In order to identify noise sources in a diesel engine, specifically exciting forces such as combustion and piston slap, the so-called coherence method which utilizes relationships between the auto and cross spectra of cylinder pressure, cylinder liner acceleration and engine noise has been examined. Also, as an alternative, a multivariable regression analysis in one-third octave band auto spectra of each signal mentioned above has been made.
It was shown that the simple coherence model studied does not seem useful for this particular type of noise source identification problem. However, the multivariable regression approach has yielded fairly reasonable results, though some problems have been found in accuracy.
From this research, it was found that combustion noise is predominant for heavier engine load conditions, though at lower load and high speed conditions, piston slap noise becomes appreciable.