【作者】 田江平；

【导师】 隆武强； 西田惠哉；

【作者基本信息】 大连理工大学 ， 动力机械及工程， 2009， 博士

【摘要】 相对于进气道喷射汽油机而言,缸内直喷汽油机具有燃油经济性好、响应快、冷启动容易、空燃比控制精确、EGR率大、排放低等优点,将成为进气道喷射汽油机的替代产品。而其点火可靠性和燃烧稳定性的提高一直是研究的关键点。为此,作者提出了点火室式直喷(DISC:Direct-Injection Sub-Chamber)汽油机燃烧系统的概念,即:在缸盖上设置一点火室,点火室与主燃烧室由通道相连,火花塞位于室内,喷油嘴置于室内或室外。通过利用机械式三次喷射油嘴的喷射特性,或利用进气气流和压缩挤流运动的燃油喷射策略,或利用喷雾在不同背压下的贯穿距特性和压缩挤流运动的燃油喷射策略,来保证在不同工况下,点火室内的混合气浓度都在可点燃范围内,从而确保点火可靠性。由于点火室喷出的火焰能量大大超过火花塞的点火能量,提高了燃烧稳定性,可以进一步扩大缸内直喷汽油机的稀薄燃烧范围,提高经济性,并能降低控制系统的难度。对一台直喷柴油机的供油系统和燃烧系统进行了改造,增设了电控点火系统,使其成为点火室式直喷汽油机的原理样机。在这台样机上,进行了点火可靠性和怠速排放性能实验。结果表明:经过改装后的发动机各部件工作正常,电控点火系统工作稳定,满足燃烧系统初步验证试验的要求。发动机可以在空燃比48下稳定运行,并具有400r/min的超低怠速。经过优化的点火室式直喷汽油机具有低于国3标准的排放性能。为了保证点火室内混合气的当量比浓度,设计了机械式三次喷射油嘴,并完成了其工作过程的数值模拟,分析了各喷孔的喷射特性。计算结果表明:三次喷射油嘴能够保证从恒量喷孔喷射出的燃油量不受循环供油量的影响,其恒量喷孔及主喷孔的喷射性能均能够满足点火室式直喷汽油机的要求。三次喷射油嘴的喷雾喷入到点火室内,是一个近距离多次碰撞的复杂过程。使用激光吸收散射法和CFD计算程序对点火室内的混合气形成过程进行了实验和模拟研究,并对目前可获得的喷雾/壁面作用模型进行了评价。实验结果表明,不同的碰撞角度对喷雾在点火室内的发展和混合气形成过程有显著影响。高的喷射压力会提高喷雾的汽化速度,但是对喷射终了时刻气相质量的绝对值影响不大。当环境温度和压力较低时,喷雾的汽化速度非常低,会造成点火室内火花塞的湿润现象,因此喷雾不宜过早喷入点火室。点火室深度较小时可以缩短其内部形成均匀混合气的时间,但是也会造成火花塞的湿润现象。不同的喷雾/壁面作用模型计算结果有很大的差别,很难各个方面都与实验结果相吻合。由于对喷油时刻和雾化的要求,应用于本燃烧系统的汽油多孔喷油嘴不能像柴油机用多孔喷油嘴那样采用较大的喷孔夹角。为了研究小夹角喷雾之间的相互干涉现象,加工了小喷孔轴线夹角的双孔喷嘴。使用激光吸收散射法对双孔喷雾特性进行了测量,并将结果与作为基准的单孔喷雾进行了比较。与实验相配合,使用三维CFD计算程序AVLFIRE对单孔及双孔喷雾进行了数值模拟。结果表明:当喷孔轴线夹角小于25°时,由于两喷束内的液滴之间的碰撞和聚合,使喷雾的汽化速度降低。小液滴聚合成为大液滴,增加了液滴的动量,有使双孔喷雾贯穿距增大的趋势。另一方面,由于两喷束内液滴具有较高的轴向速度,而周围空气未能及时补充到这个区域,使得在喷雾内部形成了相对压力较小的区域,使得双孔喷雾贯穿距有减小的趋势。双孔喷雾的贯穿距是以上两种因素相互作用的结果。存在于喷雾中心的低压区域,使得双孔喷雾的两喷束有向喷雾中心偏移的趋势,偏移量随喷孔轴线夹角的变化而变化。喷雾撞壁对于喷雾的汽化速度有显著的阻碍作用,需要尽量避免。在本燃烧系统中,指向活塞远端的喷雾可以采用较小的喷孔轴线夹角,以获得较长的贯穿距,来增加空气利用率;而指向活塞近端的喷雾应采用相对较大的喷孔轴线夹角,以减少或避免喷雾撞壁现象。更多还原

【Abstract】 Compared with PFI gasoline engine,GDI engine has become its substitute for higher fuel economy,faster engine response,easier cold-start,precise air/fuel ratio control,larger EGR rate,and lower emissions.However,it is difficult to realize the ignition reliability and combustion stability in GDI engines.Therefore,the concept of Direct-Injection Sub-Chamber (DISC) gasoline combustion system is advanced.A sub-chamber is set in the cylinder head, and the sub-chamber is connected with the main combustion chamber by a channel.The spark plug is located in the sub-chamber,and the injector is mounted inside or outside the sub-chamber.Under all working conditions,the mixture inside the sub-chamber is in the range of easy to be ignited and the ignition reliability is realized.In the three executive schemes,the injection characteristics of the mechanical 3-injection nozzle,inlet airflow and squish,and spray tip penetration under different back pressure and squish are utilized to ensure the mixture concentration inside the sub-chamber.Due to the much larger energy of the flame compared with the spark plug discharging,the combustion stability is enhanced and the lean-bum range can be expanded.This combustion system can improve the fuel efficiency and decrease the difficulty of the control system.A DISC prototype engine is rebuilt from a DI diesel engine by modifying the fuel delivery system and combustion system,and adding electronic control system.After that,the preliminary feasibility and the emission experiments under idle working conditions are conducted.The following conclusions can be concluded from the experiments:all parts of the engine and the electronic control ignition system work well,and they can fulfill the demands of feasibility experiments of the DISC gasoline engine.The engine can run at the air/fuel ratio of 48 with the super low idle speed of 400r/min smoothly.The emissions of the optimized engine are lower than the National 3 emission regulations.In order to keep the mixture inside the sub-chamber at the range within which the mixture is easy to be ignited,a mechanical 3-injection nozzle is designed.Numerical simulation about its performance is conducted and the characteristics of the injection from each kind of nozzle hole are analyzed.The computational results show that the injection quantity from the constant nozzle hole is almost independent with the fuel injection quantity per cycle per cylinder,and the performance of both the constant nozzle hole and the major nozzle holes can fulfill the demands of this DISC gasoline engine. When the spray from the 3-injection nozzle is injected into the sub-chamber,it is a complex process including near-impingement and multi-impingement.The mixture formation process inside the sub-chamber is studied with the Laser Absorption-Scattering(LAS) technique and CFD code experimentally and numerically.After that,the spray/wall interaction models available are estimated using experimental results.Experimental results show that:the spray and the mixture formation process are influenced by the impinging angle. A higher injection pressure can increase the vaporization rate,while there is no big difference on the absolute value of the vapor phase mass around the EOI.Under low ambient pressure and temperature,the vaporization rate is very low,and it will cause spark-plug wetting.So,it is not suitable to inject fuel into the sub-chamber too early.When decreasing the sub-chamber depth,the time interval from the start of injection to a homogeneous mixture is formed inside the sub-chamber,but that will endanger the possibility of spark-plug wetting.Calculation results varies very much when adopting different spray/wall interaction models,and none of these models can simulate all the aspects of the impinging sprays perfectly by now.According to the demands of the combustion system on atomization and injection advance angle,the hole-axis-angle(HAA) among the spray jets in the DISC gasoline engine cannot be so large as typical multi-hole nozzles in diesel engines.Two-hole nozzles with small HAA were manufactured in order to study the interactions between the neighbor jets. The characteristics of the two-hole sprays are measured using LAS technique,and comparisons are made with the single-hole spray which is considered to be the baseline.At the same time,numerical simulations about the single-hole and two-hole sprays were made using a 3-D CFD code AVL FIRE.Experimental and numerical results show that:when the HAA is smaller than 25°,the vaporization of the spray will be decreased because of the collisions and coalescences of the droplets in the neighbor jets.The two-hole spray has the tendency to have a longer penetration due to the behavior of smaller droplets forming larger droplets,while the lower relative pressure region inside the spray formed by the high axial spray velocity and lack of air entrained into this area makes its penetration to decrease.So the spray tip penetration results from the co-effect of these two factors.The low relative pressure region inside the spray also causes the two jets to incline to the center of the spray,and the offset is determined by the HAA of the spray.In the DISC gasoline engine,a relative smaller HAA is favorable for the jets pointing the far end of the piston,so that it will result in a longer penetration,which can enhance the air utilization.A relative larger HAA is favorable for the jets pointing the near end of the piston,in order to decrease or avoid impingement.更多还原