燃烧边界条件对车用重型天然气发动机燃烧及排放的影响
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摘要
能源的匮乏和全球环境的恶化已成为世界关注的焦点，开发和使用节能清洁燃料已刻不容缓。在内燃机燃料中，天然气以其资源丰富、排放友好等优点得到了高度重视和运用。然而由天然气理化特性所决定的燃烧速度慢的问题，阻碍了天然气发动机进一步向高效超低排放的发展。另外，由于稀燃天然气发动机不能通过三效催化转化器TWC(Three WayCatalysts)来控制排放，因此需要合理组织机内净化措施，使其满足日益严格的排放法规。
为了解决以上问题，本文针对电控多点喷射重型稀燃天然气发动机，采用试验与数值模拟相结合的手段，从缸内气流运动、混合气浓度分布及燃烧过程三个方面入手，详细分析了影响天然气发动机火焰传播速度的物理及化学因素，对比研究了不同稀燃方式对天然气发动机工作过程的影响，提出了有利于天然气发动机高效清洁燃烧的缸内流动分布、混合气浓度分布及燃烧模式，为开发高效清洁天然气发动机提供了理论依据和数据支持。
为选取合理的燃烧边界条件值，首先通过试验研究了点火及喷气时刻对发动机缸内燃烧过程和排放特性的影响规律。在对喷气时刻的研究中结合了数值模拟结果的分析，更直观清晰的解释了试验燃烧结果规律，研究结果表明：
1.在一定范围内，点火时刻的提前和喷气时刻的滞后能够提高燃烧等容度，获得更多工质做功能力，提高燃油经济性，但NOx排放增加；模拟结果显示喷气时刻的滞后使燃气浓度在气缸轴线方向上的分布状态由上稀下浓变换为上浓下稀，且混合气浓度分布梯度增大。
2.通过调节点火及喷气时刻能够实现基于NOx排放或燃油经济性的不同优化控制策略，在基于NOx排放控制策略下的欧洲稳态标准13工况测试循环ESC13(EuropeanStationary Cycle)值可以使NOx排放满足国4排放法规限值。
为了实现天然气发动机的高效燃烧，改善其火焰传播速度慢的问题，提出利用压缩后期流体湍动能来促进燃烧速度的思想。采用STAR-CD三维流体软件模拟计算明确了发动机缸内气流运动的演变历程，采用新型燃烧室及不同燃气喷射位置实现了压缩后期湍动能的改变。利用纵横截面图及对关键区域采样统计的方法揭示了不同燃烧室形状及燃气喷射位置下，缸内气流运动特性，流体运动速度场，湍动能场及混合气浓度分布状态，结合试验数据分析了缸内流动对燃烧过程及有害物排放的影响规律，研究结果表明：
1.压缩后期缸内湍动能峰值相位与放热率峰值相位距离越短越有利于提高燃烧等容度，增加发动机热效率。十字形燃烧室所对应的湍动能与放热率峰值相位距离比原机圆柱形燃烧室的缩短了10.5°CA。
2.缸内流动的微观参数场显示原机的圆柱形燃烧室保持了流体的高速大尺度涡流运动，径向涡流比较改进后的十字形燃烧室高31.7%；而十字形燃烧室剪切大尺度涡流运动产生更多小尺度湍流，使压缩后期的质量平均湍动能峰值增加了78.2%。在对均布肋圆周半径上的采样点统计分析得知，在燃烧过程中，十字形燃烧室内流体平均速度虽然比圆柱形的低，但速度大小分布更紊乱，速度标准差更高，所以不规则非稳定气流运动更多。
3.结合试验结果研究发现，在火花塞周围混合气浓度基本一致的情况下，十字形燃烧室由于强的湍动能不利于初期稳定火核的快速形成，表现为滞燃期加长；而在主要燃烧过程中，十字形燃烧室高的湍动能促使快速燃烧的产生，缩短了燃烧持续期，使发动机燃油经济性在外特性下平均提高了5.2%，排气温度平均降低了23.7℃。对比ESC13工况测试值得出，十字形燃烧室所对应的NOx排放比原机燃烧室高40%，HC和CO排放有所下降。
4.基于4种不同燃气喷射位置的模拟结果发现，喷气位置影响局部流动状态，使压缩后期缸内混合气在径向上的浓度分布发生改变，合适的喷气位置可以在火花塞周围形成有利于快速燃烧的湍动能和燃气浓度分布。试验所得燃烧和排放结果，很好的验证了对模拟结果的流动及混合气浓度分析。
为了使天然气发动机在只采用机内净化措施的情况下大大降低NOx排放，试验研究了三种不同稀燃方式（当量比燃烧+废气再循环EGR(Exhaust Gas Recirculation)稀释、空气稀释、空气+EGR双重稀释）对天然气发动机燃烧过程，排放特性及稀燃极限的影响规律，研究结果表明：
1.在当量比燃烧+EGR稀释方式下，通过点火时刻、喷气时刻及EGR率本身的优化，可以使NOx排放降幅达90.2%。而在当量比燃烧情况下，EGR对降低排气温度的贡献较小，与双重稀释燃烧方式的排气温度对比发现，前者在平均EGR率为21%时所对应的平均排气温度比后者在平均EGR率为6.5%时所对应的平均排气温度还高出9.7%。
2.在空气稀释燃烧方式下，随着额外空气量的增多，缸内压力和放热率减小，峰值相位滞后，最高燃烧压力循环变动系数增大。NOx排放在过量空气系数λ=1.1-1.2区间达到巅峰，而后随着λ的继续增大而急剧下降；HC排放表现出与NOx排放相反的变化趋势；燃气消耗率随着λ的增加而降低。相比于双重稀释燃烧方式，空气稀燃对应的稀释极限较小，当过量空气系数较大时容易出现燃烧不稳现象。在ESC13各转速下，同等燃气消耗率时，双重稀燃对应的NOx排放比空气稀燃分别降低43.2%，47.1%和50.9%。
3.对比三种燃烧方式ESC13测试循环值得出，当量比燃烧+EGR方式可以达到更优的NOx排放；空气稀释燃烧方式可以达到更优的燃油经济性；双重稀释燃烧方式则结合了以上二者的优点，能够达到更优的NOx-be折衷关系。在对点火时刻和喷气时刻等燃烧边界条件进行优化后，三种燃烧方式均可以使天然气发动机的NOx排放满足国4和国5排放限值，且双重稀释燃烧方式具有最优的燃油经济性。
为了发挥EGR降低NOx排放的功效，减小其对燃烧速度的恶化作用，以达到在保证天然气发动机热效率的同时进一步降低NOx排放的目的，提出了EGR分层方式。为了将传统EGR回流方式与发动机本身解耦，避免其引入方式对发动机运转过程造成的影响，以及EGR量和引入压力等的复杂控制问题，同时为了解决EGR引入压力与燃气气轨压力存在的大压差问题，采用了CO_2代替EGR的方法。研究基于三维数值模拟和发动机台架试验分析了EGR均质分布（空气掺混CO_2）与EGR分层分布（天然气掺混CO_2）对天然气发动机缸内混合气形成、燃烧过程及排放特性的影响规律，研究结果表明：
1.天然气掺混CO_2方式下缸内CO_2浓度呈现分层分布，且分布规律与天然气浓度分布一致。相比于空气掺混CO_2方式，天然气掺混所对应的缸内总稀释比峰值更小，实现了燃气浓的区域CO_2多，燃气稀的区域CO_2少的理想分布状态。
2.试验结果表明无论何种掺混方式下，CO_2的加入都使整个燃烧速度变缓，滞燃期，主燃期均随着CO_2掺混率的增加而延长，CA50后移，循环变动增大。而天然气掺混CO_2方式对缸内燃烧速度影响较小，其对应的排气温度也较空气掺混的低。在1750r/min-75%负荷工况下，通过优化燃烧边界条件后发现无论何种掺混方式，CO_2的加入都能够使be-NOx折衷区域优于无CO_2掺混所得区域，天然气掺混CO_2方式能够得到比空气掺混CO_2方式和无掺混都更优的be-NOx折衷区域，改善了be-NOx的矛盾关系，为实现重型稀燃天然气发动机的高效清洁燃烧提供了可行燃烧技术。
World attention on energy crisis and environmental pollution leads a quest for energyconversation and clean burning fuels in the internal combustion engines. Compressed naturalgas (CNG), regarded as one of the most promising alternative fuels, is widely used inautomotive engines due to its rich resource and friendly emissions. Howerver, flamepropagation velocity is slow during the combustion process in engines when fueled with CNGdue to its physiochemical properties, which restricts the development of high efficiency andultralow emissions. In addition, the reasonable in-engine emission control technologies shouldbe proposed to meet tightening emission regulations due to the unavailability of Three WayCatalysts (TWC) on lean-burn CNG engine.
In order to solve those problems, the in-cylinder flows and the distribution of mixtureconcentration as well as combustion process were investigated in the electronic controlled andmulti-point injection lean-burn CNG engine by means of experiment and3D numericalsimulation. The physical and chemical factors were analyzed which influence the flamepropagation velocity of CNG engine. The influence of various lean-burn modes on workingprocess of CNG engine has been comparatively studied. Based on these researches, advancedin-cylinder flows and mixture concentration distribution as well as combustion mode wereproposed to support the development of high efficiency and cleaning burning CNG engine.
For the reasonable combustion boundary conditions, the influences of ignition timing andinjection timing on combustion process and emission characteristic were experimentalinvestigated firstly. Combine the analysis of numerical simulation results in the injection timingresearch, which could visually explain the experimental burning tendency. The research resultsshowed that: within certain limits, the constant volume ratio increased during combustion whenthe ignition timing was ahead and the injection timing was behind, which could obtain moreworking power so that the fuel economy and NOx emission were increased. The laggedinjection timing changed the fuel concentration distribution in the direction of cylinder axisfrom “up thin down thick” to “up thick down thin”. The various optimal control strategies basedon NOx emission or fuel economy could be realized by adjusting ignition and injection timing.The European Stationary Cycle13(ESC13) results satisfied China stage4emission regulationsbased on the NOx emission optimal control strategy.
In order to solve the problem of slow flame propagation in the CNG engine, the enhancedturbulence intensity during the late period of compression stroke was required to promoteburning velocity. The evolution of in-cylinder flows was described by3D computational fluid dynamic tool STAR-CD, and the methods based on changed shape of combustion chamber andfuel injection location were proposed to enhance the turbulent kinetic energy (TKE). The crossand vertical sections coupled with statistics results of critical area sampling revealedcharacteristics of in-cylinder flows, like velocity vectors, TKE and distribution of mixtureconcentration. The influence of in-cylinder flows on combustion process and emissions wasstudied combined with experimental results. The study results showed that:
1. The shorter phase position distance between the peak TKE and the peak heat release rateduring the late period of compression stroke was more beneficial to the enhanced burningconstant volume ratio and then increased thermal efficiency, which the distance of crosscombustion chamber was10.5°CA shorter than that of cylindrical combustion chamber.
2. The analysis of in-cylinder microphenomenon indicated that the original cylindricalcombustion chamber maintains high speed large scale vortex motion so that the horizontalvortex ratio is31.7%higher than that of cross combustion chamber. On the contrary, the crosscombustion chamber sheared the large scale vortex motion into micro scale turbulence, its peakTKE was78.2%higher than that of original combustion chamber. According to the statisticresults of sampling points on the ribs circle radius, during the combustion process, even thoughthe average velocity of cross combustion chamber flows was lower than that of cylindricalcombustion chamber, the velocity was more disordered so that the velocity standard deviationwas larger, which existed more turbulence.
3. The numerical and experimental results indicated that large TKE was disadvantage tothe early flame development under the same mixture concentration, which led to a longerignition delay period. However, the large TKE was required during the main combustion periodto form rapid burning. As a result, the large TKE in the cross combustion chamber acceleratedthe burning velocity, enhanced the combustion quality and then increased the fuel economy5.2%higher than that of the cylindrical combustion chamber at WOT as well as the23.7℃decreased discharge temperature. The ESC13results showed that the NOx emission of crosscombustion chamber was40%higher than that of the cylindrical combustion chamber, whereasthe HC and CO emissions were decreased.
4. Studies on the4kinds of fuel injection locations, the results showed that the influence offuel injection location on the average TKE was unobvious, but the local flow state was changed.The fuel injection location changed the distribution of mixture concentration in the radialdirection. The reasonable distribution of TKE and mixture concentration could be formedaround the spark plug. The experimental combustion and emission results efficiently validatedthe numerical flow and mixture concentration analysis.
The potential of various dilution methods to control NOx emission of CNG engine wasinvestigated experimentally by engine internal purification. Three kinds of dilution methodswere stoichiometric with Exhaust Gas Recirculation (EGR) dilution combution, air dilutioncombustion and twofold dilution combustion that combined extra air and EGR. The influenceof them on combustion process, emission characteristic and lean-burn limitation on CNGengine was researched, the results showed that:
1. In the combustion method of stoichiometric with EGR, based on the optimal ignitiontiming and injection timing as well as EGR rate, the NOx emission could decreased by90.2%.However, the contribution of EGR on decreased exhaust temperature was limited whilestoichiometric combustion. Compared with twofold dilution method, its exhaust temperaturewas9.7%higher while the average EGR rate was21%, yet the average EGR rate was6.5%inthe two-fold dilution.
2. In the combustion method of air dilution, the increased extra air amount led to decreasedin-cylinder combustion pressure and heat release rate, and to lagging corresponding phasepositions as well as the increased coefficient of cyclic variation of the peak combustion pressure.The NOx emission reached their peak between the excess air coefficient named λ from1.1to1.2, and then the increased λ resulted in the sharp decreased NOx emission, increased HCemission and reduced CNG consumption. Compared with the twofold dilution method, thedilution limit of the air dilution method was narrow so that unstable combustion could appear.The NOx emission of twofold dilution method was separately43.2%,47.1%and50.9%lowerthan that of air dilution method under the ESC13speeds while the CNG consumptions wereequal.
3. Compared the ESC13test emission results of that three kinds of dilution methodsindicated that, stoichiometric with EGR could obtain the optimal NOx emission and the airdilution method could obtain the optimal fuel economy while the twofold dilution combined theadvantages of those two methods so that obtained the optimal NOx-be relationship. The NOxemission could meet China stage4and5emission regulations through these three kinds ofdilution methods while the combustion boundary conditions were optimized. Moreover, thetwofold dilution method had the optimal fuel economy.
In order to utilize the great ability of EGR to control NOx emission and to alleviate thedisadvantage in burning velocity, to realize the further decreased of NOx emission whilemaintained the thermal efficiency, the tratified EGR was proposed. For decoupling theinteraction of traditional EGR loop and CNG engine, avoiding the complex control on theamount and introduced pressure, and sloving the large differential pressure between traditional EGR and fuel gas rail, the CO_2was adopted to replace the EGR. The traditional EGR is equal toair blending CO_2and the stratified EGR is equal to CNG blending CO_2. The influence of thesetwo CO_2blending methods on the mixture formation and mixture formation as well as emissionwas numerical and experimentral investigated, and the results indicated that:
1. The CO_2concentration distribution of CNG blending method was the same as the CH4concentration distribution. Compared with air blending CO_2, the peak of total dilution rate inthe CNG blending CO_2method was smaller, which realized the ideal distribution of that thickregion with large CO_2and thin region with less CO_2.
2. The experimental results indicated that the increased CO_2delayed the burning velocityand CA50, increased the ignition delay period, combustion duration and cyclical variation. TheCNG blending CO_2has weaker influence on the burning velocity. The optimal be-NOxrelationships of those three CO_2blending methods (no blending, air blending and CNGblending) were obtained under the optimized combustion boundary conditions at1750r/min-75%load. Blending CO_2could get better be-NOx relationship than that of no CO_2blending method. CNG blending CO_2obtained the optimal be-NOx relationship region than theothers. It improved trade-off between NOx and be, which was advantaged to the high efficiencycleaning combustion of the heavy-duty lean-burn CNG engine.
为了解决以上问题，本文针对电控多点喷射重型稀燃天然气发动机，采用试验与数值模拟相结合的手段，从缸内气流运动、混合气浓度分布及燃烧过程三个方面入手，详细分析了影响天然气发动机火焰传播速度的物理及化学因素，对比研究了不同稀燃方式对天然气发动机工作过程的影响，提出了有利于天然气发动机高效清洁燃烧的缸内流动分布、混合气浓度分布及燃烧模式，为开发高效清洁天然气发动机提供了理论依据和数据支持。
为选取合理的燃烧边界条件值，首先通过试验研究了点火及喷气时刻对发动机缸内燃烧过程和排放特性的影响规律。在对喷气时刻的研究中结合了数值模拟结果的分析，更直观清晰的解释了试验燃烧结果规律，研究结果表明：
1.在一定范围内，点火时刻的提前和喷气时刻的滞后能够提高燃烧等容度，获得更多工质做功能力，提高燃油经济性，但NOx排放增加；模拟结果显示喷气时刻的滞后使燃气浓度在气缸轴线方向上的分布状态由上稀下浓变换为上浓下稀，且混合气浓度分布梯度增大。
2.通过调节点火及喷气时刻能够实现基于NOx排放或燃油经济性的不同优化控制策略，在基于NOx排放控制策略下的欧洲稳态标准13工况测试循环ESC13(EuropeanStationary Cycle)值可以使NOx排放满足国4排放法规限值。
为了实现天然气发动机的高效燃烧，改善其火焰传播速度慢的问题，提出利用压缩后期流体湍动能来促进燃烧速度的思想。采用STAR-CD三维流体软件模拟计算明确了发动机缸内气流运动的演变历程，采用新型燃烧室及不同燃气喷射位置实现了压缩后期湍动能的改变。利用纵横截面图及对关键区域采样统计的方法揭示了不同燃烧室形状及燃气喷射位置下，缸内气流运动特性，流体运动速度场，湍动能场及混合气浓度分布状态，结合试验数据分析了缸内流动对燃烧过程及有害物排放的影响规律，研究结果表明：
1.压缩后期缸内湍动能峰值相位与放热率峰值相位距离越短越有利于提高燃烧等容度，增加发动机热效率。十字形燃烧室所对应的湍动能与放热率峰值相位距离比原机圆柱形燃烧室的缩短了10.5°CA。
2.缸内流动的微观参数场显示原机的圆柱形燃烧室保持了流体的高速大尺度涡流运动，径向涡流比较改进后的十字形燃烧室高31.7%；而十字形燃烧室剪切大尺度涡流运动产生更多小尺度湍流，使压缩后期的质量平均湍动能峰值增加了78.2%。在对均布肋圆周半径上的采样点统计分析得知，在燃烧过程中，十字形燃烧室内流体平均速度虽然比圆柱形的低，但速度大小分布更紊乱，速度标准差更高，所以不规则非稳定气流运动更多。
3.结合试验结果研究发现，在火花塞周围混合气浓度基本一致的情况下，十字形燃烧室由于强的湍动能不利于初期稳定火核的快速形成，表现为滞燃期加长；而在主要燃烧过程中，十字形燃烧室高的湍动能促使快速燃烧的产生，缩短了燃烧持续期，使发动机燃油经济性在外特性下平均提高了5.2%，排气温度平均降低了23.7℃。对比ESC13工况测试值得出，十字形燃烧室所对应的NOx排放比原机燃烧室高40%，HC和CO排放有所下降。
4.基于4种不同燃气喷射位置的模拟结果发现，喷气位置影响局部流动状态，使压缩后期缸内混合气在径向上的浓度分布发生改变，合适的喷气位置可以在火花塞周围形成有利于快速燃烧的湍动能和燃气浓度分布。试验所得燃烧和排放结果，很好的验证了对模拟结果的流动及混合气浓度分析。
为了使天然气发动机在只采用机内净化措施的情况下大大降低NOx排放，试验研究了三种不同稀燃方式（当量比燃烧+废气再循环EGR(Exhaust Gas Recirculation)稀释、空气稀释、空气+EGR双重稀释）对天然气发动机燃烧过程，排放特性及稀燃极限的影响规律，研究结果表明：
1.在当量比燃烧+EGR稀释方式下，通过点火时刻、喷气时刻及EGR率本身的优化，可以使NOx排放降幅达90.2%。而在当量比燃烧情况下，EGR对降低排气温度的贡献较小，与双重稀释燃烧方式的排气温度对比发现，前者在平均EGR率为21%时所对应的平均排气温度比后者在平均EGR率为6.5%时所对应的平均排气温度还高出9.7%。
2.在空气稀释燃烧方式下，随着额外空气量的增多，缸内压力和放热率减小，峰值相位滞后，最高燃烧压力循环变动系数增大。NOx排放在过量空气系数λ=1.1-1.2区间达到巅峰，而后随着λ的继续增大而急剧下降；HC排放表现出与NOx排放相反的变化趋势；燃气消耗率随着λ的增加而降低。相比于双重稀释燃烧方式，空气稀燃对应的稀释极限较小，当过量空气系数较大时容易出现燃烧不稳现象。在ESC13各转速下，同等燃气消耗率时，双重稀燃对应的NOx排放比空气稀燃分别降低43.2%，47.1%和50.9%。
3.对比三种燃烧方式ESC13测试循环值得出，当量比燃烧+EGR方式可以达到更优的NOx排放；空气稀释燃烧方式可以达到更优的燃油经济性；双重稀释燃烧方式则结合了以上二者的优点，能够达到更优的NOx-be折衷关系。在对点火时刻和喷气时刻等燃烧边界条件进行优化后，三种燃烧方式均可以使天然气发动机的NOx排放满足国4和国5排放限值，且双重稀释燃烧方式具有最优的燃油经济性。
为了发挥EGR降低NOx排放的功效，减小其对燃烧速度的恶化作用，以达到在保证天然气发动机热效率的同时进一步降低NOx排放的目的，提出了EGR分层方式。为了将传统EGR回流方式与发动机本身解耦，避免其引入方式对发动机运转过程造成的影响，以及EGR量和引入压力等的复杂控制问题，同时为了解决EGR引入压力与燃气气轨压力存在的大压差问题，采用了CO_2代替EGR的方法。研究基于三维数值模拟和发动机台架试验分析了EGR均质分布（空气掺混CO_2）与EGR分层分布（天然气掺混CO_2）对天然气发动机缸内混合气形成、燃烧过程及排放特性的影响规律，研究结果表明：
1.天然气掺混CO_2方式下缸内CO_2浓度呈现分层分布，且分布规律与天然气浓度分布一致。相比于空气掺混CO_2方式，天然气掺混所对应的缸内总稀释比峰值更小，实现了燃气浓的区域CO_2多，燃气稀的区域CO_2少的理想分布状态。
2.试验结果表明无论何种掺混方式下，CO_2的加入都使整个燃烧速度变缓，滞燃期，主燃期均随着CO_2掺混率的增加而延长，CA50后移，循环变动增大。而天然气掺混CO_2方式对缸内燃烧速度影响较小，其对应的排气温度也较空气掺混的低。在1750r/min-75%负荷工况下，通过优化燃烧边界条件后发现无论何种掺混方式，CO_2的加入都能够使be-NOx折衷区域优于无CO_2掺混所得区域，天然气掺混CO_2方式能够得到比空气掺混CO_2方式和无掺混都更优的be-NOx折衷区域，改善了be-NOx的矛盾关系，为实现重型稀燃天然气发动机的高效清洁燃烧提供了可行燃烧技术。
World attention on energy crisis and environmental pollution leads a quest for energyconversation and clean burning fuels in the internal combustion engines. Compressed naturalgas (CNG), regarded as one of the most promising alternative fuels, is widely used inautomotive engines due to its rich resource and friendly emissions. Howerver, flamepropagation velocity is slow during the combustion process in engines when fueled with CNGdue to its physiochemical properties, which restricts the development of high efficiency andultralow emissions. In addition, the reasonable in-engine emission control technologies shouldbe proposed to meet tightening emission regulations due to the unavailability of Three WayCatalysts (TWC) on lean-burn CNG engine.
In order to solve those problems, the in-cylinder flows and the distribution of mixtureconcentration as well as combustion process were investigated in the electronic controlled andmulti-point injection lean-burn CNG engine by means of experiment and3D numericalsimulation. The physical and chemical factors were analyzed which influence the flamepropagation velocity of CNG engine. The influence of various lean-burn modes on workingprocess of CNG engine has been comparatively studied. Based on these researches, advancedin-cylinder flows and mixture concentration distribution as well as combustion mode wereproposed to support the development of high efficiency and cleaning burning CNG engine.
For the reasonable combustion boundary conditions, the influences of ignition timing andinjection timing on combustion process and emission characteristic were experimentalinvestigated firstly. Combine the analysis of numerical simulation results in the injection timingresearch, which could visually explain the experimental burning tendency. The research resultsshowed that: within certain limits, the constant volume ratio increased during combustion whenthe ignition timing was ahead and the injection timing was behind, which could obtain moreworking power so that the fuel economy and NOx emission were increased. The laggedinjection timing changed the fuel concentration distribution in the direction of cylinder axisfrom “up thin down thick” to “up thick down thin”. The various optimal control strategies basedon NOx emission or fuel economy could be realized by adjusting ignition and injection timing.The European Stationary Cycle13(ESC13) results satisfied China stage4emission regulationsbased on the NOx emission optimal control strategy.
In order to solve the problem of slow flame propagation in the CNG engine, the enhancedturbulence intensity during the late period of compression stroke was required to promoteburning velocity. The evolution of in-cylinder flows was described by3D computational fluid dynamic tool STAR-CD, and the methods based on changed shape of combustion chamber andfuel injection location were proposed to enhance the turbulent kinetic energy (TKE). The crossand vertical sections coupled with statistics results of critical area sampling revealedcharacteristics of in-cylinder flows, like velocity vectors, TKE and distribution of mixtureconcentration. The influence of in-cylinder flows on combustion process and emissions wasstudied combined with experimental results. The study results showed that:
1. The shorter phase position distance between the peak TKE and the peak heat release rateduring the late period of compression stroke was more beneficial to the enhanced burningconstant volume ratio and then increased thermal efficiency, which the distance of crosscombustion chamber was10.5°CA shorter than that of cylindrical combustion chamber.
2. The analysis of in-cylinder microphenomenon indicated that the original cylindricalcombustion chamber maintains high speed large scale vortex motion so that the horizontalvortex ratio is31.7%higher than that of cross combustion chamber. On the contrary, the crosscombustion chamber sheared the large scale vortex motion into micro scale turbulence, its peakTKE was78.2%higher than that of original combustion chamber. According to the statisticresults of sampling points on the ribs circle radius, during the combustion process, even thoughthe average velocity of cross combustion chamber flows was lower than that of cylindricalcombustion chamber, the velocity was more disordered so that the velocity standard deviationwas larger, which existed more turbulence.
3. The numerical and experimental results indicated that large TKE was disadvantage tothe early flame development under the same mixture concentration, which led to a longerignition delay period. However, the large TKE was required during the main combustion periodto form rapid burning. As a result, the large TKE in the cross combustion chamber acceleratedthe burning velocity, enhanced the combustion quality and then increased the fuel economy5.2%higher than that of the cylindrical combustion chamber at WOT as well as the23.7℃decreased discharge temperature. The ESC13results showed that the NOx emission of crosscombustion chamber was40%higher than that of the cylindrical combustion chamber, whereasthe HC and CO emissions were decreased.
4. Studies on the4kinds of fuel injection locations, the results showed that the influence offuel injection location on the average TKE was unobvious, but the local flow state was changed.The fuel injection location changed the distribution of mixture concentration in the radialdirection. The reasonable distribution of TKE and mixture concentration could be formedaround the spark plug. The experimental combustion and emission results efficiently validatedthe numerical flow and mixture concentration analysis.
The potential of various dilution methods to control NOx emission of CNG engine wasinvestigated experimentally by engine internal purification. Three kinds of dilution methodswere stoichiometric with Exhaust Gas Recirculation (EGR) dilution combution, air dilutioncombustion and twofold dilution combustion that combined extra air and EGR. The influenceof them on combustion process, emission characteristic and lean-burn limitation on CNGengine was researched, the results showed that:
1. In the combustion method of stoichiometric with EGR, based on the optimal ignitiontiming and injection timing as well as EGR rate, the NOx emission could decreased by90.2%.However, the contribution of EGR on decreased exhaust temperature was limited whilestoichiometric combustion. Compared with twofold dilution method, its exhaust temperaturewas9.7%higher while the average EGR rate was21%, yet the average EGR rate was6.5%inthe two-fold dilution.
2. In the combustion method of air dilution, the increased extra air amount led to decreasedin-cylinder combustion pressure and heat release rate, and to lagging corresponding phasepositions as well as the increased coefficient of cyclic variation of the peak combustion pressure.The NOx emission reached their peak between the excess air coefficient named λ from1.1to1.2, and then the increased λ resulted in the sharp decreased NOx emission, increased HCemission and reduced CNG consumption. Compared with the twofold dilution method, thedilution limit of the air dilution method was narrow so that unstable combustion could appear.The NOx emission of twofold dilution method was separately43.2%,47.1%and50.9%lowerthan that of air dilution method under the ESC13speeds while the CNG consumptions wereequal.
3. Compared the ESC13test emission results of that three kinds of dilution methodsindicated that, stoichiometric with EGR could obtain the optimal NOx emission and the airdilution method could obtain the optimal fuel economy while the twofold dilution combined theadvantages of those two methods so that obtained the optimal NOx-be relationship. The NOxemission could meet China stage4and5emission regulations through these three kinds ofdilution methods while the combustion boundary conditions were optimized. Moreover, thetwofold dilution method had the optimal fuel economy.
In order to utilize the great ability of EGR to control NOx emission and to alleviate thedisadvantage in burning velocity, to realize the further decreased of NOx emission whilemaintained the thermal efficiency, the tratified EGR was proposed. For decoupling theinteraction of traditional EGR loop and CNG engine, avoiding the complex control on theamount and introduced pressure, and sloving the large differential pressure between traditional EGR and fuel gas rail, the CO_2was adopted to replace the EGR. The traditional EGR is equal toair blending CO_2and the stratified EGR is equal to CNG blending CO_2. The influence of thesetwo CO_2blending methods on the mixture formation and mixture formation as well as emissionwas numerical and experimentral investigated, and the results indicated that:
1. The CO_2concentration distribution of CNG blending method was the same as the CH4concentration distribution. Compared with air blending CO_2, the peak of total dilution rate inthe CNG blending CO_2method was smaller, which realized the ideal distribution of that thickregion with large CO_2and thin region with less CO_2.
2. The experimental results indicated that the increased CO_2delayed the burning velocityand CA50, increased the ignition delay period, combustion duration and cyclical variation. TheCNG blending CO_2has weaker influence on the burning velocity. The optimal be-NOxrelationships of those three CO_2blending methods (no blending, air blending and CNGblending) were obtained under the optimized combustion boundary conditions at1750r/min-75%load. Blending CO_2could get better be-NOx relationship than that of no CO_2blending method. CNG blending CO_2obtained the optimal be-NOx relationship region than theothers. It improved trade-off between NOx and be, which was advantaged to the high efficiencycleaning combustion of the heavy-duty lean-burn CNG engine.
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