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EGR diesel engines are used in an underwater environment, or in terrestrial applications that demand low exhaust emissions. In the underwater mode the intake mixture may contain up to 30% CO2 whereas with land-based EGR diesels the percentage will be much lower. In both applications noise is an important emission parameter for not only is it a pollutant but in the underwater environment a primary means of detection. Thus, in the research reported here the combustion noise levels and spectra have been measured for a diesel engine using a variety of precisely proscribed intake mixtures containing levels of CO2. It has been found that the presence of CO2 alters both the sound level and frequency spectra of the combustion noise and that in general, although not in all circumstances, the sound pressure levels are increased.