Analysis of Turbulent Jet Ignition (TJI) Single-Cylinder Engine through 3D-CFD Simulation

In the framework of a continuous strive for better engine efficiency, the Turbulent Jet Ignition technology presents itself as a valid option to decrease fuel consumption and emissions. Nevertheless, careful design and optimization is required to fully exploit the advantages of TJI combustion, with 3D/CFD analysis acting as a fundamental tool in this regard. The present work aims at the describing with 3D/CFD modelling and simulation a Turbulent Jet Ignition single cylinder engine. After describing the main features in TJI operation, a comparison between stochiometric and lean mixture conditions is presented. A calibration campaign, aimed at improving the reliability of the model, is then performed. This was realized by changing the turbulent Prandtl number inside the cylinder. The effect of pre-chamber geometry was also analyzed. A pre-chamber hole diameter sweep is thus investigated, highlighting the benefits and flaws of each layout on engine performance. It was confirmed how the pre-chamber hole diameter is a fundamental design parameter for TJI operation. Finally, a new configuration using rotated holes was tested. The interaction between turbulent jets and exhaust valves for the rotated holes design was investigated and it consistently proved to be a source of Heat Release Rate loss due to two effects. In order to characterize the potential flame passage/extinction event, a fundamental aspect of TJI operation, a study on flame and hole interaction is lastly carried out: the methodology, which is based on the ratio between Laminar Flame Thickness and hole diameter, proposes a threshold value of said ratio dividing a flame passage event from a flame extinction event. The interaction between flame and hole was studied on a 3D/CFD model representing a real experimental setup, as well as on a different set of experimental evidence. Finally, an application on an engine case was performed. The results suggest that the proposed methodology, which is based on the measurement of Laminar Flame Thickness, is able to qualitatively capture the difference between flame and jet ignition, showing promising evidence in the analytical characterization of flame/hole interaction; nevertheless, there is room for improvement, in order to correct the dependency of the proposed threshold value on temperature and pressure of the unburned gases.