Diverse techniques have been developed for spray investigations, especially high speed cinematography and microphotography. Recently, the progresses obtained in the application of laser techniques to simultaneous measurement of droplet size and velocity have opened new perspectives for in situ investigations of Diesel fuel jets.In this study, a laser measuring system, based on the Phase Doppler Anemometry method (PDA), is applied to examine the dynamic behaviour of the fuel jet in an experimental direct injection Diesel engine fitted with large optical accesses. The local measurements have been performed for two cases: with combustion and without combustion. The experimental set-up and the elaboration of a procedure for both data acquisition and treatment allow investigations about different aspects of fuel spray such as the nature of atomisation process, the influence of injection conditions (advance of injection and injected fuel quantity) and the spatial and time-resolved evolution of droplet diameters in the combustion chamber. Finally, the experimental data about fuel droplets have been exploited in order to specify drop size distribution laws.The recent progress made in the field of Diesel injection has improved engine performance as well as reducing exhaust gas emissions. The research of a more efficient and less polluting combustion requires a better knowledge of break-up, evaporation and coalescence processes in the fuel jet. Moreover, all spray modelling needs information about drop size distribution. Those are the main objectives of fuel spray investigation which attract the attention of many researchers in the Diesel engine field.For Diesel type fuel sprays, the following experimental techniques were usually used for drop size measurement:Liquid Immersion Sapling Technique [1, 2]. The fuel is injected into a jet receiver, filled with a special liquid. The collected droplets will be observed by a microscope in order to make microscopic size counts. The key problem as well as the greatest difficulty in the application of this method is the selection of an appropriate receiving liquid, which must retain the initial characteristics of liquid droplets and make optical observations of the samples easier. The droplet counts and statistics make long and tedious work, with a considerable risk of mistake. Moreover, this technique cannot be used for instantaneous measurement or spatial and time-resolved distribution study of the droplets.Microphotography [3, 4]. This technique consists of the photography or cinematography of a small part of the fuel jet during a very short lighting time provided by a micro-flash. The visual field of spray can be enlarged before and/or after the spray imaging. In order to make observation of individual droplets in the micro-photograph possible, the spray imaging could be performed only in the locations where droplet concentrations were not of great importance (usually in the spray circumference). The precision of this method depends on the optical system and the film resolution. The task of observation and the statistical analysis of the photographs is also time-consuming.Holography [5, 6]. This method allows one to reconstruct entirely the spray image with differential focusing through the various planes of droplets between the forward and back plane of two grating. Thus, the size and location of droplets can be determined. Making holograms requires a relatively complex optical system and a delicate manipulation of lighting sources, the result of which is a double recording of the unknown object wave, generated, by illuminating the spray with a coherent laser beam, and a second known reference wave. The two basic types of this technique are in-line holography, where the object wave is almost parallel to the reference wave, and off-axis holography where there is a relatively large angle between these two lighting waves.Malvern's Technique . This method is based on light scattering theory: the apparatus measures simultaneously the intensity of the forward scattered laser light at several scattering angles. It is a line-of-sight measuring technique, which can be used to evaluate the drop size distribution, for example the Sauter mean diameter (SMD) within a laser beam. However, up to now the Malvern's technique seems to have had only limited applications in Diesel spray investigations.Phase Doppler Anemometry (PDA) This is an extended version of the Laser Doppler Anemometry (LDA) technique, also based on light scattering principles and well known in velocity measurements. The PDA apparatus works upon simultaneous detection of the frequencies and phases of the Doppler signals, provided by the scattered lights from a droplet moving through the measuring volume. This measuring volume, an ellipsoid, is formed at the intersection of two laser beams. Thus, the obtained Doppler frequency indicates the droplet velocity, while the drop size is a function of phase differences between the signals detected at different locations in the detecting plan. More details about PDA principles can be found in the bibliography, for example in the works of Bachalo and Houser . The PDA method, which seems to be the more advanced experimental technique for two-phases flow (gas/particle) investigations, allows both the instantaneous and simultaneous measurement of droplet velocity and diameter. It is also easy to make a very local test, because of the small measuring volume (generally less than 1 mm3 ).The above analysis of different drop size measuring techniques leds us to choose the PDA method for this study. Referring to the bibliography, the application of this advanced technique to in-cylinder spray investigation still remains a new study area. Since 1990, few results were published [9, 10, 11, 12, 15] and furthermore, the combusting fuel spray was seldom examined.The present study follows our preliminary application of the PDA technique to Diesel spray investigation [13, 14]. Some efforts have been made to ameliorate experimental apparatus and test conditions, as well as to enlarge test matrix. The droplets velocities have been discussed in these above studies, and are not the subject of the present paper.