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The engine is an important source of diesel powered vehicle noise, and becomes dominant after proper treatment of exhaust and cooling system noise at vehicle speeds below fifty miles per hour. This paper presents the results of a comprehensive study dealing with surface noise radiation characteristics and the ranking of sources on three truck diesel engines. A special acoustic facility constructed to emphasize the acquisition of diesel engine noise data is describad. Information is presented on measurement methodology and its viability for quantification of engine noise, directivity, and radiation patterns from engine surfaces. Acoustic measurements, conducted over a range of engine loads and speeds, include spectral analysis of the sound pressure field derived from a thirtytwo microphone spherical array. Various noise identification techniques are explored under comparative testing to ascertain potential candidates for quantification and ranking of engine external surface radiation.