Analysis of In-Cylinder Flow and Cycle-to-Cycle Flow Variations in a Small Spark-Ignition Engine at Different Throttle Openings

Flow variations from one cycle to the next significantly influence the mixture formation and combustion processes in engines. Therefore, it is important to understand the fluid motion and its cycle-to-cycle variations (CCVs) inside the engine cylinder. Researchers have generally investigated the cycle-to-cycle flow variations in moderate- to large-sized engines. In the present work, we have performed the flow measurement and analysis in a small spark-ignition engine. Experiments are conducted in an optically accessible, single-cylinder, port-fuel-injection engine with displacement volume of 110 cm³ at different throttle openings (i.e. 50% and WOT) using particle image velocimetry. Images are captured at different crank angle positions during both intake and compression strokes over a tumble measurement plane, bisecting the intake and exhaust valves and passing through the cylinder axis. The histograms of vorticity are used as a metric for the quantification of cycle-to-cycle flow variations. It is found that for wide-open (i.e. 100%) throttle, cycle-to-cycle variations first increased from 76 CAD (after TDC of intake) to a maximum value at about 118 CAD, and then decreased during the late intake and early compression to a minimum at about 232 CAD for measured crank angle degrees. Results also showed that cycle-to-cycle variations for 50% and wide-open throttle conditions were comparable for all measured CADs. This similarity between 50% and WOT conditions based on histograms of vorticity was found to be consistent with turbulent kinetic energy (TKE) results. In addition, CFD simulations are also performed using CONVERGE software, and a great resemblance is observed between CFD simulations and experimental results for both 50% and WOT conditions.