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The effect of intake duct length on volumetric efficiency and in-cylinder flow field is investigated in a four-stroke single cylinder research DI Diesel engine, motored at 1000-3000 rpm. Three lengths of straight duct upstream of the helical inlet port of the engine are considered and measurements of instantaneous mass flow rate and pressure drop across the port are reported as a function of duct length and engine speed. The in-cylinder flow field is characterised by LDV measurements of swirl velocity at inlet valve closure and inside the piston bowl at top dead centre of compression. The results show a significant increase of swirl velocity magnitudes as resonance of the induction system is reached. Turbulence intensity is shown to be less affected by the inlet conditions.