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.
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