Two-Dimensional In-Cylinder Flow Field in a Natural Gas Fueled Spark Ignition Engine Probed by Particle Tracking Velocimetry and Its Dependence on Engine Specifications

An experimental study was made to investigate in-cylinder flow field in a natural gas fueled spark ignition engine and the effects of engine specifications on in-cylinder flow field. The instantaneous two-dimentional flow fields in a single-cylinder visualization engine, which has 75mm bore and 62mm stroke, were measured in various cross sections perpendicular to the cylinder axis by using the laser light sheet PTV method at various crank angles during intake, compression, and expansion strokes over the wide range of piston combustion chamber configuration, top clearance, and nominal swirl ratio. Flow fields during compression and expansion strokes were also calculated using KIVA2 simulation code for better understanding of the measured results. The results showed that induction-generated swirl is getting concentric to the cylinder center in compression stroke, and is shifted in the radial direction in expansion stroke. Swirl velocity increases with the lapse of time, and shows its maximum around 20°BTDC in expansion stroke. Squish velocity, which is approximately 0 at TDC, shows its inward maximum and outward maximum around 20°BTDC and 20°ATDC, respectively. The maximum outward squish velocity in expansion stroke is about twice as large as the maximum inward squish velocity in compression stroke. Turbulence energy increases, after showing its maximum around 20°BTDC in compression stroke. Smaller piston cavity diameter resulted in larger squish velocity, actual swirl ratio, and turbulence energy. Top clearance had little effect on in-cylinder flow field. Higher nominal swirl ratio resulted in larger actual swirl ratio and turbulence energy, while it had little effect on squish velocity.