Numerical Investigation of the Effects of Rate-Shaped Main Injection on Combustion and Emission in an OPOC Two-Stroke Diesel Engine

The effects of various split injection strategies on the opposed-piston opposed-cylinder(OPOC)diesel engine combustion and emission characteristics have been studied numerically using AVL-Fire CFD tools.The five rate-shaped main injections were used in split injection strategies.The results show that ignition delay from a rectangular injection rate is the shortest.Maximum pressure of the trapezoid injection rate is the largest.And the NOx emission of the rectangular injection rate is the largest.Meanwhile,the soot emission of the trapezoid injection rate is the least among the five injection rates.

Effects of different combustion modes on the thermal efficiency and emissions of a diesel pilot-ignited natural gas engine under low-medium loads

Research on dual-fuel(DF)engines has become increasingly important as engine manufacturers seek to reduce carbon dioxide emissions.There are significant advantages of using diesel pilot-ignited natural gas engines as DF engines.However,different combustion modes exist due to variations in the formation of the mixture.This research used a simulation model and numerical simulations to explore the combustion characteristics of high-pressure direct injection(HPDI),partially premixed compression ignition(PPCI),and double pilot injection premixed compression ignition(DPPCI)combustion modes under a low-medium load.The results revealed that the DPPCI combustion mode provides higher gross indicated thermal efficiency and more acceptable total hydrocarbon(THC)emission levels than the other modes.Due to its relatively good performance,an experimental study was conducted on the DPPCI mode engine to evaluate the impact of the diesel dual-injection strategy on the combustion process.In the DPPCI mode,a delay in the second pilot ignition injection time increased THC emissions(a maximum value of 4.27g/(kW·h)),decreased the emission of nitrogen oxides(a maximum value of 7.64 g/(kW·h)),increased and then subsequently decreased the gross indicated thermal efficiency values,which reached 50.4%under low-medium loads.

Numerical study of the effects of split injection strategy and swirl ratio for biodiesel PCCI combustion and emissions

New methods have been studied in the past few decades to decrease exhaust emissions and fuel consumption of diesel engines.Using new combustion methods as low-temperature combustion(LTC)strategies and also utilizing new fuels as bio-diesel are beneficial ways to overcome diesel engine issues.This numerical study focused on the effect of split injection strategies as well as swirl ratio(SR)on the biodiesel premixed charge compression ignition(PCCI)combustion process and the related emissions.In this regard,KIVA-3V code as a CFD tool has been utilized to analyze the combustion and emission characteristics of the diesel engine.Single and split injection strategies have been executed on biodiesel PCCI combustion and have been compared in order to probe and examine the injection strategies.The results show that in both strategies,advancing the injection timing can lower the combustion emission and improve the engine performance by optimizing the start of injection(SOI)as sweet spot value.Utilizing a high swirl ratio with the split injection strategy has a negative effect on overall combustion,emission,and performance in comparison with the single injection strategy.For multi-injection scheme,SOI=−35 ATDC,SR=1.1,and for single injection scheme,SOI=−40 ATDC,SR=1.1 are considered as optimum cases regarding minimum total exhaust emissions and highest performance.

EFFECTS OF INJECTION STRATEGY ON D.I. DIESEL ENGINE COMBUSTION

Experiments are conducted to develop an understanding of how split injections can affect the combustion and emission characteristics of a D.I. diesel engine with a common-rail injection system. The ratio of the amount of fuel injected between two injection pulses and the injection interval is varied keeping the injected fuel quantity constant. Results show that under the 70D90-10 injection pattern, the engine achieves the lower NOx-smoke emissions and BSFC compared with the single injection pattern. The heat release rate and the temperature show that the split injections increase the initial premixed burn and retards the diffusion burn. With the balance of these two effects, the maximum in-cylinder temperature decreases while the 50% heat release point is held at almost the same crank angle. Therefore, both NOx emission and BSFC are improved while keeping the smoke emission at the same level.

Combustion Characteristics and Emission of a Two-Stroke Compression Ignition Engine with Two-Stage Injection

In order to study the effect of two-stage injection on two-stroke diesel engines, a well characterized research engine equipped with electronically controlled common rail system and scavenging system was constructed. Through analysis of combustion and emissions, two-stage injection shows its advantages. Compared with the standard injection, it produces less emissions, while compared with single early injection, it expands engine operation range. Further experiments were carried out to study the influence of several injection control parameters on two-stage injection. The fuel in the first injection is used for forming homogeneous mixture. The fuel in the second injection keeps combustion, and it is the main source of smoke emissions. NO_x is formed in both combustion process caused by these two injections, and there is an optimum fuel allocation ration to produce minimum NO_x. The cylinder pressure decreases, and the combustion is depressed with the increasing of scavenging pressure. By optimizing the injection control parameters of two-stage injection, NO_x and smoke can be reduced beyond 30% simultaneously.

Complete Modeling for Systems of a Marine Diesel Engine

This paper presents a simulator model of a marine diesel engine based on physical, semi-physical, mathematical and thermodynamic equations, which allows fast predictive simulations The whole engine system is divided into several functional blocks： cooling, lubrication, air, injection, combustion and emissions. The sub-models and dynamic characteristics of individual blocks are established according to engine working principles equations and experimental data collected from a marine diesel engine test bench for SIMB Company under the reference 6M26SRP1. The overall engine system dynamics is expressed as a set of simultaneous algebraic and differential equations using sub-blocks and S-Functions of Matlab/Simulink. The simulation of this model, implemented on Matlab/Simulink has been validated and can be used to obtain engine performance, pressure, temperature, efficiency, heat release, crank angle, fuel rate, emissions at different sub-blocks. The simulator will be used, in future work, to study the engine performance in faulty conditions, and can be used to assist marine engineers in fault diagnosis and estimation （FDI） as well as designers to predict the behavior of the cooling system, lubrication system, injection system, combustion, emissions, in order to optimize the dimensions of different components. This program is a platform for fault simulator, to investigate the impact on sub-blocks engine＇s output of changing values for faults parameters such as： faulty fuel injector, leaky cylinder, worn fuel pump, broken piston rings, a dirty turbocharger, dirty air filter, dirty air cooler, air leakage, water leakage, oil leakage and contamination, fouling of heat exchanger, pumps wear, failure of injectors （and many others）.

大缸径柴油机燃烧系统优化模拟

为提高某缸径200 mm船用发电柴油机的燃油经济性,本文设计了活塞燃烧室和燃油喷射系统的升级方案并进行了模拟优化。升级方案提高了压缩比和燃油喷射压力,采用大径深比浅ω燃烧室配合158°喷油夹角喷油嘴。对不同方案下发动机的缸内工作过程进行了计算流体力学模拟,计算了高压指示功和放热率相位,分析了缸内温度、反应过量空气系数和速度分布及演化。模拟结果表明:升级方案能够提高发动机热效率。增加喷孔数并减小孔径,可以在保持NOx排放基本不变的条件下提高高压指示功4.5%,降低碳烟排放约60%。采用“平顶”浅ω燃烧室与158°喷油夹角喷雾配合,油气混合气快速进入余隙并形成逆时针的漩涡流动,能够加速油气混合和燃烧过程,提高热效率。

掺烧不同比例的异戊醇对船舶柴油机性能的影响

为了研究不同比例的异戊醇/柴油混合燃料在4190Z L C-2型船用中速柴油机中的燃烧和排放特性,运用AVL_FIRE仿真软件建立柴油机燃烧室模型,并通过台架实验验证其仿真模型的准确性。在仿真软件中通过调整异戊醇在柴油中的掺混比来研究混合燃料对柴油机燃烧和排放性能的影响。结果表明:掺混异戊醇有助于改变NO的生成规律,使NO在速燃期的生成量上升,在缓燃期与后燃期的生成量减少,并且总体生成的NO质量分数降低;随着异戊醇掺混比的上升,CO最终生成质量分数下降,Soot最终生成质量分数上升,同时生成的Soot质量分数峰值降低,在缓燃期的氧化速率增快,并因为较快的耗氧量使得最终生成的Soot质量分数比纯柴油的高。

无人机用点燃式二冲程柴油机燃烧特性研究

针对柴油燃料雾化蒸发困难导致的发动机燃烧效果差、易爆震的问题,本文采用空气辅助缸内直喷方案,对一台点燃式二冲程柴油发动机开展燃烧特性研究。建立了点燃式二冲程柴油发动机的三维仿真模型,对混合气特性和燃烧特性进行仿真研究,并通过台架试验进行了验证。研究结果表明:点燃式二冲程柴油发动机在热机工况下,点火时刻有58%的燃油蒸发,缸内平均当量比由冷起动工况下的0.2提升至0.8,混合气品质较高,缸内燃烧情况良好。通过燃用汽油、柴油两种燃料进行发动机整机性能对比试验发现,在高转速大负荷工况下,两种燃料的燃烧特性曲线十分接近,柴油燃烧的指示平均有效压力(IMEP)的分布均值仅比汽油低1.2%。总体而言,柴油燃料应用于点燃式二冲程发动机的燃烧性能与汽油燃料相似,且使用柴油燃料时缸内压力最大震荡幅值仅为0.02 MPa,远小于爆震阈值,不会出现爆震现象。

氢气掺混甲烷对湍流燃烧及NO生成排放的影响

采用非绝热火焰面-反应进度变量湍流燃烧模型和非绝热、非稳态NO计算预测模型,针对DLR双旋流燃烧室中的旋流喷射、混合和燃烧过程,开展了大涡模拟研究,获得了纯甲烷、体积分数30%和60%氢气掺混甲烷湍流燃烧的流场结构、火焰特性和NO分布的详细计算结果.分析表明,由于燃烧室中的流场主要由旋流空气所决定,掺氢燃烧的速度分布与纯甲烷燃烧没有明显区别.掺氢燃烧的外回流区温度降低,中心回流区温度升高,放热区域向上游移动,放热速率增大而放热区域减小.NO主要产生于内剪切层,外剪切层和高速剪切流的下游区域;在纯甲烷算例中,快速型和热力型机理所生成的NO数量相当,而掺氢燃烧中快速型机理所生成的NO减少、热力型机理所生成的NO则明显增多.

基于PISO算法的汽车喷油器优化仿真研究

通过三维数值模拟软件Converge结合PISO算法,针对重型直喷柴油机,研究喷油器的不同孔径、孔深和喷油倾角对发动机燃烧和排放性能的影响,得到柴油发动机最佳热效率的喷油器参数设置方案。结果表明:相较于喷孔深度和喷孔倾角,喷孔直径的改变对发动机热效率的影响更大,且可有效改善发动机的NO_(x)和碳烟排放。当喷孔直径为0.179 mm、0.219 mm及0.259 mm时,分别能实现1.4%、3.4%、和2.8%的增幅,热效率提升较为显著。

改变喷油压力对柴油机掺烧水/生物柴油性能的影响

基于某常规增压柴油机,研究了掺烧代用燃料生物柴油对柴油机性能的影响。但是,相比柴油,生物柴油黏性较大,含氧量高,对柴油机影响较大。因此对掺烧水/生物柴油做进一步研究,通过提高喷油压力进行优化。在柴油机75%中高负荷下,选择以下工况进行研究:生物柴油掺混比为40%,掺水率为2%~8%,喷油压力为31 MPa~61 MPa。结果表明:随着掺水率增加,缸内爆压下降幅度较小,NO排放降幅较大,能有效缓解生物柴油富氧性带来的NO废气增加问题;随着喷油压力升高,燃油雾化效果变好,最高爆发压力也有所提高,CO与碳烟排放得到优化,这改善了掺烧生物柴油带来的燃油雾化效果变差的不利影响。最终选择生物柴油掺混比为40%,掺水率为8%,组合喷油压力为41 MPa为最佳组合,该组合下的NO排放及缸内爆压与原机相当,CO和碳烟的排放优于原机。仿真结果可为生物柴油的实际应用提供解决措施。

柴油机主喷提前角对燃烧过程改善的模拟

多次燃油喷射的灵活调节对改善柴油机燃烧过程具有重要影响。利用CONVERGE模拟研究不同主喷提前角对燃烧过程和产物分布的影响,表明缸内燃空当量比为0.6~1.0的稀混合气容易自燃,引起缸内混合气温度的升高,自燃混合气浓度范围扩大,促使燃空当量比为1.0~1.3的浓混合气发生燃烧。当主喷提前角为12°CA BTDC时,通过改变稀混合气、浓混合气的自燃时刻,燃烧放热更加集中在压缩上止点附近,使得平均指示压力达到最大并提升3.8%。

喷射参数对柴油机燃烧与排放特性的影响

在一台共轨柴油机上进行了喷射压力、喷射定时、后喷量及后/主间隔角等喷射参数影响作用的试验研究,利用FIRE软件数值模拟获取微观场变化信息,综合分析了喷射参数对缸内燃烧与有害排放物生成的影响机理及规律.结果表明,提高喷射压力和提前喷射定时有利于改善燃油经济性及碳烟排放,但在喷射压力较高(120～130,MPa),喷射定时提前到上止点前2°CA时,再提前喷射定时对碳烟的影响不大,而NOx的生成量显著增加,同时会引起燃烧粗暴.当后/主间隔角一定(15°CA),后喷量为1.5～2.0,mg时,烟度值达到最低,降幅为28%左右;后喷量为1.5,mg时,后/主间隔角在25,oCA附近,烟度达到最低,降幅达到20%.因此,引入后喷时需要选取适当的后/主间隔角和后喷油量,既要增强后喷燃油对缸内流场的扰动效果,使得更多的氧气进入主喷的燃烧产物区,加速碳烟的氧化又要避免进入高温缺氧区域内的后喷燃油量过多以及燃烧过于拖后等负面影响.

燃烧室形状对增压中冷柴油机燃烧和排放影响的数值模拟

在保持压缩比不变的条件下,基于原机燃烧室,新设计了5种不同形状的燃烧室,应用CFD数值模拟软件STAR-CD模拟研究了6种不同形状的燃烧室对柴油机燃烧过程的影响.结果表明,喷雾和深坑缩口型燃烧室壁面碰撞后能产生较强的湍流运动,提高混合气的形成速率.当燃烧室喉口直径相同时,增加燃烧室凹坑深度有利于实现快速燃烧.随着燃烧室凹坑加深,最高爆发压力升高,燃烧持续期缩短;深坑缩口型燃烧室燃烧速度快,Soot排放低,但NOx排放高,而浅盘缩口型燃烧室则相反.

柴油机双卷流燃烧系统的排放特性

为研究双卷流燃烧系统对直喷式柴油机有害排放物生成的影响,该文采用试验和计算流体力学CFD仿真分析方法,探讨燃油喷射系统中喷油提前角、喷孔直径、油束夹角和涡流比对双卷流燃烧系统(DSCS)排放特性的影响。研究结果表明:随着喷油提前角的增加和喷孔直径的减小,NOx生成量增加,碳烟生成量降低;随着油束夹角的增加,碳烟生成量降低,NOx生成量先增加后降低;涡流比和弧脊的匹配直接影响着双卷流燃烧系统中燃油的雾化、混合和燃烧过程,为了降低有害污染物的排放应采用较小的涡流比与双卷流燃烧室,确定了与双卷流燃烧室相匹配的满足NOx、碳烟排放最佳涡流比为1。该研究可为柴油机双卷流燃烧系统优化设计提供参考。

直喷式柴油机燃烧室几何形状对排放影响的多维数值模拟研究

在保证压缩比相同的情况下,设计了3种不同形状的燃烧室。运用AVL公司开发的FIRE软件,对3种不同形状燃烧室进行了三维数值模拟,分析了上止点附近气缸内的气体流动情况,并对3种燃烧室内的排放情况及影响排放的主要因素进行了比较分析,得出:径深比过小,即燃烧室过深,不利于空气与油气的良好混合,易造成局部油气过浓,从而局部缺氧,产生较多碳烟;径深比过大时,油气雾化好、燃烧早、温度上升快,NOx生成量增加。最后,通过模拟计算与试验研究相结合,验证了模型并选取了最优形状的燃烧室。

柴油-丙烷发动机不同喷油提前角时的燃烧和排放特性

在一台单缸直喷式柴油机上开展了不同喷油提前角下燃用柴油和柴油-丙烷混合燃料时的燃烧和排放特性研究.研究结果表明:柴油-丙烷混合燃料的燃烧与排放参数随喷油提前角的变化趋势与燃用纯柴油时基本相同;对于给定的平均有效压力,柴油-丙烷混合燃料的放热率峰值和NOx排放随喷油提前角的推迟而减小,而总燃烧持续期、CO、HC和碳烟排放随喷油提前角的推迟而增加;对于给定的平均有效压力和喷油提前角,放热率峰值和NOx排放随混合燃料中丙烷含量的增加而增大,而总燃烧持续期、CO、HC和碳烟排放随丙烷含量的增加而减小.

F-T柴油在直喷式柴油机中燃烧与排放特性的研究

煤通过FischerTropsch(FT)合成可得到十六烷值高、硫和芳香烃质量分数极低的FT柴油。研究分析了未作改动的单缸直喷式柴油机燃用FT柴油时的燃烧和排放特性。结果表明,与燃用0号柴油相比,燃用FT柴油时的滞燃期平均缩短18.7%,预混燃烧放热峰值降低26.8%,扩散燃烧放热峰值较高,燃烧持续期相当。燃用FT柴油时的最高燃烧压力略低,最大压力升高率显著下降,机械损失和燃烧噪音较小,燃油消耗率和热效率都得到显著改善。燃用FT柴油可同时降低CO、HC、NOx和炭烟排放,其中NOx和炭烟分别平均降低16.7%和40.3%。研究表明,FT柴油是柴油机良好的清洁代用燃料。

柴油机分段喷射及涡流匹配燃烧和排放特性

为探究柴油机分段喷射及涡流改善燃油经济性的潜力和机理,在单缸机上进行了有、无进气涡流时不同分段喷射策略的试验,5%,预喷比例、10°,CA间隔工况的燃油消耗率比单次喷射降低6.1,g/(k W·h),NO_x排放增加200×10^(-6)(体积分数);有、无涡流对比试验中,在过量空气系数为1.6时,加入涡流比为1.0的涡流,燃油消耗率仍降低2.0,g/(k W·h),NO_x排放增加120×10^(-6).内窥镜试验利用KL因子表征碳烟浓度,上止点后16°,CA,单次喷射工况KL因子为1.22,分段喷射工况KL因子为0.75,有涡流分段喷射工况KL因子为0.21.仿真分析表明,小预喷、短间隔分段喷射和适当进气涡流的加入,均使得主喷油束可以利用预喷产生的热氛围且不被其束缚,从而改善燃油经济性,降低缸内碳烟浓度.