An optical investigation into the effect of fuel spray, turbulent flow and flame propagation on DISI engine performance

There is currently considerable interest in new engine technologies to assist in the improvement of fuel economy and the reduction of carbon dioxide emissions from automotive vehicles. Within the current automotive market, legislative and economic forces are requiring automotive manufacturers to produce high performance engines with a reduced environmental impact and lower fuel consumption. To meet these targets, further understanding of the processes involved in in-cylinder combustion is required. This thesis discusses the effect of fuel spray structure, flame propagation and turbulent flow on DISI engine combustion. To investigate these flow processes within the fired single cylinder Jaguar optical engine a number of optical measurement techniques have been used, including high speed laser sheet flow visualisation (HSLSFV) and high speed digital particle image velocimetry (HSDPIV). Results obtained from dual location flame imaging has provided further understanding of the relationship between flame growth, engine performance and cycle-to-cycle variation. Detailed correlation analysis between flame growth speed and engine performance parameters demonstrated that it is the flow conditions local to the spark plug at the time of spark ignition that have greatest influence on combustion. It was also demonstrated that further gains in engine performance and stability can be achieved by optimising the fuel injection timing. The temporal and spatial development of flow field structures within the pent-roof combustion chamber at the time of spark ignition were quantified using HSDPIV. Decomposition analysis of the raw velocity data enabled the relationship between specific scales of turbulent flow structure and engine performance parameters to be investigated. Correlations between the high frequency turbulence component and pressure derivatives are shown, demonstrating that it is the frequencies of motion >600 Hz that have the greatest influence on early flame development and therefore rate of charge consumption, engine performance and combustion stability. A series of double fuel injection strategies were devised to investigate the potential for using the fuel injection event to influence flow field structures within the cylinder. Results demonstrated that while the fuel injection event had limited impact on bulk flow structures, there was an increase in turbulence post fuel injection, depending on the timing of the second injection pulse. However, this advantage was not sustained throughout the compression stroke to have significant impact on combustion. The final stage of research investigated fuel spray structure, flame propagation and charge motion at fuel impingement locations, comparing a single and triple injection strategy. A triple injection strategy is proposed that results in an improvement in the levels of fuel impingement on combustion chamber walls and a reduction in the high luminosity regions within the flame. Consequently, adopting the multiple injection strategy highlighted the potential for reducing unburned HC emissions and soot formation within homogeneous charge DISI engines.