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THE MAIN AIMS of the experiments reported in this paper are to obtain a quantitative comparison between the noise ranking of engine components using intensity and lead wrapping techniques, and to analyze the causes of the sudden increase in noise level observed during the initial part of the acceleration process. The results indicate that the surface intensity technique agrees well with the lead wrapping sound power results and, although the surface intensity technique in its present state of development, is still time consuming, it does also produce information about the vibration levels of the surface which is essential when the component has to be redesigned to reduce noise. The acceleration test results indicate that the increased, sound pressure level is directly related to changes in the combustion process while turbocharger lag appears to have negligible effect.

This paper reviews some previous findings about the combustion noise of diesel engines and compares them with the authors' findings. The reason why the noise of most turbocharged diesel engines is more sensitive to speed than load is explained. The findings of this research suggest that the frequency content of the combustion gas pressure up to about 300 Hz is related to the maximum cylinder pressure, Ṗmax. Between about 300 Hz and 2 kHz it is related to the maximum rate of cylinder pressure rise Ṗmax and above about 2 kHz it is related to both the magnitude and the duration of the second derivative of the cylinder pressure P̈. Pmax is more sensitive to load than speed. No correlation was found between Ṗmax and engine speed or load. The variation in the magnitude of P̈ is thought to be a random process, but the frequencies of the cylinder pressure fluctuations are closely related to the cavity resonance frequencies.

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.

Structure borne sound is one of the most important reasons of noise pollution in the automobiles and aircraft's. Noise is mostly generated by the vibrating panels excited by either a mechanical or an acoustical excitation. Examples of the typical vibrating structures in automobiles are engine cylinder, gearbox cover, transmission system covers, panels of the body etc. Sound radiation characteristics are also important in the phenomenon of resonant sound transmission through a panel. Resonant sound transmission occurs because of resonant modes of the panel within the frequency bandwidth of interest. Typical example of resonant sound transmission is the transmission through a firewall of an automobile, which forms the partition between the engine compartment and the cabin interior. Radiation characteristics can be typically defined by radiated sound power, radiation efficiency and space average mean square velocity of the panel.