Modern spark ignition engines are downsized and boosted to meet stringent emission standards and growing customer demands on performance and fuel economy. They operate under high intake pressures and close to their limits to engine knock. As the intake pressure is increased knock becomes the major barrier that prevents further improvement on downsized boosted spark ignition engines. It is generally accepted that knock is caused by end gas autoignition ahead of the propagating flame. The propagating flame front has been identified as one of the most influential factors that promote the occurrence of autoignition. Systematic understanding and numerical relation between the propagating flame front and the occurrence of knock are still lacking. Additionally, knock mitigation strategy that minimizes compromise on engine performance needs further researching. Therefore the objectives of the current research consist of two steps: 1). study of turbulent flame propagation in both naturally aspirated SI engine. 2) study of the relationship between flame propagation and the occurrence of engine knock for downsized and boosted SI engine. The aim of the current research is, firstly, to find out how turbulent flames propagate in naturally aspirated and boosted S.I. engines, and their interaction with the occurrence of knock; secondly, to develop a mitigation method that depresses knock intensity at higher intake pressure. Autoignition of hydrocarbon fuels as used in spark ignition engines is a complex chemical process involving large numbers of intermediate species and elementary reactions. Chemical kinetics models have been widely used to study combustion and autoignition of hydrocarbon fuels. Zero-dimensional multi-zone models provide an optimal compromise between computational accuracy and costs for engine simulation. Integration of reduced chemical kinetics model and zero-dimensional three-zone engine model is potentially a effective and efficient method to investigate the physical, chemical, thermodynamic and fluid dynamic processes involved in in-cylinder turbulence flame propagation and knock. The major contributions of the current work are made to new knowledge of quantitative relations between intake pressure, turbulent flame speed, and knock onset timing and intensity. Additionally, contributions have also been made to the development of a knock mitigation strategy that effectively depresses knock intensity under higher intake pressure while minimizes the compromise on cylinder pressure, which can be directive to future engine design.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:654790 |
| Date | January 2015 |
| Creators | Gu, Jiayi |
| Publisher | Loughborough University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://dspace.lboro.ac.uk/2134/17356 |
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