Appl. Phys. B 64, 717–724 (1997) C  Springer-Verlag 1997 Imaging and post-processing of laser-induced fluorescence from NO in a diesel engine Th.M. Brugman, G.G.M. Stoffels, N. Dam, W.L. Meerts, J.J. ter Meulen Department of Molecular and Laser Physics, University of Nijmegen, Toernooiveld, 6525 ED Nijmegen, The Netherlands (Fax: +31-24/365-3311) Received: 11 July 1996/Revised version: 12 November 1996 Abstract. The imaging of in-cylinder NO distributions in a steadily running optically accessible diesel engine operated on standard diesel fuel is performed by means of the 2D- LIF technique and a tunable ArF excimer laser at 193 nm. Simultaneous excitation at 226 nm showed that no sizable photo-chemical effects are induced by the excimer laser. In order to account for the gradually decreasing transparency of the windows due to soot deposits and for the in-cylinder atten- uation of the laser intensity, signal processing procedures for the imaged distributions are presented. In addition to the more realistic depictions of NO distributions at selected crank an- gles and loads resulting from these procedures, the latter are an essential step towards a quantitative interpretation of the corresponding in-cylinder NO concentrations. PACS: 42.30.Va; 82.20.Wt Diesel engines are becoming increasingly popular because of their highly efficient combustion in comparison to spark ig- nited engines. This high efficiency generally leads to a lower emission of CO 2 and as a consequence diesel-fuel-driven en- gines tend to contribute less to the greenhouse effect. The high temperature that results from the high pressure in the combus- tion chamber of a diesel engine causes the auto-ignition of the fuel/air mixture but also gives rise to the subsequent forma- tion of NO and NO 2 . Furthermore, the largest of the organic molecules making up the complex chemical composition of diesel fuel are easily transformed into carcinogenic coatings on the soot particles that are produced during the combustion as well. For environmental reasons the emissions of NO x and soot must be reduced drastically in order to meet the ever more stringent emission standards. However, in spite of the progress made in the modelling of the physics and the chemistry in the combustion chamber [1] the diesel combustion process is still too complex to be fully understood. In recent years the applica- bility of several non-intrusive laser diagnostic techniques such as LIF (laser induced fluorescence) and Mie scattering to the study of the combustion processes inside diesel engines has been widely recognized. When the fuel is doped with fluoresc- ing tracers, 2D-LIF can be successfully applied in the study of the fuel distribution in a spark-ignition engine [2] and in the combustion chamber of a directly injected diesel engine [3]. In-cylinder soot distributions may be studied by a combination of the 2D-LII (laser induced incandescence) technique and res- onant (Mie) scattering [4]. In-cylinder NO distributions have been visualized by means of 2D-LIF by Arnold et al. [5], Ala- ta¸ s et al. [6] and Brugman et al. [7]. Alata¸ s et al., using a 50/50 mixture of iso-octane and tetradecane in a square combustion chamber, found evidence that the NO formation ceases at 30 ◦ to 40 ◦ ATDC (after top dead centre). In these experiments it was assumed that the LIF detection method is non-intrusive. The high excimer laser intensities used, however, may give rise to photo-chemical processes by which NO is produced or destroyed. This might result in erroneously measured NO distributions. In this work a two-colour experiment was per- formed on NO in a running diesel engine in order to check for the occurrence of these photo-chemical effects. In most of the above mentioned studies the test engines were driven by low-sooting substitute fuels and operated in skip-fired mode (i.e., the combustion in the motored engine takes place only every selectable number of cycles) in order to prolong the optical transparency of the windows of the combustion chamber. However, if the measurements are to be performed in a standard diesel-fuel-driven engine under steady operating conditions, the decreasing window transparency as a result of soot deposition should be taken into account. Also, the in-cylinder attenuation of the laser sheet intensity needs to be accounted for. The objective of this study was to investigate the extent to which the applied 2D-LIF in-cylinder imaging technique in itself is capable of producing reliable and accurate results. Therefore, methods are explored by which the imaged NO-fluorescence distributions can be adjusted for the gradual- ly decreasing transparency of the windows and the in-cylinder attenuation of the laser intensity, in order to obtain more realis- tic NO distributions. In addition, as a check of the reliability of the obtained NO LIF images, measurements were performed in which the laser-beam direction was reversed. When correct- ed for window transmission losses and laser-beam extinction, the obtained NO images should be independent of the laser beam direction. 1 Experimental The laser, the engine and the intensified CCD camera are the principal elements of the experimental setup as depicted in Fig. 1. The tunable ArF excimer laser (Lambda Physik