Marchitto, Luca (2015) Characterization of air/fuel mixture and combustion processes in a DISI engine through advanced optical and X-ray based diagnostics. [Tesi di dottorato]

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The present dissertation has investigated the influence of injection parameters on air/fuel mixture and combustion processes in a DISI engine, through advanced diagnostics. It is possible to distinguish three parts of the work: the spray macroscopic parameters characterization, the spray break-up investigation and the study of the effects of injection timing and duration on the combustion process. In the first stage, the influence of injection pressure and duration on the fuel mass rate and the spray morphology has been analysed. A fuel injection rate meter based on the Bosch Tube principle has provided useful information about the time resolved behaviour of the delivered mass highlighting both transient, such as needle lift and nozzle closing operation, and quasi steady stages of the injection event. 2D Mie-scattering imaging and Particle Image Velocimetry technique have been used for characterizing the liquid spray morphology in terms of tip penetration, cone angle and velocity vector distribution. Even if the injection parameters affect the overall spray geometry, the characterization of the inner structure of the spray is fundamental for featuring the atomization phenomena which influence the fuel - air interaction. Therefore, Phase Doppler Anemometry (PDA) and Laser Doppler Velocimetry (LDV) have been applied for providing information about the velocity and size of droplets. Trials have been performed at different injection pressures and distances from the nozzle, along two different axes: the first one corresponding to the jet axis, the second one on the jet edge. Especially at the higher injection pressure, accurate tests have been possible only at a certain distance from the nozzle, where the spray is more dilute and the laser beam can cross the jet core. For this reason X-ray absorption measurements have been performed in order to investigate high-dense regions of fuel sprays, immediately downstream of the nozzle, providing quantitative measurements of the fuel. X-rays penetrate the dense part of fuel spray because of its weak interaction with the hydrocarbon chain due to their low atomic number. X-ray radiography and tomography have been used to investigate the spray core and reconstruct the 3D inner structure. The information about the spray development has been used to optimize the injection strategies of an optically accessible DISI engine operating at mid load condition, at a fixed speed of 2000 rpm. UV-visible imaging has been performed on the engine for investigating the influence of the injection duration and timing on the flame front propagation, while natural emission spectroscopy has been applied to characterize the formation and the evolution of the main compounds featuring the spark ignition and combustion processes. Exhaust emission measurements (HC, CO and NOx) have been correlated with pressure related data and optical results.

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