Investigation of the 3D flow field in an IC engine using tomographic PIV

In-cylinder measurements of the instantaneous 3D flow field are required for further understanding of turbulent mixing and combustion processes in internal combustion (IC) engines as well provide valuable data for the development of large eddy simulation (LES) models for combustion simulation. Tomographic particle image velocimetry (TPIV) is applied within a motored direct-injection spark-ignition (DISI) optical engine to measure the instantaneous volumetric flow field at selected crank-angle degrees (CAD) during the intake and compression stroke. Measurements were conducted using a double-pulse Nd:YAG laser with an average pulse energy of 375 mJ for illumination and four CCD cameras arranged circularly around the optically accessible cylinder. A volume of 47 × 35 × 4 mm 3 was imaged located centrally between the intake valves. The uncertainty of the 3D velocity data was addressed using the principle of mass conservation for the inlet flow. A precision within 9% was found for the calculation of the velocity gradient tensor and is comparable to experiments in generic configurations, indicating that TPIV measurements within a motored IC engine are feasible. Based on the volumetric velocity data, the average structure of the flow field was analyzed showing a clear orientation of the average velocity, which changes during the different phases of the cycle. The 3D turbulent kinetic energy (TKE) is computed and compared to the 2D-TKE determined from in-plane velocity components and reveals distinct regions of large out-of-plane velocity fluctuations. Results from volumetric velocimetry furthermore provides insight into the occurrence and orientation of vortical structures that here are identified by using the Q-criterion. Differences between flow structures during intake and compression strokes are discussed on basis of ensemble-averaged and individual cycle velocity maps as well as the full vorticity vector.