Laser Diagnostics for Applications to In-Cylinder Engine Investigations

Abstract: In the work presented in the thesis laser diagnostics was employed primarily in the investigation of engines, but was also used to study soot in flames. Quantitative measurements were performed for visualizing both in-cylinder soot formation and DME sprays in a combustion vessel. The DME sprays inside a diesel truck engine were also qualitatively investigated. The combustion process in the HCCI engine was studied as well. Two species, formaldehyde and OH, were detected there simultaneously. Laser-induced incandescence (LII) was used in the work with soot diagnostics to measure soot concentrations, the potential for soot particle sizing also being investigated. Early in the work, various fundamental studies were performed in well-characterized flames in which results using the LII technique were compared with measurements performed by means of the scattering-extinction technique. As an extension of this work, LII was combined with an extinction measurement to provide on-line calibration of quantitative measurements of soot volume fractions. LII was also employed to investigate soot production in the cylinder of a gasoline direct-injection engine. Quantitative soot volume fraction distributions were obtained from the engine after calibrating the LII technique against a small, premixed well-defined flame. In the second part of the thesis, the use of dimethyl ether (DME) sprays as fuel for diesel engines was investigated. Laser-Rayleigh imaging was applied to obtain quantitative images of the local equivalence ratios for the DME sprays. Before any of the engine measurements were performed, the mixing conditions of both burning and non-burning DME sprays were investigated in a chemically pre-heated combustion vessel. These measurements were followed up by in-cylinder investigation of the use of DME spray in a diesel truck engine. The development of the spray was studied from the start of injection until all the fuel was consumed. In the final part of the work, ignition and combustion processes in the HCCI engine were investigated by means of laser-induced fluorescence (LIF). Two species, formaldehyde and OH, were imaged simultaneously. The formaldehyde formed as an intermediate during an early stage of the combustion process served as an indicator of low-temperature reactions. The OH radical, in turn, an important intermediate formed in high-temperature regions during combustion, was used as an indicator of regions in which HCCI combustion was underway. How the combustion process is affected by changes in the homogeneity of the fuel-air mixture that arises from early versus late injection timing, was investigated. During the data evaluation there was a need of defining an index of homogeneity in order to compare the homogeneity of the signal distribution in the different measurements. The effect of fuel volatility on HCCI combustion was also investigated, fuels of low and high volatility, respectively, being employed.