Valve Flow Coefficients under Engine Operation Conditions: Pressure Ratios, Pressure and Temperature Levels

Engine valve flow coefficients are not only used to characterize the performance of valve/port designs, but also for modelling gas exchange in 0D/1D engine simulation. Flow coefficients are usually estimated with small pressure ratios and at ambient air conditions. In contrast, the ranges for pressure ratio, pressure and temperature level during engine operation are much more extensive. In this work the influences of these three parameters on SI engine poppet valve flow coefficients are investigated using 3D CFD and measurements for validation. While former investigations already showed some pressure ratio dependencies by measurement, here the use of 3D CFD allows a more comprehensive analysis and a deeper understanding of the relevant effects. At first, typical ranges for the three mentioned parameters during engine operation are presented. A preliminary study for a simple nozzle geometry shows the suitability of the utilized 3D CFD code for partially overcritical flow, and demonstrates the limits of the fundamental nozzle flow equation. Steady flow simulations of two different four-stroke SI engine cylinder head geometries reveal that valve flow coefficients show noticeable dependencies on all three named parameters especially for small valve lifts, and flow direction from port into cylinder. Pressure recovery within and downstream valve gap influenced by valve gap boundary layer height is identified to be responsible for this behavior. In the reverse flow direction the existence of pressure recovery can also be confirmed, but its effects are found to be superimposed by flow necking and separation effects which especially depend on valve design. For validation, the 3D CFD results are compared with valve flow coefficients measured at flow test bench with extended pressure ratios and pressure levels. The measurements confirm the valve flow coefficient dependencies found by simulations.