Optimization of unthrottled continuously variable valve lift (CVVL) engine combustion chamber with gasoline-ethanol blend: A computational fluid dynamics (CFD) study on in-cylinder flow and combustion

Abstract

Unthrottled Continuously Variable Valve Lift (CVVL) engines adopting the Early Intake Valve Closure (EIVC) strategy exhibit weak in-cylinder turbulence under low loads, leading to prolonged combustion duration. This study optimizes the combustion chamber of an E10-fueled CVVL engine (equipped with advanced intake port) to resolve this low-load issue while preserving high-load efficiency. It incorporates three core innovations: (1) For the first time, the performance of cylinder wall-side and spark plug-side valve masking structures is systematically compared under identical CVVL operating conditions; (2) It innovatively investigates the interaction between an advanced high-tumble intake port design and the CVVL operating mechanism; (3) It explores masking technology application in E10-fueled CVVL systems. The research employs an experimentally validated CFD approach. At 2000 r/min, simulations were performed for two masking structures (Cases 1 and 2) and a baseline (no masking) under partial-load (brake mean effective pressure, BMEP = 0.4 MPa) and high-load (BMEP = 1.1 MPa) conditions. Key results show: under partial load, the low-lift characteristic of CVVL conflicts with the advanced intake port’s flow-guiding principle, inhibiting tumble formation. Among the configurations, spark plug-side masking outperforms cylinder wall-side masking and the baseline significantly: it increases turbulent kinetic energy (TKE) at ignition timing by 89% (vs 6% for Case 1), shortens 10%–90% combustion duration by 26.4%, raises in-cylinder peak pressure by 13.3%, and has lower volumetric efficiency loss (9.4% vs 13.4% for Case 1) and reduced pumping losses. Under high load, the advanced intake port dominates in-cylinder flow, making masking’s effect on engine performance negligible. This study confirms that the synergistic effect of spark plug-side masking and the advanced intake port effectively alleviates low-load combustion deterioration in E10-fueled unthrottled CVVL engines, providing a feasible solution to balance combustion stability and efficiency.