Valve Flow Coefficients under Engine Operation Conditions: Piston Influence and Flow Pulsation 2019-24-0003

Engine valve flow coefficients are used to describe the flow throughput performance of engine valve/port designs, and to model gas exchange in 0D/1D engine simulation. Valve flow coefficients are normally determined at a stationary flow test bench, separately for intake and exhaust side, in the absence of the piston. However, engine operation differs from this setup; i. a. the piston might interact with valve flow around scavenging top dead center, and instead of steady boundary conditions, valve flow is nearly always subjected to pressure pulsations, due to pressure wave reflections within the gas exchange ports. In this work the influences of piston position and flow pulsation on valve flow coefficients are investigated for different SI engine geometries by means of 3D CFD and measurements at an enhanced flow test bench. In the past, most research work on valve flow coefficients left aside possible piston influence and, for dynamic boundary conditions, it largely omitted subtraction of the gas inertia effects, which are already covered by 1D simulation. In this work, concerning piston influence, various valve overlap situations are investigated and compared with current 0D/1D simulation. It is shown that common piston shapes can cause masking effects for open valves and reduce their mass flow noticeably. Subsequently a modeling approach is presented to consider the piston influence in 0D/1D engine simulation. Besides this, the influence of flow pulsation on flow coefficients is investigated using pressure ratio profiles, which are derived from engine test bench measurements at different engine speeds. These profiles, one with, and one without change in flow direction, are applied to a 1D and a 3D CFD flow test bench model at various valve lifts. It is shown that 1D simulation can mainly provide a decent pulsation mass flow prediction, but it depends on the geometrical accuracy of the port model.