This and the accompanying article present an experimental study of diesel sprays under current direct injected diesel engine operating conditions. In this article (Part I) the study is focused on the flow behavior inside the nozzle, and the characteristics of the sprays injected into a high-density gas at low temperature so that fuel evaporation is avoided. A complete characterization study has been performed for five different nozzles, with nominal hole diameter ranging from 115 to 200 mu m, in different injection conditions, in order to evidence the influence of nozzle geometry and injection parameters on major flow features, both inside and downstream of the nozzle orifices. The experimental methodology used in this work includes a characterization of the internal nozzle geometry on the basis of microscopic visualization of silicone molds of the nozzle, a hydraulic characterization of the nozzle, measurement of the spray momentum, and spray visualization and image processing. Combining these techniques makes it possible to determine the discharge, velocity, and contraction coefficients, and the critical cavitating conditions, all of which are necessary for a proper analysis of the injection process as well as for modeling purposes. Moreover, the accurate determination of these coefficients have made it possible to determine, on the basis of the spray visualization results, a unique proportionality constant of the expression for spray tip penetration derived from dimensional analysis for rectangular injection rates, valid for all the nozzles and conditions evaluated. In a second publication (Part II),fuel evaporation will be analyzed for the same set of nozzles used in this article, making use of the results presented here. Liquid spray penetration will be measured under both reacting and nonreacting atmospheres for the five nozzles and the dependence of liquid length with injection and ambient conditions will be analyzed by applying a simple spray model and the hypothesis of mixing limited vaporization.