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Interest on the issue of diesel injector nozzle deposits is rising in the last years due to its effects on engine performance. The alteration of nozzles geometry can cause a difference in fuel mass flow and influence smoke emission. Investigation on the effects of nozzle coking in a diesel injector has been the topic of this paper. The experiments have been carried out in a single cylinder optical engine operating in premixed mode. The head of a Euro 5 production engine has been mounted on an elongated cylinder and the production CR injection system has been used. A sapphire window has been set in the piston head in order to have visible access to phenomena occurring in the combustion chamber. Three injectors with decreasing flow number (FN) have been tested. Engine has been fed with commercial diesel fuel. High spatial and temporal resolution camera has been used for the acquisition of in-cylinder injection and combustion images.

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 behavior 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.