Experimental interpretation of compression ignition in-cylinder flow structures

Understanding and predicting in-cylinder flow structures that occur within compression-ignition engines is vital if further optimisation of combustion systems is to be achieved. To enable this prediction, fully validated computational models of the complex turbulent flow-fields generated during the intake and compression process are needed. However, generating, analysing and interpreting experimental data to achieve this validation remains a complex challenge due to the variability that occurs from cycle to cycle. The flow-velocity data gathered in this study, obtained from a single-cylinder CI engine with optical access using high-speed PIV, demonstrates that significantly different structures are generated over different cycles, resulting in the mean flow failing to adequately reflect the typical flow produced in-cylinder. Additionally, this high level of variability is shown by the work to impact the assessment of turbulence throughout the cycle, influencing the values often used to validate mathematical models. The original work in this paper analyses experimental PIV data from the single cylinder engine, to characterise the differences between individual cycles’ bulk flow structures and the resultant turbulent fields. The analysis approach presented uses proper orthogonal decomposition (POD) and spatial filtering to interpret the progression of the flow structures and energy throughout compression, giving an understanding of the actual flow structures that are most likely to be produced in the engine. This analysis of the data provides a meaningful understanding of the nature of the bulk flow variations and how the turbulent field develops over a given cycle, from the intake stroke to the end of compression.