In-cylinder turbulence and cycle variations in an optical compression-ignition engine

Increasingly stringent emissions regulations have created the need for ever increasing improvements to be made to the reduction of compression ignition engine emissions. Understanding and predicting the possible in-cylinder flow structures that occur within compression-ignition (CI) engines is vital if further optimisation of combustion systems is to be achieved. To enable this prediction, fully validated computational models of the complex in-cylinder flows from intake through to the combustion 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 thesis over a vast range of spatial locations and test condition, obtained from a single-cylinder CI engine with optical access using high-speed PIV, demonstrates that significantly different structures are generated over different cycles. This results 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 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 is applied to flows produced from two different cylinder heads; a high swirl head and a low swirl head. It was shown that the strongly organised flow induced from the high swirl head decays at a lower rate resulting in the flow energy dissipating to turbulence at the end of compression.The work goes on to investigate the high pressure fuel spray structure in the single cylinder engine using high speed images to determine the information needed for model validation. This data provided quantitative validation to CFD results on the levels of variations which could occur. Finally, the combustion flame is captured to quantify the variations in the structure and timing of the combustion event. This data showed that the higher engine speed experiences less variations in the combustion, which can be related to the trend of higher turbulence and less variations in the air flow for higher speeds.The differences in air flow, fuel spray from a mean condition have been shown to result in changes in the combustion which will influence the production of emissions. Therefore, it is important that the variations quantified in this thesis are considered in simulations.