The application of technologies such as direct injection, turbo charging and variable valve timing has caused a significant evolution of the gasoline engine with positive effects on fuel consumption and emissions. The current developments are primarily focused on the realization of improved full load characteristics and fuel consumption reduction with stoichiometric operation, following the downsizing approach in combination with turbo charging and high specific power. The requirements of high specific power in a relatively small cylinder displacement and a wide range of DI injection specifications lead to competing development targets and to a high number of degrees of freedom during engine layout and optimization. One of the major targets is to assess the stability of the combustion system in the early development phase.
The in-cylinder charge motion generation by different intake port concepts has been identified as a potential source of cyclic fluctuations due to dynamic instabilities. The identification of cyclic fluctuations of the in-cylinder charge motion has become possible by the application of high speed optical diagnostic methods, which enable a cyclic resolved analysis of the charge motion development.
In this paper the charge motion generation and evolution of cyclic fluctuations are investigated by high speed particle image velocimetry (HSPIV) for two different intake port designs with individual tumble levels. The optical diagnostics are performed on a flexible motored engine, which is capable to be operated with metal flow boxes instead of prototype cylinder heads. The results show that cycle resolved HSPIV enables to provide an improved understanding of the evolution of cyclic instabilities of the primary tumble charge motion as well as secondary charge motion, such as omega tumble structures and fluctuations due to large scale turbulence. The fluctuation intensity can be expected to have a significant impact to the mixture formation and at the end of the compression to the combustion stability.
This present study shows that in order to change the tumble intensity a switch between different intake port designs is a trade off between high tumble intensities leading to fast burning rates and strong cyclic fluctuations decreasing the combustion stability.