PROPER ORTHOGONAL DECOMPOSITION ANALYSIS OF IN-CYLINDER FLOW IN A SMALL SPARK-IGNITION ENGINE AT DIFFERENT THROTTLE OPENINGS WITH PIV DATA

In-cylinder fluid flow has been studied intensively due to its well-known impacts on combustion and pollutant formation. The variation of the in-cylinder flow is a major impediment in achieving higher performance characteristics, particularly at low engine speeds and low to moderate loads. A comprehensive study with primary emphasis on flow evolution and its variations at different throttle openings has not been undertaken in the context of small dual-valve spark-ignition (SI) engines. Therefore, a detailed study of tumble motion in a small port-fuel-injection spark-ignition engine with displacement volume of 110 cm3 is carried out in the present work at three different throttle openings (viz. 25%, 50%, and wide-open throttle) using cross-correlation particle image velocimetry (PIV) measurement technique and proper orthogonal decomposition (POD). The flow was recorded over a tumble measurement plane at various crank angle positions during both intake and compression strokes at an engine speed of 1,200 rpm. Flow variations are quantified with the histogram of averaged vorticity from all instantaneous cycles using PIV data. Turbulent kinetic energy (TKE) is calculated at every measured crank angle position for all three throttle conditions. Triple decomposition of the velocity fields is performed to separate coherent structures and quantify the variations of large-scale structures. The flow variation is related to the turbulent kinetic energy and the variations of domi-nant POD modes.