An experimental and numerical characterization has been conducted of a high-pressure common rail diesel fuel injection system. The experimental study was performed using a common rail system with the capability of producing multiple injections within a single cycle. The injector used in the experiments had a single guided multi-hole nozzle tip. The diesel sprays were injected into a pressurized chamber with optical access at ambient temperature. The gas density in the chamber was representative of the density in an HSDI diesel engine at the time of injection. Single, pilot, and multiple injection cases were studied at different rail pressures and injection durations. Images of the transient sprays were obtained with a high-speed digital camera. From these images spray tip penetration and cone angles were obtained directly. Also spray droplet sizes were derived from the images using a light extinction method (LEM).The quantitative and qualitative information from the experimental measurements helped test extensively and assess spray models used in KIVA-II within the context of injection in a high speed small bore diesel engine. A breakup model which has been previously used successfully to predict spray behavior in heavy duty diesel engine was considered in this paper: Kelvin-Helmholtz and Rayleigh-Taylor instabilities were considered for modeling spray breakup, and a breakup length accounted for the intact core region. An additional drop collision regime (coalescence followed by separation for near head-on collision) was also tested in this study to see its importance in high pressure diesel sprays. Its effect is to reduce the droplet size by about 8% during the injection. The computations are in fairly good agreement with the spray tip penetrations, and provide trends about drop sizes measurements.