Combustion analysis in an optical Diesel engine operating with low compression ratio and biodiesel fuels

In this paper we report how optical techniques were applied in the cylinder of an optically accessible engine equipped with latest-generation EURO V diesel engine head. The injection strategy with high percentage of EGR, characteristic of real engine operating point, was adopted. In particular, the combustion behaviour at 1500 rpm \ 2 bar BMEP was investigated. Alternative diesel fuels were used. In particular, rapeseed methyl ester (RME) and gas to liquid (GTL) were selected as representative of 1st and 2nd generation alternative diesel fuel, respectively. Combustion analysis was carried out in the engine combustion chamber by means of visible digital imaging. These measurements helped to analyze the chemical and physical events occurring during the mixture preparation and the combustion development. Ultraviolet (UV) digital imaging was also performed and the presence of characteristic radical, like OH, in the various phases of combustion was detected as well. OH spatial distribution and temporal evolution were computed and compared to soot volume fraction measured in the combustion chamber by means of the two colour pyrometry methods. The effect of fuels on the combustion process and pollutants formation was evaluated. The GTL fuel showed better performance in terms of indicated mean effective pressure (IMEP) with respect to the diesel reference fuel with different effects on particulate matter (PM) and gaseous emissions. On the other hand, the RME fuel showed a decrease in IMEP that can be adjusted with a little increase of fuel injected quantity, and very low production of soot in the cylinder and PM at the exhaust compared to the diesel reference fuel.