A new approach to diagnostics of the combustion process in diesel engines using vibration measurements. Part 1 : Reconstruction of cylinder pressure from vibration measurements

There is a steadily growing demand for reliable, non-invasive measurement methods which can be used to monitor combustion in diesel engines. An effective, non-invasive method would make it possible for those using diesel engines to economically detect malfunctions during combustion. The main objective of this paper is to show how, through reconstruction, it is possible to generate data on combustion parameters, which can then be used for engine diagnostics. The combustion parameters are the maximum cylinder pressure (pmax, ) and the derivative of the pressure rise (dp/da). The idea is based on reconstruction by using the transfer function, TF, from the combustion chamber to the engine surface and the measured vibration response signal of the engine surface. The analysis is based on a non-linear method called complex cepstrum and signal processing techniques. The TFs were modified to fit slightly different situations such as other cylinders of the same engine; where use can be made of symmetry. A new approach based on a new tailor-made window for reconstruction of the cylinder pressure is also presented. A matrix with engine TFs for varying speed and load was obtained. The matrix can be used as a data bank of TFs for reconstruction of the cylinder pressure at different operating conditions. An extensive analysis of the cycle-to-cycle variations was carried out, both for the measured and reconstructed cylinder pressure. The main parameters of the cylinder pressure; the maximum cylinder pressure (pmax, ) and the derivative of the pressure rise (dp/da), are thoroughly examined in order to validate the procedure.