Vibro-acoustic characterisation of a D.I. Diesel engine by the use of the scanning laser vibrometer technique

The objective of this work is represented by a laser vibrometer application on a diesel engine for the identification of noise sources associated with its structural vibration. The first investigation has been concentrated on the possibility to identify as well as quantify the structural noise emissions of a direct injection diesel engine, used mainly for agricultural application as well as small off-road vehicles. In fact, the quantity of noise emitted by the structure represents a large part of overall noise level. The structural noise is directly, in fact, associated with the surface velocity of the engine structure and/or of its components and for this reason is very important to correlate the vibration and acoustic analyses. Through the use of a scanner laser vibrometer (mod. PSV400 Polytec), the vibration maps have been determined of the four accessible investigated surfaces around the engine (according to the standard ISO 3744). In particular, a set of 48 scanning data have been executed: one scanning for each surface of the engine and for 6 different engine speeds in motored and firing operating conditions. The motored condition has been investigated for different engine speeds in order to quantify the mechanical noise associated with the rotating components and finally to compare with the overall noise in the firing condition. Vibration analysis has been conducted in the investigated frequency range 1 Hz - 4000 Hz. In the last step of the work the vibration data coming from the laser scanning vibrometer have been compared with the acoustic data previously determined through an intensity analysis in order to firstly identify the noise sources and secondly to better understand the nature of the noise, which represents an important information for future optimization.