The present work is framed in the field of maintenance engineering. Its primary aim is the application of two statistical techniques and the evaluation of their effectiveness on monitoring performance and health condition of complex equipment of a cogeneration plant. More specifically, the case study deals with one internal combustion diesel engine. The first part of the thesis is dedicated to data processing and to the creation of a reference model for the internal combustion engine, on the basis of the available operational data. The data to investigate are concerning signals whose acquisition is achieved through the continuous service of implemented systems of measurement.