Investigations of the Vibroacoustic Signals Sensitivity to Mechanical Defects not Recognised by the OBD System in Diesel Engines

Operations of the on board diagnostic (OBD) system, is directed to ensuring the correct engine operation in respect of toxicity exhaust gases. Thus, when the parameters analysed by this system are within the permissible values, the system controls them in such a way as to obtain the optimal working conditions from the point of view of the exhaust gases toxicity. The user can be completely unaware of any problem occurring until exceeding of the limiting values causes the MIL lamp to switch on or the system transfers into the failure mode operation or even engine shut-down. The previous investigations indicated explicitly that there is such group of defects (mainly mechanical), at which the OBD system performing self-regulation does not provide the user with the information concerning a defect and, in addition, weakens typical external symptoms such as e.g. power drop. Current works of the authors [1] performed for petrol engines indicated that there is representation between VA signal symptoms and mechanical defects, which at their initial phases are not detected by the OBD system. Thus, the authors decided to verify the sensitivity to VA signals of Diesel Engines.