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The present work deals with the understanding and optimisation of air-EGR mixture phenomena since EGR strategy already showed its benefits on combustion and particularly on NOx emissions. Such benefits can be only observed with an accurate regulation system limiting EGR cylinder-to-cylinder maldistribution, otherwise a risk of opposite effect may appears (no NOx reduction with increasing of particulate emission). Given the complexity to get EGR cylinder-to-cylinder distributions on « real » engine working conditions and to understand mixing process which occurs upstream cylinder head, a simplify bench has been built in co-operation with the Institute of Fluid Mechanics from Toulouse (IMFT) in order to characterise Air/EGR mixing owing to optical diagnostic (PIV, PLIF). EGR cylinder-to-cylinder distribution is also measured by a thermal method.