【作者】 韩立伟；

【导师】 洪伟；

【作者基本信息】 吉林大学 ， 动力机械及工程， 2010， 博士

【摘要】 为实现“节能减排”的发展目标,起动-停止技术和缸内直喷汽油机均具有巨大的发展潜力。在缸内直喷汽油机上可以很好地实现起动-停止技术,因此本文研究在缸内直喷汽油机应用起动-停止技术。其中重点研究其起动过程,优化起动控制策略,使发动机起动时得到良好的起动转速特性、排放性能和较小的振动。首先分析了国内外起动-停止技术、缸内直喷技术和缸内直喷发动机起动过程CFD分析的研究现状;然后选择三菱的4G15GDI发动机作为样机建立研究起动-停止技术试验平台,设计开发喷油器驱动电路和点火驱动电路,编写起动过程的喷油点火控制程序、怠速旁通阀控制程序,安装了针对起动过程的排放采集系统。采用Fire软件对该发动机起动时首循环的喷雾进行模拟计算,分析研究第一缸活塞(即处于压缩冲程的活塞,本文称之为第一缸活塞)初始位置、喷油量和喷油正时对其缸内温度场、浓度场和流场的影响。当第一缸活塞处于60°CA BTDC起动,混合气偏稀时,无论喷油正时为何时均能在上止点火花塞附近形成可燃混合气;混合气较浓时,如果喷油正时距离上止点较远,油束将会湿润火花塞以致淹缸,相反如果喷油正时在上止点附近则不会淹缸并且能够在上止点火花塞附近形成可燃混合气。当第一缸活塞处于120°CA BTDC起动,无论混合气较浓还是偏稀,喷油正时距离上止点较远还是在上止点附近,均能在上止点火花塞附近形成可燃混合气。当第一缸活塞处于180°CA BTDC起动,混合气偏稀时,无论喷油正时为何时,压缩到上止点容易发生自燃;混合气较浓时,控制合适的喷油正时可以避免自燃。研究了发动机停机后的状态。发动机停机后燃油轨道压力下降速度先迅速后缓慢,约30～45分钟后降为与低压油管相同的压力;发动机停机后处于压缩冲程的活塞停在压缩上止点前80°CA附近的概率较大,几乎不可能停在0～30°CA BTDC之间和150～180°CA BTDC之间;发动机是否发生反转取决于转动动能衰减到零时缸内气体负力矩的大小;采用两个具有一定相位差的光电传感器可以判断曲轴是否反转并检测停机后曲轴的位置。研究了电机拖动起动模式起动中的三种起动模式,即起动机起动模式、膨胀缸喷油起动机辅助起动模式和高速电机起动模式。对于起动机起动模式首循环第一缸的燃烧,当活塞初始位置处于压缩上止点前60°CA起动时,在混合气偏稀一侧和较浓一侧分别存在一块失火区域;当活塞初始位置提前到上止点前120～150°CA某一位置起动时,出现了自燃区域,而且随着活塞初始位置继续提前,自燃区域面积逐渐增大;起动时第二缸比较容易发生自燃,只有选择适当的喷油量及其对应适当的喷油正时才能防止自燃。研究了起动机起动模式起动过程的燃烧特性、起动转速特性和排放性能。通过起动过程转速和进气压力把起动过程分为A、B和C三个阶段。喷油正时对起动过程中燃烧影响很大,喷油正时在310°CA BTDC和260°CA BTDC之间时,其A、B和C各个阶段中的燃烧相对较为稳定,没有失火发生,HC排放较低;喷油正时分别在330°CA BTDC和240°CA BTDC时燃烧恶劣,有很多缸发生失火。点火正时对起动过程中燃烧影响很大。随着点火正时的提前,A阶段燃烧始点逐渐提前,B和C阶段燃烧恶化,尤其B阶段燃烧较差。此外,起动机起动模式起动过程中,活塞初始位置处于120°CA BTDC时起动速度较快。对于膨胀缸喷油起动机辅助起动模式,相比起动机起动模式有更快起动反应速度,尤其是活塞初始位置在上止点前105°CA时,达到800r/min时所用的时间仅为0.26s。对于高速电机起动模式,电机拖动转速在800～1400r/min范围内时,均能使发动机稳定地过渡切换到发动机自行着火运转模式。拖动转速相对较高时(1200r/min和1400r/min),模式切换与过渡时转速相对较平稳;但是拖动转速过高时(1400r/min),HC排放略有增加。研究了直接喷油起动模式中的两种起动模式,即反转起动模式和正转起动模式。正转与反转阻力矩不同,因为驱动各种附件的损失不同,反转阻力矩小于正转阻力矩。对于反转起动模式,压缩缸(即第一缸)的过量空气系数控制在0.84左右时可以获得最高燃烧压力;压缩缸(即第一缸)的活塞位置控制在80°CA BTDC时较为理想,并且在该位置反转时膨胀缸可以获得较大的压缩比,有利于其燃烧做功。此外,膨胀缸(第四缸)过量空气系数为0.72和0.63之间时,其最高燃烧压力最高,能够推动活塞转动360°CA,从而使得发动机起动成功。对于正转起动模式,第四缸(膨胀缸)的过量空气系数控制在0.84左右时可以获得最高燃烧压力;由于正转和反转的配气相位不同,使得正转起动时首缸很难转过其下止点。对于正转起动模式,第四缸(膨胀缸)活塞初始位置为105°CA ATDC时较为理想,能保证膨胀缸(第四缸)活塞和接下来的第一缸活塞均能转过各自的下止点,从而发动机起动成功。总的来说,对于以上五种起动模式,电机拖动发动机起动最快,只需0.25s拖动发动机达到800r/min;膨胀缸喷油起动机辅助起动模式0.3s内使发动机达到800r/min。无论哪种与传统汽油机起动相比,起动速度要快很多。对于五种不同起动模式起动过程的HC排放,电机拖动起动模式的HC排放相对较低,而其它四种起动模式在起动最初阶段均存在一个小峰值,HC排放相对略高。研究了起动过程的振动。对于起动机起动模式,起动过程振动峰值出现在起动初始阶段(前两个循环)。对于电机拖动阶段,活塞初始位置对其振动有一定的影响。最小振动发生在进气门关闭之后接近上止点的位置,可以通过控制活塞的初始位置来减小发动机起动初始阶段的振动。在相同活塞初始位置时,热机(冷却液温度85℃)起动振动较冷机(冷却液温度20℃)起动振动略有降低。高速电机起动模式相比起动机起动模式,前者起动时的振动要小很多。更多还原

【Abstract】 In order to achieve saving energy and reducing emissions, start-stop technology and gasoline direct injection engine both have great potential for development. Start-stop technology can be realized well in the gasoline direct injection engine (GDI). Consequently, the application of start-stop technology was investigated in gasoline direct injection engine in this paper. Starting process was studied especially, and control strategies of starting process were optimized for making the engine achieved good starting performance, low emissions and vibration. At first, start-stop technology, GDI engine technology, and CFD simulation of its starting process in China and other country were analyzed. Then a Mitsubishi 4G15GDI engine was selected as experimental engine for studying start-stop technology. The drive circuits of spark ignition and fuel injection were designed and developed. Some soft-wares of controlling injection, ignition and idle bypass valve in the starting process were written. Emissions collection system in the start process was designed and erected.The spray process of first cycle in starting process of the GDI engine was simulated by AVL Fire software. Influences of initial position of the compression-stroke piston, injection quantity and injection timing on the first cylinder concentration field, temperature field and flow field were investigated. The piston begins to compress from 60°CA BTDC to TDC, when the mixture is weaker and the piston reaches TDC, no matter when injection timing is, the combustible mixture can be formed near spark plug area. When the mixture is richer and injection timing is far from TDC, pocket of fuel will wet the spark plug that lead to misfire. On the contrary, if the injection timing is close to TDC, it will not lead to misfire and form combustible mixture near spark plug area. The piston begins to compress from 120°CA BTDC to TDC,when the piston reaches TDC, no matter how rich and weak mixture is, and no matter when injection timing is, the combustible mixture can form near spark plug area. The piston begins to compress from 180°CA BTDC to TDC,when the mixture is weaker and the piston reaches TDC, no matter when injection timing is, self-ignition is easy to happen. Both richer mixture and appropriate injection timing can avoid self-ignition.The engine situation after stopped was studied. Fuel rail pressure after engine stopped first decreases quickly and then decreases slowly. Its pressure decreases to the same low pressure as fuel tube within 30 to 45 minutes. The piston in the compression stroke after engine stopped tends to stop at around 80 ? BTDC and hardly stops at around 0~30 ? CA BTDC or 150~180°BTDC. When engine rotational kinetic energy decays to zero, the size of cylinder gas torque determine whether engine will reverse. Two optimatic sensors were installed at a certain phase, which could determine whether the crankshaft will reverse and detect the crankshaft position after engine stopped.The motor drive modes which include start mode of starter, start mode of expansion cylinder injection with starter assisting and start mode of high-speed motor were investigated on the bench test.In terms of the start mode of starter, the compression cylinder’s combustion in the first circle was studied. When the compression-stroke piston’s initial position is at 60°BTDC, there are misfire region as the injection quantity is greater and lesser. When the compression-stroke piston initial position is advanced to 120～150°CA BTDC, a region of self-ignition occurs and the self-ignition region enlarges gradually as the pistons initial position advances. Careful consideration must be paid to the combustion in the second combustion cylinder otherwise self-ignition is likely to occur in the cylinder. Its self-ignition can be avoided only if appropriate injection quantity and injection timing to be set. Combustion characteristics, starting speed characteristic and emission of the start process were studied. The starting process is subdivided into A, B, C stages by the inlet pressure and speed of start process. The injection timing has a huge effect on the combustion of starting process. As the injection timing is at between the 310°CA BTDC and 260°CA BTDC, the combustion of each stage is relatively stable, misfire is not occurred and the emission of HC is low. As the injection timing is at 330°CA BTDC and 240°CA BTDC, the combustion is bad and misfire happens in many cylinders. Ignition timing also has great influence on the combustion of starting process. As the ignition timing advances, the combustion of stage A advances as well, and the combustion of stage B and C, especially the B stage, become worse. Besides, in terms of start mode of starter,the starting speed is quick as the compression-stroke piston initial position is at around 120°CA BTDC.Compare to start mode of starter, using start mode of expansion cylinder injection with starter assisting can make engine start quickly, especially 105°CA BTDC where the compression-stroke piston initial position is, engine needs only 0.26s for reaching 800r/min.In terms of start mode of high-speed motor, engine can switch steadily from start mode of high-speed motor to the mode of engine catching fire of itself when the dragging speed of motor is at 800~1400r/min. The interim speed and switching mode are steady when the dragging speed of high-speed motor is at 1200r/min和1400r/min, but the HC is higher when the dragging speed is 1400r/min. The start modes of direct injection which include start mode of natural rotation and start mode of reverse rotation were investigated on the bench test.In terms of start mode of reverse rotation, the maximum combustion pressure can be achieved when excess air factor is around 0.84 in compressed cylinder. It is desirable for the compression-stroke piston initial position to be at 80°CA BTDC, which can benefit the expansion cylinder’s combustion and working because the expansion cylinder achieves a bigger compression ratio. When the expansion cylinder’s excess air factor is between 0.72 and 0.63, its maximum combustion pressure is highest, which can drive the piston to rotate over 360°CA so that engine starts successfully.In terms of the start mode of natural rotation, the highest combustion pressure can be obtained when the excess air factor is controlled around 0.84 in the fourth cylinder. Because of the different valve timing of natural and reverse rotation, the expansion cylinder’s piston can hardly pass through the BDC when forward-turning. In terms of the start mode of natural rotation, it is satisfactory for the compression-stroke piston’s initial position to be at 105°CA BTDC, which can ensure the compression-stroke piston and the next stroke piton to pass through their own BDC, so that engine starts successfully.In summary, among five start modes mentioned above, the start speed of high-speed motor is quickest, and the engine can reach 800r/min within 0.25s. The engine can reach 800r/min within 0.3s in the start mode of expansion cylinder injection with starter assisting. The five start modes cost lesser time than the traditional start mode. In terms of the emission of HC in the five start modes, the result of high-speed motor mode is relative better than the others’, while there is a peak value of emission at the first start stage in the other start modes whose emissions are a little higher.The vibration of start process was studied. In terms of the start mode of starter, the maximum vibration of start process appears during the initial start’s stage which is the first two cycle. For the stage of starter dragging, the piston’s initial position has great effect on the vibration, and the lowest vibration happens where the piston stops near to the TDC after the intake valve closes. So, the vibration of start process can be reduced by varying the piston’s initial position. When the position of initial piston is the same, hot starting (85℃) vibration is somewhat smaller than cold starting (20℃) vibration. Compared with the start mode of starter, the start process vibration of high-speed motor mode is much lower.更多还原