| 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. |
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