| Abstract: | High-speed particle image velocimetry (HSPIV) was applied to an optical spark-ignition direct-injection engine in order to analyse various turbulent properties of the flow-field. The engine was motored at 1200 RPM with an intake pressure of 100 kPa, while HSPIV images were acquired at a sampling frequency of 5 kHz on both a vertical (tumble) plane and a horizontal (swirl) plane. The flow was decomposed in mean and fluctuating components via three different methods — ensemble averaging, spatial filtering, and temporal filtering. It was found that the velocity fluctuations calculated via the ensemble average method were more closely linked to low-frequency rather than high-frequency fluctuations, suggesting that they are more representative of cycle-to-cycle variation rather than true turbulence. Visual inspection of the high-frequency fluctuating flow-fields derived through the two filter based approaches revealed turbulent structures of similar size, shape and distribution. To equate the two filtering methods quantitatively, a spatial filter was designed with a mean flow speed scaled cut-off length, which was tuned in order to match the turbulent kinetic energy (TKE) of a 300 Hz temporal filter. A brief case study was then performed on a fuel-injected operating condition, run at the same 1200 RPM engine speed and 100 kPa intake pressure. A 1:1 split ratio dual-injection strategy was employed, with the first injection at 300°CA bTDC and the second injection at 110°CA bTDC. The relatively late second injection was found to significantly increase both the mean and turbulent velocities present in the flow-field in comparison to the motored condition, with TKE magnitudes being ～5 to 10 fold higher, depending on the choice of cut-off length. |
