Abstract
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.
Highlights
As modern engines increasingly become diverse and complex in their configuration, component design, and manufacturing process for multiple purposes of high efficiency, low emission, and/or high power, it is being complicated to optimize their design as well as driving strategy
For a sufficiently large cylinder volume, the assumption of pancake geometry provides a highly accurate moment of inertia, but a distinct error increase was observed as compression proceeds a significant error in the calculation of tumble decay rate would follow
Several different approaches to model tumble decay rate could be found in literature, but most of them require a calibration process corresponding to any change in engine geometry [14,15]
Summary
As modern engines increasingly become diverse and complex in their configuration, component design, and manufacturing process for multiple purposes of high efficiency, low emission, and/or high power, it is being complicated to optimize their design as well as driving strategy. Turbulence plays a critical role in determining the engine performance, especially in spark-ignition engines where the combustion takes place based on flame propagation This is mainly because a higher turbulence level can increase the turbulent flame speed, which may result in improved thermal efficiency (by bringing it close to the constant-volume combustion) [1] as well as mitigation of knocking phenomena (in case the flame propagates ahead of the auto-ignition of the end gas) [2,3]. The details in the modeling method, as well as its validation process, are discussed
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