Impact of Fuel Sprays on In-Cylinder Flow Length Scales in a Spark-Ignition Direct-Injection Engine

The interaction of fuel sprays and in-cylinder flow in direct-injection engines is expected to alter kinetic energy and integral length scales at least during some portions of the engine cycle. High-speed particle image velocimetry was implemented in an optical four-valve, pent-roof spark-ignition direct-injection single-cylinder engine to quantify this effect. Non-firing motored engine tests were performed at 1300 RPM with and without fuel injection. Two fuel injection timings were investigated: injection in early intake stroke represents quasi-homogenous engine condition; and injection in mid compression stroke mimics the stratified combustion strategy. Two-dimensional crank angle resolved velocity fields were measured to examine the kinetic energy and integral length scale through critical portions of the engine cycle. Reynolds decomposition was applied on the obtained engine flow fields to extract the fluctuations as an indicator for the turbulent flow. For the early injection condition, the average kinetic energy is increased by the fuel injection for 40 CAD, but the turbulent kinetic energy is not enhanced significantly. When the fuel is injected in the compression stroke, both average and turbulent kinetic energy are elevated for 70 CAD after start of injection (aSOI). The spatial mean of the turbulent integral length scale varies from 2-10 mm during intake and compression stroke without fuel injection. Strong spatial variations of the turbulent integral length scale are observed. Early fuel injection lowers the spatial mean integral length scale by as much as 2 mm within 40 CAD aSOI, while the maximum decrease of that scale during late injection is about 3 mm within 30 CAD aSOI.