Investigation of In-Cylinder Soot Formation and Oxidation by Means of Two-Dimensional Laser-Induced Incandescence (LII)

Increasing requirements on the environmental acceptability of internal combustion engines led to strong efforts to meet the continuously getting stricter emission regulations. New concepts of combustion and injection have been developed together with appropriate techniques of exhaust gas aftertreatment. For diesel engines especially modern high pressure injection systems, such as the application of distributor pump and common rail have been applied successfully. However, internal mixture formation, ignition and combustion for this new systems are not completely understood so far. Optical measurement techniques offer the possibility of the in-situ investigation of the combustion process. An appropriate method for soot measurements, which is without source of interference also applicable to technical systems, is given by laser-induced incandescence (LII). The principle of this technique is to heat up the soot particles by a highly energetic laser pulse to temperatures well above ambient temperature and to detect the resulting thermal radiation. For appropriate experimental conditions, the signal can be shown to be proportional to the soot mass concentration within the detection volume. In this work, basic features of the technique are discussed in detail, especially with respect to the application within diesel engines. One aspect of particular interest is the range of detection wavelengths, which has to ensure a mass proportional signal of sufficient magnitude, but also the suppression of flame luminosity and especially elastic scattering of walls, droplets and dust. Another important point to consider is attenuation of both the incident beam and the signal due to light absorption, scattering and blinding of the optical access. Measures to minimize these influences are discussed. The soot formation and oxidation process inside the combustion chamber of a DI diesel engine was investigated by means of two-dimensional LII. For this purpose, a thin light sheet was introduced into the piston bowl of an optically accessible diesel engine, which is very close to the serial standard and driven with standard diesel fuel. The detection was performed perpendicularly to the incident beam through a transparent piston window and a mirror which was mounted inside the slit of the elongated piston. First results of a common rail injection system are presented for a mini-sac-hole nozzle. Sequences of the LII signal and flame luminosity were measured in a time interval from before top-death-center (TDC) to about 30 degrees crank angle after TDC. The comparison to investigations of natural flame luminosity, which can be detected simultaneously, is shown to give valuable additional information. It is, e.g., possible to detect a LII signal without the simultaneous occurrence of flame luminosity, which can be interpreted as originating from soot, which will not be further oxidized due to the end of the diffusion flame. This portion will be emitted later on within the exhaust stroke. Generally, the flame does not cover the complete piston bowl for the operation conditions investigated in this study. Schraml et al., 10 International Symposium on Applications of Laser Techniques to Fluid Mechanics Introduction The development of new combustion concepts, which is enforced by continuously stricter demands on the environmental acceptability of internal combustion engines, heavily relies on an improved knowledge of internal combustion phenomena. High pressure common rail systems, which offer additional flexibility of injection parameters connected with improved mixture formation, are increasingly applied to modern diesel engines in order to lower both pollutant emission and fuel consumption. However, the mechanisms of spray propagation, mixture formation, ignition and combustion are not completely understood up to now. The application of optical measurement techniques as an extension of conventional techniques allows a deeper insight into these basic mechanisms. Investigations can be performed with high spatial and temporal resolution and without disturbance of the in-cylinder flow and combustion. Regarding soot formation and oxidation laser-induced incandescence (LII) has been shown to be an appropriate tool even for technical systems. This technique offers the possibility of the quantification of soot mass concentration and can be performed to obtain two-dimensional information within a given detection plane. It has been shown to give reliable results within laboratory flames by many researchers, e.g. by Shaddix et. al. [1994] and Quay et. al. [1994]. Even some in-cylinder measurements have been published, e.g. by Dec [1992] and Inagaki et. al. [1999], within the last few years, which, however, rely on considerable modifications of the engine or the application of substitute fuels. Recently, the LII technique was firstly applied to exhaust gas measurements by Schraml et. al. [1999, 2000] and the determination of additional quantities, like the soot primary particle size or the agglomerate size was added to gain additional information concerning soot morphology by Will et. al. [1995, 1996, 1998]. In this work, LII is successfully applied to a research engine, which is very close to the series standard and is driven with standard diesel fuel. The results show almost no interference with reflections from walls and droplets and demonstrate the usefulness of this technique. Theory Basic principle of the laser-induced incandescence (LII) technique is to heat up the soot particles within the probe volume to about vaporization temperature by means of a highly energetic laser pulse and to detect the strongly enhanced thermal radiation with an appropriate detector. The relevant mechanisms of power absorption and heat loss have to be included into a power balance to calculate the resulting particle temperature T after laser irradiation. The differential equation reads [Will et. al., 1998] , 0 6 3 ) , ( ) , ( 2 2 ) 0 ( 4 2 = − − ⋅ ∆ + − Λ − ∫ dt dT C d d T M d d dt dm M H d T T E d Q p b p p v p i p abs ρ π λ λ λ ε π π π