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Title: Flow and combustion in direct injection spark ignition engines
Author: Scott, Blane
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2019
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The goal of producing more efficient internal combustion engines has led to the use of more advanced technologies and operating strategies. As a result, Computational Fluid Dynamics (CFD) is now a standard tool that is used to optimise the design of new combustion systems. Validation of these numerical simulations is essential, especially for the prediction of in- cylinder flow, which is known to have a profound effect on mixture preparation and combustion. Therefore, high quality, in-cylinder flow measurements are required to aid the validation process and the design of new combustion systems. This work outlines the development of an optical engine test facility and the installation of a high-speed Particle Image Velocimetry (PIV) system that provides in-cylinder flow measurements with high spatial and temporal resolution over a large range of crank angles. The system is capable of producing measurements at a maximum rate of 3.7 kHz at full resolution and a total of 6000 frame pairs in a single experiment. Additional improvements to the system for specific application to IC engines have been outlined, including the use of variable inter-frame delay and methods of scatter minimisation. To aid the validation of CFD simulations, metrics known as the Weighted Relevance Index (WRI) and the Weighted Magnitude Index (WMI), have been developed to quantify differences between flow fields in terms of both alignment and magnitude. These have been combined to produce a third metric, the Combined Magnitude and Relevance Index (CMRI), that produces a single value that rates the similarity of two flow fields. The application of these metrics has been demonstrated by investigating the differences between velocity measurements and CFD RANS simulations in the central tumble plane for three test conditions. The metrics were able to determine regions of the flow field that were significantly different between simulation and experiment, which would not be highlighted by conventional metrics. Flow field measurements were also made during the induction and compression strokes of firing cycles to investigate the effect of in-cylinder flow structures on cycle-by-cycle variations in combustion. The cycles were separated into subsets conditioned on burn rate, as indicated by in-cylinder pressure measurements. The WRI and WMI were then used to compare the conditionally averaged flow fields of fast and slow burning cycles. From this analysis, it was possible to determine regions of the flow that have a significant effect on the rate of combustion. Simultaneous combustion imaging showed that the flames for slow burning cycles tended to grow asymmetrically towards the exhaust valves. In contrast, the flames of fast burning cycles were convected away from the spark plug and grew rapidly in all directions.
Supervisor: Stone, Richard Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Engineering