Analysis of Cyclic Variation using Time-Resolved Tomographic Particle-Image Velocimetry
To achieve the strict legislative restrictions for emissions from combustion engines, vast improvements in engine emissions and efficiency are required. Two major impacting factors for emissions and efficiency are the reliable generation of an effective mixture before ignition and a fast, stable combustion process. While the mixture of air and injected fuel is generated by highly three-dimensional, time-dependent flow phenomena during the intake and compression stroke, the turbulent flame propagation is directly affected by the turbulence level in the flow close to the advancing flame front. However, the flow field in the combustion chamber is highly turbulent and subject to cycle-to-cycle variations (CCV). To understand the fundamental mechanisms and interactions, 3D flow measurements with combined high spatial and temporal resolution are required. Furthermore, most studies utilize velocity decompositions based on the standard Reynolds decomposition that yields a mean flow velocity and a fluctuation velocity, which comprises of fluctuations from CCV and turbulence. Therefore, time-resolved tomographic PIV measurements are coupled with a triple-velocity decomposition method based on a multiple-snapshot POD to investigate in-cylinder velocity fluctuations from cyclic variability. The method is extensively discussed and the most important steps are highlighted by examples. The results illustrate that the method yields valid findings, which allow a comparison of the impact of CCV between two parameter sets or between engine cycles.