),The method developed to measure CN uses a standardized single-cylinder engine with variable compression ratio The operating condition is: Inlet temperature (oC) 65.6 Speed (rpm) 900 Spark advance (oBTC) 13 Coolant temperature (oC) 100 Injection pressure (MPa) 10.3 With the engine running at these conditions on the test fuel, the compression ratio is varied until combustion starts at TC, ignition delay period of 13o. The above procedure is repeated using blends of cetane and HMN. The blend that gives a 13o ignition delay with the same compression ratio is used to calculate the test fuel cetane number.,Cetane Number Measurement,Cetane vs Octane Number,The octane number and cetane number of a fuel are inversely correlated.,Gasoline is a poor diesel fuel and vice versa.,Cetane number,Cetane motor method octane number,Factors Affecting Ignition Delay,Injection timing At normal engine conditi