| It is a great challenge for the automotive industry development to meet the requirement for more and more stringent environment pollution regulations and severe shortage of petroleum oil. As NOx emission from SI engine is one of main components to form optical-chemical smoke, it is very important to restrict NOx emissions. In the existing methods, EGR is relatively effective technology not only to reduce the emission of NOx but also to save fuel consumption at part load. The conventional EGR directly entering intake manifold is limited by cycle-by-cycle variation and further decreasing NOx emission, if large ratio of EGR leads to engine, which will increase fuel consumption and CO and HC emissions. However, the stratified EGR with decreasing of the cycle-by-cycle variation and increasing of the ratio of EGR, not only reduces the emission of NOx further and the pumping loss at part load but also improves the fuel economy. So the stratified EGR is taken as a important means to reduce the emission of NOx.The study focuses on the technique of stratified EGR, which consists of the mechanism of stratification, the form of stratification and the influence on mixture form, combustion process and engine performance under the condition of stratification.The research selects a four-cylinder SI engine with four valves for each cylinder, which produces strong tumble motion and devises special stratified configuration of EGR. The exhaust gas is leaded to single intake port by the guided jet and achieves stratification with fresh mixture by using tumble motion of the engine in the cylinder. The different EGR modes are realized by changing the location of the guided jet.The result of the test demonstrates that the modes of EGR have obvious influence on engine performance. At part load, the jet by the side of intake port attains better economy than conventional mode with the fuel consumption reduction by 4.1%. Simultaneously, NOx and CO as well as HC reduces mostly with the increasing of EGR ratio. The whole EGR ratios with the mode that the jet locates by the side of port are larger than that of the conventional mode and the maximum ratio reaches to 43%. At the condition of same EGR ratio, cylinder pressure of the side-jet mode is bigger than that of conventional mode and the former doesn't require cooled EGR. By using the side-jet mode, EGR needs appropriate ignition timing.Using the advanced laser measure method and the visualization technology of in-cylinder gas flow, under the different EGR modes investigated in-cylinder gas flow and the concentration distribution of the exhaust gas. The results show that adding EGR through the side- jet can make the exhaust gas stratified in cylinder with fresh charge. The stratified phenomenon is obvious at the start point of the intake cycle, and the exhaust gas and the pre-mixed gas distribute on each side in cylinder of the engine respectively, which almost lasts to the end of compression stroke. And near the end of the compress stroke, the exhaust gas mixes with the fresh charge gradually, however, the distribution of the exhaust gas and the fresh charge is heterogeneous as a whole. The tumbling flows stratify of the side jet EGR is more obviously as the speed of the engine increases. Based on the exhaust gas stratified rule and the results obtained from the bench tests and the optical engine, builds up the side-jet stratified-EGR physical mode.The new EGR mode is an effective means to reduce the NOx emission and improves the fuel consumption at the same time for future use in SI engines, which has simply designed and reliable construction. Therefore both the theory and the application prospect of this research are very significant. |
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