GTL and RME combustion analysis in a transparent CI engine by means of IR digital imaging

In the present paper, infrared (IR) measurements were performed in order to study the behaviour of biofuels combustion in a transparent Euro 5 diesel engine operating in premixed mode. Commercial diesel fuel (REF) Gas To Liquid (GTL) and Rapeseed Methyl Ester (RME) biofuels have been used. An elongated single cylinder transparent engine equipped with the multi-cylinder head of commercial passenger car and common rail (CR) injection system was used. A sapphire window was set in the bottom of the combustion chamber, and a sapphire ring was placed in the upper part of the cylinder. Measurements were carried out through both accesses by means of high-speed infrared digital imaging system. IR camera was able to detect the emitted light in the wavelength range 1.5-5 ?m. In a previous paper UV and visible cameras were used, infrared imaging allowed acquiring larger amount of information than those experiments. In particular the IR camera was used for the characterization of injection and combustion process. Analysing the IR images, it was possible to identify clearly the seven jets of vaporized fuel that react with air in the bowl. During the late combustion phase, the IR image showed a good capability to follow the hot burned gas both in the bowl and above the piston. The IR camera has shown high sensibility permitting to follow carefully the soot oxidation process within the cylinder. The GTL shows an advance of about 2°CA in the evolution of combustion process with respect to the RME. On the contrary a longer chemical activity has been detected for the latter biofuel. Finally, the IR camera was revealed very useful tool to characterize the combustion process for long time allowing high quality of the results. Images of the reactions that happen in the combustion chamber and above the piston head were clearly acquired even if the optical windows were obscured by the soot produced from the previous combustion cycles.