四气门汽油机分层EGR的试验研究

针对可以产生较强滚流运动的1.6 L四气门产品汽油机,设计了一套独特的分层废气再循环(EGR)装置。通过废气导入管将废气引到进气阀处,利用缸内气体滚流运动,实现废气与新鲜可燃混合气分层。在发动机台架上通过试验详细地研究该分层EGR装置对汽油机性能和燃烧的影响。试验结果表明,采用该装置后,相比于传统的EGR方式,EGR 率有所增加,在大量降低NOx排放同时,经济性有所改善, CO和HC排放有所降低,燃烧效率提高,但点火时刻应适当提前。     

基于废气管理的汽油机缸内温度不均匀分布对自燃着火的影响

基于废气-负荷控制策略的汽油机HCCI燃烧,利用排气门管理内部残余废气率,控制发动机负荷,进气门升程和相位调节缸内状态,控制着火和优化燃烧相位。本文利用KIVA对全可变气门机构单缸机进行CFD仿真,研究缸内温度、残余废气分布特征,以及温度、废气不均匀分布对着火的影响。结果表明,采用废气-负荷控制策略,改变进气门相位和升程可以调节缸内温度和残余废气的不均匀分布。随着内部残余废气率增大,进气门对缸内温度和废气率分布不均匀度的调控能力增强。大废气率条件下,温度和废气率不均匀度对着火时刻调节作用较明显,可以实现10°曲轴转角的有效调节;小废气率条件下,由于缸内温度和废气不均匀分布差别较小,其对于着火时刻的调节作用表现得不明显。     

柴油燃料HCCI燃烧影响因素的试验研究

本文采用在进气上止点附近进行柴油喷射,利用缸内高温残余废气促进燃油蒸发形成均质混合气,实现了柴油燃料的均质压燃(HCCI)。试验结果表明柴油燃料HCCI燃烧的放热规律呈现低温和高温放热两个阶段,并且NOx排放可以降低95%-98%。本文主要研究了影响HCCI燃烧的因素,指出负荷增大、进气温度增加和负气门重叠期的增加使HCCI着火提前,而外部EGR率的增大可以推迟着火。因此对于低温自燃性好的燃料,冷EGR是控制其HCCI着火燃烧过程的有效措施。     

甲醇裂解气对点燃式发动机性能影响研究

以一台点火式电控发动机为研究对象,采用废气余热制氢装置,研究了甲醇裂解气对点火式发动机性能的影响.研究表明:裂解气的稀燃范围十分宽泛,可实现稀薄燃烧;通过稀燃可使NOx排放较原汽油机降低90%,CO几乎接近零排放,而HC排放接近汽油.同时,稀燃也使裂解气发动机在保持原汽油机动力性的条件下,获得了较好的燃料经济性,容积替换比最小可以达到了1.48.该方法为实现甲醇高效清洁燃烧提供了切实可行的技术方案.     

PODE/汽油双燃料RCCI大负荷扩展的试验研究

采用新型含氧燃料PODE作为缸内直喷燃料实现PODE/汽油双燃料RCCI燃烧,研究PODE在不同边界条件下对RCCI双燃料燃烧和排放的影响,结合进气门晚关策略进行了大负荷扩展的试验研究。结果表明:相对柴油燃料,PODE作为直喷燃料能够实现更低的烟度排放,同时有效地改善燃烧效率和热效率;降低λ可以有效抑制峰值燃烧速率,而PODE/汽油RCCI的烟度排放对λ不敏感,即便在λ1的富油条件下依然可以实现极低的烟度排放,但当量比附近区域会引起CO的恶化;进气门晚关可以降低缸内压力和峰值燃烧速率,但是过于推迟的进气门关闭时刻会导致进气量降低引起NO_x的升高;进气门晚关结合高增压实现当量燃烧可以将大负荷范围扩展至2.31 MPa IMEP,同时并未引起有效热效率的显著恶化。     

混氢改善汽油机稀燃性能的试验研究

针对汽油机稀燃条件下循环变动大,燃料燃烧不充分的问题,本文在一台加装了电控氢气喷射系统的四缸汽油机上就混氢对改善汽油机稀燃条件下燃烧与排放性能的作用进行了试验研究。在发动机1400r/min,进气道绝对压力为61.5 kPa的条件下,就1%与3%两种进气混氢体积分数对稀燃汽油机燃烧与排放特性的影响进行了研究。试验结果表明,稀燃时发动机制动热效率随混氢分数增加而提高;滞燃期与速燃期随混氢分数增加而缩短;发动机稀燃极限所对应的过量空气系数由原机的1.45提高至混氢1%与3%时的1.55和1.96。混氢后发动机HC与CO排放降低,但NO_x排放有所升高。     

多次喷射对重型柴油机影响的试验研究

本文在一台电控共轨柴油机上进行了多次喷射及多次喷射和EGR结合对重型柴油机影响的试验研究。EGR为基于VGT的高压冷却EGR系统。研究了不同喷射策略下喷射参数对柴油机性能和排放的影响。研究了EGR和多次喷射结合降低柴油机排放的效果;通过调整EGR率,将NO_x控制在2.0 g/(kW·h),研究喷射参数和EGR对柴油机的影响。研究结果表明,预喷射可以在小负荷时改善柴油机的NO_x、CO和比油耗,但在大负荷时没有明显的影响;两次预喷射可以促进油气混合;后喷射可以促进油气混合,提高碳烟的氧化速度,因而可以明显改善柴油机的烟度和CO;优化的喷射参数结合EGR可以显著降低NO_x和烟度。     

理论空燃比SICI组合燃烧排放特性研究

均质混合气压燃(HCCI)燃烧受爆震和NO_x排放的限制,高负荷拓展是一个难题。本文提出利用理论空燃比SICI组合燃烧作为汽油机中高负荷运行区域的高效低污染燃烧模式。在试验台架上研究了理论空燃比SICI组合燃烧的排放特性及其对三效催化剂(TWC)转换性能的影响。研究发现,通过外部EGR与点火控制相结合,理论空燃比SICI组合燃烧可以在指示平均有效压力(IMEP)0.65～0.82MPa范围内稳定实现;与传统SI燃烧相比,理论空燃比SICI组合燃烧燃油经济性提高近10%,NO_x排放降低80%以上,但HC排放有所增加;SICI组合燃烧与传统SI燃烧类似,TWC空燃比特性的高效区间在理论空燃比附近,但对HC排放的催化能力提出了更高的要求。     

直喷柴油机火焰浮起长度的数值模拟研究

采用三维CFD模型模拟了直喷柴油机缸内喷雾燃烧过程,模拟缸压曲线得到了实验的验证.通过高温区与喷嘴之间的稳定距离来确定柴油机火焰浮起长度,研究在不同进气条件下火焰浮起长度的变化情况.该模型成功地预测了火焰浮起长度随着初始进气压力的增大而减少,随着进气温度的升高出现先增大后减少的趋势.同时模拟了在不同EGR率下柴油机缸内燃烧情况,发现火焰浮起长度和燃料着火延迟时间都随EGR率的增加而增大.     

EGR率对生物柴油颗粒纳米结构的影响

为研究同时运用废气再循环(EGR)技术和燃用生物柴油对柴油机排气颗粒纳米结构的影响,分别采集0％,15％,30％ EGR率下186F柴油机燃用生物柴油时的排气颗粒,并用激光拉曼光谱仪测得颗粒光谱,使用五带法对一阶拉曼光谱进行拟合,分析拟合曲线参数,计算颗粒微晶尺寸和碳碳键长度.结果表明:EGR率为30％时,生物柴油颗粒光谱的半高宽(FWHM)最大,代表化学异相性最强并且颗粒中的物质种类最多,随着EGR率降低,半高宽逐渐减小;当EGR率从0％升高到30％,ID/IG逐渐增大,代表石墨化程度降低,颗粒中的石墨结构减少;同时,iD1/ID2从0％ EGR率的8左右降低到15％和30％ EGR率的4左右,代表EGR率升高,颗粒内部缺陷由空位缺陷向石墨烯边缘缺陷发展;随着EGR率升高,微晶尺寸逐渐减小,碳碳键长度基本不变.     

生物添加剂对汽油机燃油经济性及废气排放影响的研究

将一种棕榈油提取物作为添加剂,加入汽油中以研究其对汽油机燃油经济性和排放品质的影响。针对市场上正使用的辛烷值为93#的高清洁汽油和乙醇汽油,通过发动机台架试验对加剂前后的汽油机性能指标进行了对比和分析．结果表明:加入此种添加剂后对各种汽油的燃油经济性有大幅改善作用,对乙醇汽油的效果尤甚。加剂后可以使乙醇汽油的燃油经济性与普通汽油完全一样。对于93#高清洁汽油,除了未燃碳氢(HC)略有上升外,其他排放物如氮氧化物 (Nox)、一氧化碳等有所减少, CO2排放有明显的改善。对于乙醇汽油,则HC和CO2略有上升。     

Effects of NOx addition on autoignition and detonation development in DME/air under engine-relevant conditions

Exhaust gas recirculation (EGR) technology can be used in internal combustion engines to reduce NOx emission and improve fuel economy. However, it also affects the end-gas autoignition and engine knock since NOx in EGR can promote ignition. In this study, effects of NOx addition on autoignition and detonation development in dimethyl ether (DME)/air mixture under engine-relevant conditions are investigated. Numerical simulation considering both low-temperature and high-temperature chemistry is conducted. First the kinetic effects of NOx addition on the negative temperature coefficient (NTC) regime are assessed and interpreted. It is found that NOx addition greatly promotes both low-temperature and high-temperature ignition stages mainly through increasing OH production. Then the autoignitive reaction front propagation induced by either local NO accumulation or a cold spot within NTC regime with different amounts of NO addition is investigated. For the first time, supersonic autoignition modes including detonation induced by local NO accumulations are identified. This indicates that local accumulation of NOx in end gas might induce super-knock in engines with EGR. A new parameter quantifying the ratio of sound speed to average reaction front propagation speed is introduced to identify the regimes for different autoignition modes. Compared to the traditional counterpart parameter used in previous studies, this new parameter is more suitable since it yields a detonation development regime in a C-shaped curve which is almost unaffected by the initial conditions. The results in this study may provide fundamental insights into knocking mechanism in engines using EGR technology.     

Influence of NOx chemistry on the prediction of natural gas end-gas autoignition in CFD engine simulations

Natural gas (NG) represents a promising low-cost/low-emission alternative to diesel fuel when used in high-efficiency internal combustion engines. Advanced combustion strategies utilizing high EGR rates and controlled end-gas autoignition can be implemented with NG to achieve diesel-like efficiencies; however, to support the design of these next-generation NG ICEs, computational tools, including single- and multi-dimensional simulation packages will need to account for the complex chemistry that can occur between the reactive species found in EGR (including NOx) and the fuel. Research has shown that NOx plays an important role in the promotion/inhibition of large hydrocarbon autoignition and when accounted for in CFD engine simulations, can significantly improve the prediction of end-gas autoignition for these fuels. However, reduced NOx-enabled NG mechanisms for use in CFD engine simulations are lacking, and as a result, the influence of NOx chemistry on NG engine operation remains unknown. Here, we analyze the effects of NOx chemistry on the prediction of NG/oxidizer/EGR autoignition and generate a reduced mechanism of a suitable size to be used in engine simulations. Results indicate that NG ignition is sensitive to NOx chemistry, where it was observed that the addition of EGR, which included NOx, promoted NG autoignition. The modified mechanism captured well all trends and closely matched experimentally measured ignition delay times for a wide range of EGR rates and NG compositions. The importance of C2-C3 chemistry is noted, especially for wet NG compositions containing high fractions of ethane and propane. Finally, when utilized in CFD simulations of a Cooperative Fuels Research (CFR) engine, the new reduced mechanism was able to predict the knock onset crank angle (KOCA) to within one crank angle degree of experimental data, a significant improvement compared to previous simulations without NOx chemistry.     

DMM燃料柴油机可控预混合燃烧的研究

本文介绍了在压燃式发动机上进行的预混合燃烧研究。在柴油机的进气道入口处安装了一个电控燃料喷射系统,喷入具有低十六烷值、低沸点的甲缩醛(DMM)燃料,在压缩冲程中形成均匀的混合气,并在上止点附近喷入少量柴油来点燃混合气。本文研究了预混合燃料比、发动机负荷、进气中CO2浓度和喷孔直径对发动机燃烧和排放的影响。试验结果表明,进气道喷射DMM的预混合燃烧能同时大幅降低NOx和碳烟排放,为降低柴油机有害排放提供了一种新途径。     

Influence of intermediate temperature heat release on autoignition reactivity of single-stage ignition fuels with varying octane sensitivity

This study investigates the effects of intermediate temperature heat release (ITHR) on autoignition reactivity of full boiling range gasolines with different octane sensitivity through intake temperature and simulated exhaust gas recirculation (EGR) sweeps in a homogenous charge compression ignition (HCCI) engine. To isolate the ITHR effects, low temperature reactivity was suppressed through the use of high intake temperature and low intake oxygen mole fraction. For quantification of ITHR, a new method was applied to the engine data by examining the maximum value of the second derivative of heat release rate. Combustion phasing comparisons of fuels with octane sensitivity showed that fuel with less octane sensitivity became more reactive as intake temperature and simulated EGR ratio decreased, while fuel with higher octane sensitivity had a reverse trend. For all of the fuels that were tested, the amount of ITHR increased as the intake temperature and oxygen mole fraction increased. These ITHR trends, depending on octane sensitivity, were almost identical with the trends of combustion phasing, showing that ITHR significantly affects fuel autoignition reactivity and determines octane sensitivity.     

基于DMCC发动机台架的甲醇-SCR催化还原NO_x的研究

为了验证合成气台架上组合式催化剂((Co-Mo)/Al2O3)优异的选择催化甲醇还原NO的性能,试验在一台由增压中冷,废气再循环的电控单体泵柴油机改造的DMCC发动机上实施。结果表明:提高甲醇喷射量和EGR率,推迟柴油喷射时刻均会改善甲醇-SCR的催化效率。甲醇最优喷射量受到HC减活化作用和HC完全氧化反应的控制。各工况下甲醇-SCR对PM排放的影响较小。DMCC发动机耦合甲醇-SCR系统可以实现NOx的超低排放。A25、A75和B75工况点下的NOx体积分数的最低值分别为64×10^-6、66×10^-6和47×10^-6。     

四气门发动机可变涡流稀薄燃烧特性研究

本文研究了可变涡流对四气门发动机稀薄燃烧特性的影响情况。在稀薄燃烧情况下,发动机负荷大小对CO和HC排放的影响不大,对NOx排放的影响主要表现在对13～17空燃比范围内NOx排放的影响,负荷越大,NOx排放越大;对空燃比小于13或大于17以后的NOx排放影响较小。阀片位置对发动机排放特性的影响较小,对发动机的燃油经济性存在一定影响,这是因为不同阀片位置的进气涡流比不同所致,同时也表明较强的涡流运动对燃油经济性更有利。涡流运动在不同转速条件下对发动机燃油经济性的影响情况不同,它更有利于改善低速条件下的燃油经济性。     

高温空气低燃气浓度燃烧过程的数值模拟研究

从工程实际出发,本文提出了高温空气低燃气浓度燃烧新技术,即充分利用烟气余热提高助燃空气温度,提高热能利用率;同时通过优化喷口结构,提高燃气射流速度,使燃气射流在同空气射流混合燃烧前卷吸大量炉内烟气,从而降低燃气射流中的可燃物浓度,进而降低氮氧化物的排放。通过数值模拟研究表明,通过燃气射流速度从24.56m/s提高到55.26m/s,可以降低NOx的排放;当围绕燃气喷口的六个圆形空气喷口改为两个矩形喷口时,燃气射流可从两侧卷吸更多的炉内烟气,形成低燃气浓度燃烧,从而大大降低了NOx的排放。