A Quasi-dimensional model for Prediction of In-cylinder Turbulence and Tumble Flow in a Spark-Ignited Engine

Paper #:
  • 2018-01-0852

  • 2018-04-03
Improving fuel efficiency and emission characteristics are the major issues of the engine research. Since the engine has complex systems and various operating parameters, the experimental research is limited by cost and time. 1-dimensional (1D) simulation attracts researchers’ attentions due to its effectiveness and relatively high accuracy. In 1D simulation, the applied model is required to be accurate for the reliability of the simulation results. Because in-cylinder turbulence mainly determines combustion characteristics and mean flow velocity affects in-cylinder heat transfer and efficiency in spark-ignited (SI) engine, a number of models are developed to be sophisticated for prediction of in-cylinder turbulence and mean flow velocity. Especially, tumble is considered to be a significant factor of in-cylinder turbulence in SI engine. The existing models introduced an angular momentum for energy input and output to the cylinder, and a decay function for geometric effects of tumble change. However, this function cannot universally cover different engines which have different tumble ratio, so it should be re-calculated according to the engine. In this study, the developed quasi-dimensional (QD) turbulence model also adopts an angular momentum and a decay function. Universal correlations of a decay function are found, and the function can be utilized for different engine with the minimum tuning constant. Coefficients of the function relate to tumble ratio and stroke-to-bore (S/B) ratio. The model was validated with the results of mean flow velocity, turbulence intensity, and tumble ratio from 3D CFD. The accuracy of results was confirmed during the period from near the end of the compression stroke to the beginning of the expansion stroke which primarily affects combustion and heat transfer characteristics. Also, the overall profile of mean flow velocity, turbulence intensity, and tumble ratio are similar to 3D CFD results. This study shows that the model can be applied to engines with different tumble intensities in overall range of engine speed.
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