This numerical simulation investigated the impact of pre-chamber fuel injection timing, injection angle, and exhaust gas recirculation (EGR) on combustion and emissions in a heavy-duty diesel engine using a pre-chamber combustion system. The results illustrate that the pre-chamber combustion system exhibits the potential to enhance the indicated thermal efficiency (ITE) of a diesel engine, while specific pre-chamber configurations can concurrently lower both NOx and soot emissions. The optimized cases show that the sustained turbulence generated by the pre-chamber enhances the redistribution of the unburned mixtures in the cylinder. The sustained turbulence leads to a broader range of gas mixtures near the equivalence ratio(Phi) of 1, resulting in complete combustion and lower NOx and soot emissions. In the optimized pre-chamber combustion system (with pre-chamber injection timing (PIT) at 0 degrees CA ATDC and pre-chamber injection angle (PIA) at 54.5 degrees), introducing 3% EGR results in a reduction of 11.61% in NOx emissions and 9.97% in soot emissions compared with the baseline engine.
Accurate and comprehensive reconstruction of in-cylinder combustion process is essential for timely monitoring of engine combustion state. This article developed a method based on the zero-dimensional (0-D) physical model integrated with big data. The traditional 0-D prediction model based on cumulative fuel mass is improved, the factor of in-cylinder temperature is introduced to adjust the heat release rate, which solves the problem of difficulty in calibrating the heat release rate. Then, convolutional neural network-gated recurrent unit (CNN-GRU), as a deep neural network, including a special convolutional layer and a gated recurrent unit (GRU) neural network is designed for the parameters to be calibrated in the model. The 0-D predictive combustion model is constructed by combining the physical model with CNN-GRU, the combustion process is simplified and reconstructed. The fitting results show that the 0-D physical model based on improved cumulative fuel mass approach is an effective method to reflect the heat release law. Under non-calibration conditions, the root mean square error (RMSE) value of peak firing pressure (PFP) based on CNN-GRU prediction model is 0.5862. The prediction model is a promising method to realize online fitting and optimization of combustion process.
Combustion performance experiments were carried out on a single-cylinder diesel engine to study the influ-ences of a lateral swirl combustion system(LSCS)and a multi-swirl combustion system(MSCS)on engine performance.Combined with simulation analysis,the fuel-air mixing and combustion characteristics were revealed for the LSCS and the MSCS respectively.Experimental results show that under low loads and high excess air coefficients,the MSCS ob-tains better combustion performance than the LSCS with the maximum reduction in fuel consumption of 3.6 g/(kW·h),in soot emission of 0.13 g/(kW·h)and in combustion duration of 2.6° CA.However,under heavy loads and low excess air coefficients,the LSCS shows better combustion performance than the MSCS with the maximum reduction in fuel con-sumption of 2.6 g/(kW·h),in soot emission of 0.56 g/(kW·h)and in combustion duration of 2.8° CA.Simulation results indicate that,when the engine load decreases or the excess air coefficient increases,the fuel spray penetra-tion ability becomes weakened,and the circular ridge of the MSCS chamber improves fuel-air mixing quality more effectively.However,when the engine load increases or the excess air coefficient decreases,the fuel spray pene-tration ability is enhanced.The split-flow creation of the LSCS chamber improves the fuel-air mixing quality more evidently,while the circular ridge of the MSCS chamber hinders the diffusion of fuel spray.
The contribution of dimethyl ether(DME) to the ignition delay times(IDTs) of ammonia(NH3) was investigated behind reflected shock waves. The experiments were performed at a pressure of 0.14/1.0 MPa, temperature range of 1150-1950 K, equivalence ratio of 0.5/1.0/2.0, and NH3/DME mixing ratios of 100/0, 95/5, 90/10, and 70/30. It was observed that the addition of DME decreased the IDTs and promoted the reactivity of NH3. With the increase of DME, the effect of the equivalence ratio on the IDTs of NH3 decreased. Under higher temperature and pressure conditions, the promoting effect of DME on the ignition of NH3 was weakened. An updated mechanism is proposed to reveal the promoting effect of DME on the ignition of NH3. Mechanisms from the literature were compared against the measurements, and the updated kinetic mechanism was validated with experimental data. Good agreement between measurements and simulations were shown. Chemical kinetic analyses were performed to interpret the interactions between DME and NH3 during fuel ignition. The numerical analysis indicated that the promotion effect of DME is primarily due to an increase of the rate of production and concentration of the radical pool, especially the OH radical pool. The large number of OH radicals generated by the reaction HO2 + CH3 = OH + CH3O during the early oxidation of DME is key to the NH3 consumption and early initiation of the chain reaction.
In order to make high pressure common rail system of the marine diesel engine select injection rate in a flexible way, the double-lift electronically controlled injector, whose needle valve has two upper limit positions, was proposed. An AMESim model of the injector was established, and based on the model, injection quantity characteristics and needle displacement, injection rate, pressure of the delivery chamber with different control timing of the injector under different injection modes and rail pressure were investigated. The results show that, the lift of needle valve is capable of being switched under different rail pressure, enabling the injector to realize three basic injection modes of low lift, high lift and boot. Compared with high lift injection mode, the injector under low lift injection mode has higher control accuracy on injection quantity, and the injection quantity changes almost linearly with the IPW. When the injector is in low lift injection mode, the balance point of delivery chamber pressure fluctuation due to the needle opening will be slightly higher than that of high lift injection mode. The variation of average injection rate with rail pressure under low lift injection mode is smaller than that under high lift injection mode, thus the injection rate fluctuation of low lift is less affected by delivery chamber pressure fluctuation. When the injector is in boot injection mode, lift switching delay is used to describe the time lag of lift switching signal to injection signal. As the value of lift switching delay increases, the change of injection quantity with rail pressure and injection pulse widths transits from high lift injection mode to low lift injection mode. The start time of lift switching has significant effects on the fluctuation of delivery chamber pressure and injection rate in the subsequent fuel injection process.
The combustion characteristics of ammonia/methanol mixtures were investigated numerically in this study. Methanol has a dramatic promotive effect on the laminar burning velocity (LBV) of ammonia. Three mechanisms from literature and another four self-developed mechanisms constructed in this study were evaluated using the measured laminar burning velocities of ammonia/methanol mixtures from Wang et al. (Combust.-Flame. 2021). Generally, none of the selected mechanisms can precisely predict the measured laminar burning velocities at all conditions. Aiming to develop a simplified and reliable mechanism for ammonia/methanol mixtures, the constructed mechanism utilized NUI Galway mechanism (Combust.Flame. 2016) as methanol sub-mechanism and the Otomo mechanism (Int. J. Hydrogen. Energy. 2018) as ammonia sub-mechanism was optimized and reduced. The reduced mechanism entitled 'DNO-NH3', can accurately reproduce the measured laminar burning velocities of ammonia/methanol mixtures under all conditions. A reaction path analysis of the ammonia/methanol mixtures based on the DNO-NH3 mechanism shows that methanol is not directly involved in ammonia oxidation, instead, the produced methyl radicals from methanol oxidization contribute to the dehydrogenation of ammonia. Besides, NOx emission analysis demonstrates that 60% methanol addition results in the highest NOx emissions. The most important reactions dominating the NOx consumption and production are identified in this study. (C) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
基于化学反应动力学及三维计算流体动力学(CFD)耦合开展了低压氨/柴油双燃料低速机的燃烧和排放仿真研究.构建了氨/柴油双燃料机理,其滞燃期、层流火焰速度及重要组分浓度的计算结果与试验结果吻合良好;在CONVERGE中建立了低速船机的三维CFD模型,确定了 G方程模型中NH3燃料层流火焰速度的经验参数,研究了压缩比和当量比对氨/柴油双燃料低速机性能的影响.结果表明:适当提高压缩比可以改善氨着火燃烧的稳定性,压缩比为14.5可获得较高效率并将最大爆发压力控制在合理范围;氨燃料当量比在0.410附近性能达到最优,当量比更高使着火过于提前、燃烧温度大幅提高,导致热效率下降和NOx排放明显升高,而当量比更低时指示热效率降低.在当量比为0.410、压缩比为14.5时氨/柴油双燃料低速机获得了效率及排放相互折衷下的最优值.
开展了多种燃油喷射策略对乙醇汽油发动机颗粒物排放影响的试验.结果表明:与单次喷射和2次喷射相比,3次喷射的颗粒物数量(PN)降低近50％,尤其是核态颗粒物降低更多.就降低颗粒物(PM)质量而言,采用多次喷射比增加乙醇比例的效果更明显.燃用3种乙醇体积分数为0(E0)、30％(E30)和85％(E85)的燃料下,总颗粒物排放随着乙醇比例增加而下降.对比两种瞬态工况发现,在绝大多数工况下3次喷射和5次喷射都比单次喷射的颗粒物浓度低.尤其是冷启动和大负荷工况下,多次喷射最多可将颗粒物浓度降低80％.随着乙醇体积分数增加,颗粒物粒径分布在绝大多数尺寸上都有明显下降,并通过降低爆震趋势可有效避免发动机性能下降和排放恶劣的现象.乙醇混合燃料结合多次喷射策略,在几乎不降低发动机动力性能的同时显著改善了颗粒物排放.
研究了波动背压对柴油机性能响应特性的影响,分析探讨了不同波动特征下性能参数的响应机制.结果表明:波动背压环境下,增压柴油机功率、增压压力、涡轮前排气压力及排气温度均随背压明显波动,呈现显著的迟滞环形状.背压波动周期和时均值是影响性能响应特性的关键因素.波动周期缩短时,发动机功率、增压压力及涡轮前排气压力迟滞环由狭长状逐渐扩展,非定常度扩至3倍以上,非定常特征逐渐强化.背压时均值减小时,各性能参数在波动条件下的迟滞环呈膨胀趋势,低时均背压时非定常度为高时均背压的2.55倍以上,非定常特性明显加强.进气端性能参数对波动背压的响应明显滞后于排气端,前者非定常度为后者1.9倍以上,波动背压条件下发动机性能响应迟滞特征主要来源于增压器转动惯量.
声发射信号具有频率范围宽、蕴含信息丰富的优点,被广泛应用于柴油机故障诊断中,但诊断判据在多工况和变机型下适应性较差,以柴油机喷油器喷孔堵塞故障为例,分析缸盖声发射信号燃烧段特征,发现不同机型燃烧段的缸盖声发射信号包含多个对喷孔堵塞故障较为敏感的衰减振荡,研究了适应性较好的特征参数提取方法.柴油机运行工况多变,难以采集到所有工况充足的故障样本,基于实例的TrAdaBoost迁移学习算法,利用已有工况的数据作为源域辅助训练数据,结合少量目标域数据构成联合训练集,通过对各故障类别的权重迭代,提高了同机型不同工况的故障诊断算法的鲁棒性和故障识别率,并经不同型号的柴油机试验验证表明,该方法将诊断准确率从55％提高到90％以上,有较强的跨机型适用性.
通过一台新开发的超高压快速压缩机,进行了不同比例的正丁醇/生物柴油混合燃料在当量比为0.3～1.0、压力为1～6 MPa以及温度为700～975 K条件下的自着火特性研究.结果表明:混合燃料的着火延迟时间随着温度、压力和当量比的升高而减小.此外,混合燃料的着火延迟时间随着正丁醇比例的增加而增大,但随着温度的升高,着火延迟时间对正丁醇的比例不敏感.采用已有的详细动力学模型进行了模拟研究并与试验结果进行了对比.通过对正丁醇/生物柴油自着火特性的化学动力学分析,研究了自着火过程中正丁醇和生物柴油之间的化学相互作用.结果表明:正丁醇的加入极大程度抢夺了生物柴油低温反应产生的自由基,削弱了生物柴油的低温反应路径.
通过同步辐射X射线测量技术,开展了直喷汽油机喷油嘴实际针阀运动过程中的内流与液核破碎的试验观测研究.在不同X射线光源参数下对内流流场进行了连续图像拍摄,确定了最优拍摄技术方案.分析了不同喷射压力、不同喷孔结构下内流随针阀特性的变化规律,进而讨论了内流对射流液核破碎的影响.结果表明:随着针阀开度的增加,喷孔内会快速地形成挑射液流.挑射液流的宽度与喷孔入口的角度有关,与喷射压力无关.其原因与所采用喷油嘴的长径比较短有关.挑射液流会加速射流的液核破碎过程.喷孔内挑射液流的宽度增加会引起射流宽度的增大.
基于Maxwell-Mohr定理建立活塞环自由型线模型,活塞环自由型线和截面尺寸作为输入建立了活塞环/缸套的三维有限元接触模型,得到活塞环/缸套接触力.设计并搭建了活塞环周向张力测量试验台,通过试验很难测得活塞环/缸套整个接触面的接触力(分布力),而只能测得有限个点的支反力(集中力).提出了分布力与集中力的等效原理模型,实现了对活塞环/缸套接触力模型的验证.结果表明:试验结果与仿真结果在345.位置处有最大误差为8.75％,其他位置具有较好的一致性,整体误差在可接受范围内.该模型为活塞环设计提供了理论依据,且可作为活塞环摩擦、润滑分析的输入,来提高计算的准确性.
基于CONVERGE软件开展三维仿真计算,分析了定容燃烧弹内高压甲烷射流撞击当量比为1的预混气体燃烧过程中的涡量变化特征.根据射流前锋面与火焰的位置关系,将火焰发展全过程分为射流前峰面未接触火焰阶段、进入火焰阶段和离开火焰阶段3个阶段.相比于自由射流贯穿速度,在射流进入火焰阶段射流燃烧平均贯穿速度显著增加,并大于自由射流的贯穿速度和预混燃烧速度之和.产生此结果的原因是此阶段动量参数涡量的大幅增加;在轴向距离为40mm处、火焰发展时间为1.6ms时,射流燃烧模式平均涡量值与自由射流模式差值最大,约为260 s-1,是此时此截面预混燃烧模式平均涡量值6s-1的43倍.
为研究海拔对柴油机瞬态排放的影响及变化规律,试验了 0、1.0、2.0和2.4 km海拔下柴油机恒转速增转矩瞬态模式时NOx体积分数、烟度和颗粒数(PN)排放变化规律.同时,通过全球统一瞬态试验循环(WHTC)分析了不同海拔下柴油机NOx、颗粒物(PM)和PN排放变化.结果表明:当增压压力、过量空气系数和废气再循环(EGR)率趋于稳定前,NOx体积分数随海拔升高逐渐增大;当增压压力、过量空气系数和EGR率均趋于稳定后,NOx体积分数随海拔升高而降低.不同海拔下,瞬态过程中烟度和PN均出现峰值,随着油门100％开度响应时间缩短和海拔升高,烟度和PN峰值逐渐增大.WHTC结果显示:随着海拔升高,NOx、PM和PN排放均增大;与0 km海拔相比,1.0、2.0和2.4 km海拔下的NOx排放分别增大了 7.17％、12.69％和16.90％,PM排放分别增大了3.78％、13.64％和 24.88％,PN 排放分别增大了 21.49％、48.86％和 58.70％.
基于一台2.0L涡轮增压直喷氢内燃机,试验了稀薄燃烧下直喷氢内燃机的近零排放(NOx排放小于20 × 10-6)工作特性,在中高转速(2 000～3 500r/min)下,近零排放时平均有效压力均达到1MPa以上,相比于自然吸气状态,动力性提升了 2.6倍.进一步针对不同喷氢相位和喷氢压力进行了全工况研究,结果表明:当喷氢结束角为80℃ABTDC时,在近零排放边界下,可以同时达到最大的平均有效压力和最高有效热效率.优化后的近零排放氢内燃机最高转矩达到204N·m(2 500r/min),最大功率提升至72kW(3 500r/min),最高有效热效率达到41.5％(2 000r/min);可在全转速工况下实现乘用车常用工况范围的近零排放.提出的优化方法可以指导高效近零排放氢内燃机控制策略的开发.
为探究稀释燃烧改善发动机性能的潜力,通过一台1.5 L高压缩比增压直喷汽油机开展台架试验,对比研究了空气稀释、废气再循环(EGR)稀释及复合稀释燃烧在不同稀释程度下对中速、中负荷工况下发动机性能的影响规律.结果表明:稀释燃烧延长了燃烧持续期,降低了有效燃油消耗率(BSFC),减少了发动机传热损失,并降低了 CO排放,稀释方式不同会导致HC和NOx排放随稀释率的变化规律不同,但在高稀释率下,相比无稀释燃烧,HC排放升高,NOx排放降低;相较于EGR,空气稀释对燃烧的抑制更弱,稀释边界更宽,BSFC降低效果更好,CO与HC排放显著更低,未燃损失更低,NOx排放更高,且这些规律在相同稀释率、不同EGR占比的复合稀释燃烧的性能参数变化中同样存在,但有效热效率在过量空气系数φa=1.34、EGR率约为5％(稀释率为1.4)时达到最高,这与排气损失更低有关,此时相较原机,BSFC降低了 5.7％,NOx降低了 33％,均比φa为1.40时的降幅更大,证明了复合稀释燃烧具备更强的节能减排潜力.
超声波测量技术具有穿透性强的优点,是最具应用前景的柴油机活塞环油膜厚度测量手段.但由于柴油机的运行状态较为复杂,较多研究局限于超声波测量技术的理论与标定方面,实际应用于柴油机动态测量的研究较少.笔者设计并搭建了柴油机气缸油膜厚度超声波测量模拟试验台,对超声测量柴油机活塞环油膜厚度变化以及分布进行分析.通过电机模拟转速为0～100r/min下气缸与活塞的运行状态,并采用新型的数据采集策略来解决传统超声波测量过程中数据量大、有用信号占比较少的问题.结果表明:活塞环最小油膜厚度随转速的升高而增大,且测量精度随脉冲重复频率和空间分辨率的升高而增加.
为研究二聚环戊二烯(DCPD)燃料添加剂对碳烟颗粒生成演化的影响,采集掺混不同比例DCPD的正庚烷反扩散(IDF)及正扩散(NDF)火焰中的碳烟颗粒,使用高分辨率透射电子显微镜(HRTEM)、拉曼光谱(RS)和热重分析仪(TGA)分析碳烟颗粒的微观结构、石墨化程度和氧化活性.结果表明:在反扩散火焰中,随着DCPD掺混比例从0％增加到20％,颗粒中类液状物质显著减少,碳烟颗粒平均微晶尺寸增长,且平均微晶曲率降低,石墨化程度提高,氧化活性降低,表明在反扩散火焰中DCPD对初生碳烟具有促进其生长的作用.而在正扩散火焰中,随着DCPD掺混比例从0％增加到20％,碳烟颗粒粒径显著增加,微晶尺寸降低,微晶曲率升高,石墨化程度降低,氧化活性升高,表明正扩散火焰中DCPD可以通过改变碳烟颗粒的微观结构来增加其氧化活性.
针对现有基于时域特征的高压油泵故障诊断准确率低的问题,笔者提出一种参数优化变分模态分解(VMD)算法和散布熵的特征提取方法,并采用支持向量机(SVM)进行故障诊断.首先,基于对高压油泵工作原理及典型故障的分析,利用AMESim平台搭建高压油泵仿真模型进行故障模拟和信号采集.然后,针对VMD效果受限于分解个数和惩罚因子选取的问题,采用改进灰狼优化(IGWO)算法对VMD进行参数寻优.通过计算各模态的散布熵值形成故障特征向量,最后,采用SVM对故障特征向量进行训练和诊断,实现高压油泵的故障诊断.该方法的故障诊断准确率可达到95％以上,能有效地实现高压油泵故障诊断.
