Turbulence is one of the most important aspects in spark-ignition engines as it can significantly affect burn rates, heat transfer rates, and combustion stability, and thus the performance. Turbulence originates from a large-scale mean motion that occurs during the induction process, which mainly consists of tumble motion in modern spark-ignition engines with a pentroof cylinder head. Despite its significance, most 0D turbulence models rely on calibration factors when calculating the evolution of tumble motion and its conversion into turbulence. In this study, the 0D tumble model has been improved based on the physical phenomena, as an attempt to develop a comprehensive model that predicts flow dynamics inside the cylinder. The generation and decay rates of tumble motion are expressed with regards of the flow structure in a realistic combustion chamber geometry, while the effects of port geometry on both charging efficiency and tumble generation rate are reflected by supplementary steady CFD. The developed tumble model was integrated with the standard k-ε model, and the new turbulence model has been validated with engine experimental data for various changes in operating conditions including engine speed, load, valve timing, and engine geometry. The calculated results showed a reasonable correlation with the measured combustion duration, verifying this physics-based model can properly predict turbulence characteristics without any additional calibration process. This model can suggest greater insights on engine operation and is expected to assist the optimization process of engine design and operating strategies.
For past decades, substantial developments have been accomplished in internal combustion (IC) engine technology, but there still remain some possible improvements. The combustion in an IC engine is a highly intricate phenomenon, thus, numerous factors correlated with different forms of loss decides the efficiency of an engine. In spark-ignition (SI) engines, the combustion duration is considered important because it plays a key role in determining the combustion phasing for best possible energy conversion. The geometry of engine components may directly change the burning rate of air-fuel mixture, therefore, it should also be considered as significant as other aspects like exhaust gas recirculation (EGR) rate or boosting in investigation of the engine performance. This is the reason the development engineers are putting their effort to design an engine with optimized flow motion. Tweaking the flow dynamics via design modification or use of auxiliary device influences the turbulence level inside the combustion chamber, thus, the burning rate as well. Intake port orientation, masking, and piston shape are one of the typical design parameters manipulated for such purpose, and profound understanding on the effect of these design parameters on burning rate is encouraged in order to assist the optimization process.
The design optimization process should be based on a fundamental understanding of how the design parameters affect the flow motion and combustion characteristics. This study aims for a simpler and faster method to investigate the consequences of design modifications. As a base model, a physics-based quasi-dimensional (QD) engine model is developed for simulation of SI combustion phenomenon. It is modeled to consider the change in flow motion and turbulence properties via simplified modeling. The advantages of such QD model is that it requires much less computational resource compared to 3D CFD model, and allows a greater degree of freedom within the simulation process which facilitates parametric studies. A zero-dimensional (0D) turbulence submodel is used to describe energy cascade mechanism, and turbulence intensity is calculated reflecting the effect cause by design modification. According to the sensitivities drawn from parametric study, the results of each effect on burning rate and other engine performance properties are compared individually and collectively.
A qualitative analysis suggests how sensitive each effect are at given operating conditions. The result infers that the flow concentration by port design modification boosts the burning rate, but it is advantageous in terms of fuel economy to enhance the breathing ability by valve masking. The product of this comparative study assists an intuitive understanding on how the design modification would affect the engine operations, and it is encouraged to develop the model further via validation with experiment data to provide more reliable output. It is believed that it can be utilized as a good reference in engine design process.
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