Design and Demonstration of a Spark Ignition Engine Operating in a Stratified-EGR Mode

Abstract

The use of substantial amount of exhaust gas recirculation (EGR) via the technique of flow stratification in-cylinder was investigated to improve the fuel economy of spark ignition engines under part load. The fuel economy gain is achieved by reducing the pumping loss with EGR while the stratification enables stable combustion at high levels of EGR. By doing so, the ability to control NOx emissions using the three-way catalytic converter is also retained. To realize this goal, a new method of supplying EGR to achieve in-cylinder stratification was incorporated and experiments were performed to demonstrate the concept.A new transparent engine facility was constructed to assist the development of the stratified EGR strategy. In order to minimize the mixing in intake port, to maintain the stratification in cylinder, and to obtain fast burn, an intake flow control system was realized by using solenoid valve-controlled EGR injection. Qualitative measurements of in-cylinder flow motion at motoring condition were carried out with visualization techniques such as planar laser-induced fluorescence (PLIF) and Mie scattering to obtain information on the mixing process during intake and compression. The engine was operated at firing condition to assess the engine performance operating in a stratified-EGR mode. Cylinder pressure measurements were used to gain information about the combustion process. A new combustion model incorporating mixing and flame stretch factor, which is believed to be significant, especially under the diluted mixture condition, was developed to examine flame propagation properties under the stratified condition.The visualization results of PLIF showed that the stratification between air/fuel mixture and EGR gas was relatively well established during the intake stroke. There was, however, significant mixing in the late part of the compression stroke. This process may be explained by that large scale tumble motion was introduced into the cylinder during the intake stroke, and that the organized motion broke into small eddies during the compression stroke and hence resulted in the substantial mixing then. Performance comparison between the engine operating with the homogeneous mixture and with the stratified mixture illustrated that the stratified mode had greatly improved fuel consumption and had provided stable combustion at high dilution ratio. The observed trend on the burning process could be reproduced reasonably well by the model. Thesis AdvisorWai K. Cheng Associate professor of Mechanical Engineering

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