Engine downsizing and downspeeding using GTDI technology improves fuel economy while maintaining vehicle performance. The downsizing potential of an engine application is limited by engine knock at low engine speeds as well as turbocharger inlet and catalyst temperatures at high speeds, requiring high spark retard and fuel enrichment, respectively. Both spark retard and fuel enrichment reduce the overall real world fuel economy benefit. Cooled exhaust gas recirculation (EGR) has been investigated as a way of reducing knock and lowering exhaust gas temperatures.
This paper discusses the use of low-pressure route cooled EGR for knock mitigation at low engine speeds in order to improve full load performance and fuel consumption and increase the potential for engine downsizing. In particular, this study focuses on increasing the EGR tolerance of the combustion system through the use of very high tumble ratios exceeding that of current production systems, enhanced ignition and increased compression ratio. An externally boosted single cylinder research engine is used to investigate the effects of these various concepts on the EGR tolerance and the subsequent effects on low speed, full-load engine performance and fuel consumption. Computational fluid dynamics (CFD) is used to understand the mixture preparation and turbulence differences expected when using high tumble intake ports.
A high tumble combustion system was found to provide high EGR dilution tolerance, improved thermal efficiency and higher load capability through advancement of the knock limited combustion phasing. However, extremely high tumble ratios were found to increase the knocking tendency of the engine.