Refinement of a 0D Turbulence Model to Predict Tumble and Turbulent Intensity in SI Engines. Part I: 3D Analyses

Paper #:
  • 2018-01-0850

  • 2018-04-03
Recently, a growing interest in the development of more accurate phenomenological turbulence models is observed, since this is a key pre-requisite to properly describe the burn rate in quasi-dimensional combustion models. The latter are even more utilized to predict engine performance in very different operating conditions, also including unconventional valve control strategies, such as EIVC or LIVC. Therefore, a reliable phenomenological turbulence model should be able to physically relate the actuated valve strategy on turbulence level during the engine cycle, with particular care in the angular phase when the combustion takes place. Similarly, the capability to sense the effects of engine architecture and intake geometry would improve the turbulence model reliability. 3D-CFD codes are recognized to be able to accurately forecast the evolution of the in-cylinder turbulence field, taking into account both geometrical features (compression ratio, bore-to-stroke ratio, intake runner orientation, valve, piston and head shapes, etc.) and operating conditions (engine speed, boost level, valve strategy). Instead, more common 0D turbulence models usually synthesize geometrical effects in a number of tuning constants and “try” to be sensitive to operating conditions as much as possible. In this two-part paper, the final goal is the refinement of a previously developed 0D turbulence model, here extended to directly predict the tumble vortex intensity and its close-to-TDC collapse into turbulence. In addition, the model is enhanced to become sensitive to engine geometrical characteristics, such as intake runners’ orientation, compression ratio, bore-to-stroke ratio and valve number, without requiring any preliminary estimation of the tumble coefficient on a flow bench. Part I describes a background study, where 3D analyses are performed to highlight the effects of operating conditions and main engine geometrical parameters on tumble and turbulence evolution during the engine cycle. In a preliminary stage, the averaging process influence to define representative quantities of mean flow and turbulence is discussed, in order to take into account not-uniformities inside the combustion chamber. 3D simulations are carried out under motored conditions on a VVA engine, at various engine speeds. The VVA device is controlled to simulate both standard, early and late valve closures. The results highlight substantial differences in mean flow velocities, turbulent intensities and tumble ratios among the above valve strategies. To focus the engine geometry impact on the turbulence evolution, further analyses are performed on a different engine, by changing the angle between the intake runners and the cylinder axis. The geometrical compression ratio and the bore-to-stroke ratio are modified, as well. Finally, a two-valve version of this engine is also considered. The above data are discussed to widely assess the effects of valve strategy and main engine geometrical parameters on mean flow, tumble and turbulence evolution inside the combustion chamber. The presented results constitute an extended database for the development and validation of a more refined quasi-dimensional model, presented in the companion part II of the paper.
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