Acoustic Analysis of Unsteady Diesel Engine Combustion Chamber Pressure Evolution

operating conditions. Using spectral and timefrequency analyses, it is shown that stochastic fluctuations dominate the periodic part of the pressure-time trace in a range of frequencies bounded by about a few kHz. Based on this observation, a decomposition of the pressure as a sum of (i) a periodic uniform slow time varying component and (ii) a stochastic non-uniform fast time varying fluctuating component is proposed to describe the evolution of the pressure field inside the combustion chamber. Chamber volume evolution, mean periodic heat release variations and wall heat losses are mainly responsible for the low frequency content of the pressure-time trace. However, above a certain frequency value, pressure fluctuations are no longer uniform throughout the chamber. It is shown that these high frequency pressure perturbations are produced by fluctuations of the heat release. During an engine cycle and from cycle to cycle, maxima of acoustic energy density are not distributed uniformly over the frequency range, but appear at discrete frequencies corresponding to the first transverse eigenmodes of the chamber. In particular, these maxima are not always located at the same discrete frequencies and do not reach the same level depending on the cycle under consideration. Clearly, the high frequency content of the in-cylinder pressure power spectrum is dominated by high density levels around frequencies corresponding to cavity resonances, which are then randomly excited by turbulent combustion fluctuations, in agreement with theoretical findings. Giving insight into the dierent mechanisms controlling the in-cylinder pressure evolution over many cycles, this article also emphasizes the need for development of new simulation tools taking into account stochastic fluctuations of turbulent combustion.