The Effect of Different Throttle Openings on Multi-plane Analysis of In-cylinder Flow Fields in a Small-Bore Spark-Ignition Engine

To meet stringent emission norms and achieve enhanced engine performance in spark-ignition engines, in-cylinder charge motion is one of the most important factors for fuel–air mixture preparation and proper combustion. However, in small-bore spark-ignition engines, the development of tumble motion is hindered by an anticlockwise vortex located beneath the intake valve, leading to an early tumble decay during compression. Moreover, the intensity of the tumble directly depends on the intake mass flow rate, regulated by throttle valve openings. Therefore, understanding the impact of throttle openings on flow evolution in small-bore engines is essential. This study employs computational fluid dynamics (CFD) simulations, validated against experimental data of in-cylinder pressure traces and ensemble-averaged flow fields, to analyze the influence of throttle openings on flow fields. Flow evolution on multiple planes is discussed in-depth, along with the jet emanating from the intake valve curtain area, which is correlated with the formation of in-cylinder flow structures. Additionally, it is found that both intake mass flow rate and backflow intensity significantly affect the flow fields. While backflow during intake valve opening (IVO) is more pronounced under 25% throttle opening (TO) condition, it minimally impacts the flow fields on the symmetric tumble plane during the intake stroke for both the TO conditions. Conversely, backflow during intake valve closing (IVC) is more prominent under 100% TO, resulting in earlier tumble decay compared to 25% TO. Also, the effect of backflow is found to have minimal effects on the flow fields of the cross-tumble plane and offset tumble plane.