In-cylinder flow field characteristics and evolution mechanism of unthrottled gasoline engine with Early Intake Valve Closure (EIVC) under different operating conditions

The efficiency improvement of traditional gasoline engines is hindered by a limited compression ratio due to knock and severe pumping losses caused by fixed valve lift. Unthrottled technology has emerged as a solution relying on variable valve mechanisms. However, existing variable valve systems suffer from poor adaptability under full operating conditions and unsystematic research on the coupling mechanism between in-cylinder flow and combustion. In this study, an unthrottled gasoline engine with Early Intake Valve Closure (EIVC) equipped with a Fully Hydraulic Variable Valve System (FHVVS) was taken as the research object. By combining bench tests with CONVERGE simulations, the changes in the in-cylinder flow field during the intake and compression processes of the EIVC-based unthrottled gasoline engine were comparatively analyzed at different engine speeds (1000 r/min, 2000 r/min, 5000 r/min) and with different intake valve lift curves (Curve 5, Curve 6, Curve 8). The results show that: At the same engine speed, extending the intake valve opening duration (e.g., Lift Curve 8 extends by more than 10 °CA compared to Curve 5) can increase the peak tumble ratio (at 1000 r/min, Curve 6 increases by 5.4 % compared to Curve 5), and the attenuation of gas flow velocity in the late compression stage is slower (at the ignition timing, the average velocity of Curve 8 is 7.8 m/s, while that of Curve 5 is only 4.8 m/s). For the same lift curve at different engine speeds, the crank angles corresponding to the two peak tumble ratios remain basically unchanged, but the peak values increase significantly with the rise of engine speed (for Curve 5, the two peaks at 5000 r/min increase by 17.6 % and 235.7 % respectively compared to those at 1000 r/min). The turbulent kinetic energy increases markedly with the increase of engine speed (for Curve 5, the maximum value at 5000 r/min increases by 622.7 % compared to that at 1000 r/min). Additionally, the large-lift curves (e.g., Curve 8) exhibit higher turbulent kinetic energy (TKE) at the ignition timing (2.7 m 2 /s 2 at 1000 r/min, while Curve 5 only reaches 1.7 m 2 /s 2 ), which is beneficial for flame propagation.