A Study of Cycle-to-Cycle Flow Variations in a Small Spark-Ignition Engine at Low Throttle Opening

Cycle-to-cycle flow variations significantly influence the combustion variations from one cycle to the next, particularly at low operating loads in small spark-ignition engines. Hence in the present work, cycle-to-cycle flow variations are analyzed at low throttle opening of 25% in a small spark-ignition engine using particle image velocimetry (PIV) technique. Experiments are conducted in an optically accessible single-cylinder, port-fuel-injection engine (volume: 110 cm3) at 1200 rpm engine speed. 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 processed using cross-correlation method to obtain the instantaneous velocity fields considering 200 image pairs at each crank angle position considered. Turbulent kinetic energy and probability density functions of vorticity are then calculated from measured instantaneous velocity fields at different crank angle positions to quantify the cycle-to-cycle flow variations. It is found that cycle-to-cycle variations were increased from 84 CAD to 114 CAD during intake, and then were reduced during intake (from 114 CAD to BDC of intake) and compression, except a slight increase during late compression. CFD simulations are also performed using CONVERGE software, and showed good agreement with measured flow fields obtained using PIV.