In the present study an innovative diagnostic system of the combustion process, developed especially for cogenerative reciprocating engines, is introduced. This work is part of a more wide research activity, dedicated to the development of diagnostic systems for energy plants. This system is based on the evaluation of the energy content of the vibration signal directly acquired on the cylinders head through the Discrete Wavelet Transform technique and the Parseval's theorem. The system development and its test took place alongside a consistent maintenance work that has allowed to distinguish between different engine operative functioning, and, in particular, between good and bad conditions of the combustion chambers. Then, starting from the obtained results, a diagnostic system has been developed, basing on the acquired vibration signals and the operative engine load, in order to formulate for each cylinder a judgment about the quality of the combustion process. (C) 2011 Elsevier Ltd. All rights reserved.