Study On Effects Of Internal EGR Strategy On Gasoline CAI Combustion

Controlled Auto-Ignition(CAI) as well as named Homogeneous Charge Compression Ignition(HCCI) is a new combustion mode develpmented in resent years. This combustion mode combined with the benifents of traditional spark ignition combustion mode and compression ignition combustion mode, having high efficiency and extra-low NOX emmions. The mixture of flash air and fuel as well as residual gas were well mixed in intake stroke process and compress stroke process, and compressed ignitied near the TDC as the flash mixture reached the firing point. The characreristis of this ignition process are the ignition happened at many points and there is no flame diffuse, the combustion temperature very low. As this new combustion mode applied on gasoline engine, the limit of fuel utilize efficence can be conquered, and used on disiel engine, the limit of NOX emmission and smoke emission trade- off should be aproved.In this paper, a basic single cylinder engine was modified, the comshaft vavle train was replaced by a new electro-hydraulic variable valve-train system, and a new control system and data collection system were developed. With the engine testing system the sparke ignition combustion mode and CAI combustion mode can be relized. Furthermore, a gasoline CAI engine simulation mode coupled with GT-Power and CHEMKIN was setted up. The testing system and CAI simulation mode were used to study the effects of the internal EGR strategy on gasoline CAI conbustion at 1000rpm andλequal to 1 condition. The valve timing qualification used to realize CAI combustion were explored with the combustion charmber EGR strategy and exhaust port EGR strategy.The research results show that:The tseting results of combustion chamber EGR strategy. The testing results show that , when intake vavle opening time(IVO) between -38°CA ATDC and 167°CA ATDC, and exhaust vavle closing time(EVC) between 68°CA BTDC and 118°CA BTDC, stabile CAI conbustion was realized at 1000rpm. In this valve timing region, the negative valve overlap angle (NVO) can changed between 35°CA and 280°CA ,the RGR rate region between 26% and 89%, and the BMEP range is 0.177MPa to 0.495MPa, the effective efficiency was 0.21 to 0.36. But there are a half region happened knocking, in order to realize normal combustion, the EGR must be controlled by valve timing and reached 40%. On the lift of symmetrically NVO line, the EVC timing must more than 90°CA BTDC and on the right the EVC timing must more than 100°CA BTDC.And the IVO timing can be changed between -38°CA ATDC and 165°CA ATDC at all CAI combustion region. The BMEP range of normal CAI combustion is 0.177MPa to 0.45MPa. On the lift of symmetrically NVO line, beacause of the late intake valve open angle small than the early exhaust valve close angle, the residual gas flow back into the intake port, the intake port EGR strategy and combustion charmber EGR strategy exist at the same time. The intake port EGR strategy can affect the mix process and improve CAI combution. The tseting results of exhaust port EGR strategy. The gasoline CAI combustion was reached by using exhaust gas re-breathing in the intake stroke with late exhaust valve closing timing. The testing results show that, the CAI combustion region realized by exhaust port EGR strategy is smaller than the CAI combustion region with combustion charmber EGR strategy. In the CAI region, the EGR rate is between 37% and 58%, the BMEP range was 0.266MPa to 0.521 MPa, the effective efficiency was 0.284 to 0.41. Compared with the combustion charmber EGR strategy, the exhaust port EGR strategy is adapt to larger load condition.Simulate work of gasoline CAI combustion. The simulation work about combustion chamber EGR strategy and early exhaust port EGR strategy ware carried out using gasoline CAI simulation mode coupled with GT-Power and CHEMKIN software. The potential CAI combustion regions were got and had been used to guide the testing work. And then, aim at the difference of simulation results and testing results, and some abnormity combustion phenomenon, a new simulation study process was carryied on.