This work focuses on the analysis of the pilot injection in a research diesel engine by non-conventional optical diagnostics. The visualization of the pilot injection process is obtained by means of an optical access in the piston head and by the presence of an inclined mirror for the collection of the images. Due to the long residence time in the combustion chamber, the pilot injection is in vapor phase for most of the time. Hence, an appropriate technique needs to be used for its investigation. Visible imaging and an innovative, non-conventional optical diagnostics in the infrared is applied for the visualization of the liquid and vapor fuel. In particular, infrared (IR) imaging has been performed at the wavelengths of 3900 nm and 3400 nm. Experimental results of the liquid and vapor penetration have been compared to the well proofed 1d model developed by the Sandia National Laboratories and adapted for in-cylinder conditions. It is the first time that high spatial and temporal resolution images in the infrared are available for the detection of the jet penetration. Infrared measurements agree well with model prediction indicating that the vapor fuel can be reliably tracked using this technique. Finally, the infrared emission along the jet axis has been found to be related to the equivalence ratio calculated by the model. (C) 2016 Elsevier Ltd. All rights reserved.