In the present paper, infrared (IR) measurements were performed in order to study the development of injection and combustion in a transparent Euro 5 diesel engine operating in premixed mode. An elongated single-cylinder engine equipped with the multi-cylinder head of commercial passenger car and with common rail (CR) injection system, respectively, was used. A sapphire window was set in the bottom of the combustion chamber, and a sapphire ring was placed between the head and the top of the cylinder line. Measurements were carried out through both accesses by a new high-speed infrared (IR) digital imaging system obtaining information that was difficult to achieve by the conventional UV-visible camera. IR camera was able to detect the emitted light in the wavelength range 1.5-5 μm that is relevant for the emission bands of CO₂ and H₂O. The evaporation phase of pre and main injection, and subsequent combustion evolution were analyzed. Moreover, IR imaging was carried out by means of "ad hoc" filter to evaluate the spatial distribution of CO₂ inside the cylinder. Images sequence during the whole engine cycle, starting from the intake and stopping to the exhaust phase, was observed from both optical windows. A comparison with visible imaging was carried out. IR measurements showed the ability to determine temporal and spatial distribution of fuel during evaporation phase and evaluate the combustion process evolution for longer time than visible imaging. Moreover, the IR camera was revealed very useful tool to detect the emitted light after a long operation time of the engine. The camera was able to acquire images of the reactions that happen in the combustion chamber and above the piston head even if the optical windows were obscured by the soot produced from the previous combustion cycles.