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For several years, the development of combustion and injection systems has been focused on drastically reducing pollutant emissions while preserving the high fuel efficiency which characterizes Diesel engines. In the mean time, the industrial robustness and the customer reliability had to be secured for worldwide applications. Within this working frame, project investigations have shown the importance of injector nozzle characteristics as a potential to greatly improve the fuel spray quality and thereby reduce engine-out emissions. Design parameters like hole diameter, hydro grinding, conicity or inlet hole radius have shown a direct influence on the internal hydraulic flow, allowing a better trade off between high performances and emissions; at the same time, induced cavitation has been identified not only for its well known influence on the discharge coefficient but also for its key role versus coking phenomena which especially appear with small hole diameters.

The important break-through introduced by Direct Injection SI engines in the car industry has undoubtedly lead to a fantastic acceleration in using very up to date investigation tools as 3D simulation codes or advanced optical techniques, such as Laser Induced Fluorescence. The aim of this paper is to discuss and compare the conventional single tracer Fuel Air Ratio Laser Induced Fluorescence (FARLIF) and its Exciplex version. Basis of this comparison are given by a new Tumble Guided Direct Injection medium size engine developed by Renault. While using a 93% iso-octane, and an adjusted proportion of benzene and tri-ethyl-amine (TEA) mixture as fuel for an optical engine, the LIEF technique has proved to be quantitative for the lambda determination even if a high amount of liquid is present. An accuracy of ±3% can be achieved in the interesting range of 0.8 - 1.2.