【作者】 于水；

【导师】 李理光；

【作者基本信息】 上海交通大学 ， 动力机械及工程， 2009， 博士

【摘要】 怠速停机是混合动力汽车一个重要的节油方式,因此混合动力汽车在运行中会频繁起动/停机。由于进气道喷射式汽油机起动过程固有的瞬态特征,汽油机起动时燃烧和排放恶化。对于混合动力汽车发动机,起动过程是先快速拖动到怠速转速或高于怠速转速然后点火起动,因此起动过程瞬态特性更加突出,从而不利于燃烧排放控制,频繁起/停对混合动力汽车排放控制提出了新的研究课题。本文结合混合动力汽车关键技术开发项目,根据混合动力发动机测试需求设计搭建了模拟起动发电一体化电机(ISG)混合动力汽车发动机快速起动瞬态测试平台。设计了高速数据采集系统,开发了基于循环控制的发动机并行控制系统,在不影响原机ECU正常工作的前提下对喷油和点火进行并行控制。在所搭建的系统平台上对混合动力发动机起动工况瞬态燃烧和排放特性以及催化器动态特性进行了研究。基于循环的瞬态燃烧测试手段和Cambustion瞬态排放仪对发动机在提高拖动转速条件下起动时的瞬态燃烧排放特性进行了研究,并结合发动机控制策略进行了分析。结果发现提高拖动转速造成起动过程进气压力变化率升高,使得精确控制缸内混合气浓度更加困难,导致燃烧和排放恶化。由于原始标定参数不匹配,在室温冷起动时,提高拖动转速导致第一循环失火,而热机起动时,导致第3~8循环发生不同程度的部分燃烧甚至失火,使碳氢排放恶化。碳氢排放恶化的程度随拖动转速升高而增加。起动过程中只有第1~2循环会产生较多的氮氧化物,整个起动过程氮氧化物都较低,氮氧化物浓度随拖动转速升高而降低。利用瞬态碳氢测试仪对停机时碳氢排放进行了研究,当采用断点火停机时,最后一次喷油不能点燃,导致产生较高碳氢排放。当采用断油停机时,碳氢排放最多可比断点火停机时减少50%左右。断点火停机会在进气道壁面残留较多的油膜,这些额外的油膜在发动机重新起动时可能只对第一循环的燃烧造成较大影响。研究了不同边界条件对发动机起动首循环燃烧和排放的影响。混合动力模式起动时,首循环进气量比传统模式减少10%左右。混合动力起动模式下首循环缸内混合气浓度在冷起动时比传统模式稀,在热起动时与传统模式基本一致。在不同实验条件下,首循环燃烧的缸压峰值、指示平均有效压力、累积放热量、碳氢排放和氮氧化物排放都随着过量空气系数的改变而呈“窗口区域”变化规律。拖动转速、冷却水温度、进气道油膜残留和点火时刻等燃烧边界条件都对“窗口区域”造成影响。提高拖动转速后首循环燃烧的混合气稀限往浓区偏移,当水温分别为25oC、60oC、85oC时,对本文采用的发动机在混合动力起动模式下首循环燃烧稀限比传统模式分别要浓1.49倍、1.31倍和1.05倍。当混合气浓度适宜时,推迟点火时刻虽然导致放热时刻推迟、放热速度减慢,但是对最终的累积放热量影响不大。从获得较高指示平均有效压力角度来看,传统起动模式下首循环燃烧的最优点火时刻在上止点后5oCA左右,而混合动力起动模式下在上止点前5oCA左右。当点火时刻在上止点前10oCA和上止点后10oCA之间变化时,首循环燃烧排放特性的“窗口区域”的边界变化不大。但混合动力模式下提前点火时刻具有一定的扩展稀限边界的潜力。对混合动力起动过程缸内混合气形成特性进行了分析,并采用基于循环的喷油控制策略对混合动力起动过程的碳氢排放进行了优化。在发动机冷起动的瞬态过程中,缸内混合气浓度波动较小;而热起动时,第4~5循环缸内混合气过浓,然后随循环数增加而逐渐变稀,到第10循环与冷起动基本相当。在原始参数标定的基础上对起动初始的5个循环喷油量进行了调整,发现需要适当增加第一循环喷油量才能使首循环具有最高平均有效压力;而第2~5循环需要适当减少喷油量才能获得最高平均有效压力。由于进气道黏附的影响,在第4~5循环,即使不喷油仍能获得较高平均有效压力和较低碳氢排放。对起动过程前5个循环的喷油量进行优化后,使起动过程的碳氢排放大幅下降。结合混合动力汽车发动机的冷起动标定工作,对催化器在起动过程中动态特性做了研究。通过修改拖动期喷油量获得不同的空燃比变化曲线,发现在冷起动时原机标定的拖动喷油量可以获得最低的碳氢排放,热机起动时在拖转喷油量为60%于原脊标定时可以获得最低的碳氢排放。发动机停机后催化器入口温度在2分钟以内从450oC降低到250oC;催化器载体温度在15分钟以后才从450oC降低到250oC。催化器对起动初期的排放有一定的吸释能力,在催化器温度较低和碳氢排放过浓时,往往会使一定时间内催化器后测得排放高于催化器前排放。对本研究采用的发动机,要想使催化器在起动工况即时起燃(转化效率>50%),催化器载体温度要在停机时保持在至少300oC以上,同时应优化拖动起动阶段空燃比控制策略以降低发动机燃烧产生的碳氢排放。更多还原

【Abstract】 Idle stoping is one of important fuel saving methods for hybrid electric vehicle (HEV), therefore, the engine of HEV will start and stop frequently according to road condition. Due to the inherent transient behaviors during start process of port-fuel-injected (PFI) gasoline engine, combustion and emission deteriorate significantly. Moreover, as the engine was cranked to idle speed quickly when it starts in HEV, the trasneints are more dramatically than that in traditional vehicle, which are disbenefit to combustion and emission performance. Therefore, frequent start and stop operations bring new issue for optimization of emission performance in HEV. In this thesis, together with the progress of HEV key technology development project, a transient performance test system was established to simulate the engine quick start process in integrated starter and generator (ISG) HEV systems. In order to conduct cycle-by-cycle analysis and implement the control strategy optimization, a data acquisition system and a parallel control unit were designed based on cycle-by-cycle control strategy, without disturbing normal operation of the original ECU. Based on the test system, the transient characteristics of combustion and emissions during eninge quick start were investigated, as well as the dynamic performance of three way catalyst (TWC) was studied.By means of transient combustion measurement methods and Cambustion transient emission equipments based on cycle-by-cycle, the transient combustion and emission characteristics and the controls strategies during engine start process with various cranking speed were investigated. When cranking speed increased, the drop rate of intake manifold absolute pressure (MAP) is enhanced, resulting in poor control of in-cylinder mixture, therefore, the combustion and emission deteriorate significantly. Because the original calibration can not match well with quickly start condition, misfire and partial combustion occur. For cold start, misfire always occurs at first cycle. But for hot start, misfire and partial combustion always occur form the 3rd cycle to 8th cycle. During these cycles, hydrocarbon (HC) emissions deteriorate significantly, and the HC concentration increases with the rising of cranking speed. During start process, only the initial 2 cycles produce relative high level nitrogen oxide (NO) emission, and the NO concentration decreases with the rising of cranking speed. When the engine shut down by cutting off the ignition, the last injection can not be burned and leads to increase of HC emissions. Up to 50% HC can be reduced if engine was shutted down by fuel cut off. The fuel film deposit in intake port at previous stop may influence the first cycle more significantly when engine restart.The influence of various boundary conditions on the combustion and emission at first cycle was investigated. At HEV start mode, the intake air mass of first cycle will reduce about 10% compared to that at traditional mode. The fuel delivery efficiency decreases when cranking speed increased, especially at cold condition. The peak cylinder pressure, indicate mean effective pressure (IMEP), cumulative heat release, HC and NO emissions all change with the injection excess air coefficient, and exist boundary limit. The coolant temperature, intake port deposit, cranking speed and ignition timing all influence the limit with various mechanisms. For the test under coolant temperature of 25 oC, 60 oC and 85 oC, the lean limit of first cycle for HEV quick start should be enriched to 1.49, 1.31, 1.05 times than that for traditional start. When the mixture is fit to combustion, although the retardation of ignition timing could delay the combustion phase and slow combustion velocity, the cumulative heat release has little change. On the view of obtain maximum IMEP, the optimal first cycle ignition timing is at around 5oCA ATDC under traditional start mode, and around 5oCA BTDC under HEV start mode. With the ignition timing retarded from 10oCA BTDC to 10oCA ATDC, the combustion limit has no significant change. But advanced ignition timing show possibility of expanding the lean limit under HEV mode.After the analysis of mixture preparation during start process, the HC emissions during engine quick start were optimized by means of cycle-by-cycle fuel injection control. The in-cylinder mixture concentration during start transient process fluctuates more dramatically under hot start condition. Typically, the mixture at 4th and 5th cycle is over-riched at hot start. Based on the original engine calibration, the fuel injection at the initial 5 cycles was changed respectively. It is found that the first cycle need to increase fuel injection, while the 2nd to 5th cycle need to reduce fuel injection, so that maximum IMEP can be obtained for all of the initial 5 cycles. For the 4th and 5th cycle, it can still produce fairly high IMEP even there’s no fuel injection in these two cycles. By means of cycle-by-cycle fuel injection control, the HC emissions during engine quick start can be reduced significantly.Together with the calibration of HEV engine, the dynamic characteristics of TWC during engine start process were studied. The air fuel ratio (AFR) curve during cranking and starup can be changed by modifying the calibration of‘crank fuel base’. For cold start, the original calibration can obtain minimum HC concentration, for hot start, the crank fuel should be reduced to 60% of original calibration. After engine shutting down, the TWC entrance temperature decreases from 450oC to 250oC in 2 minutes, but the substrate temperature needs 15 minutes for cooling down to 250oC. The TWC have capability of absorb and release emissions during engine startup process. Under the condition that the TWC temperature is low and the emission level is very high, the emission concentration after catalyst will higher than before the catalyst. In order to guarantee the catalyst light off immediately at start, the substrate temperature should be maintained at least 300oC, as well as the AFR control strategy during startup should be optimized.更多还原