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. For the engines studied, the sound power output varies, depending on design; the loudest being the naturally aspirated; and the quietest being of prechamber design. This range varies between 8 and 10 dB (A) at maximum throttle and is speed dependent. Turbocharged engines exhibit a marked change in sound power output under increased load conditions. This is most pronounced at lower engine speeds. The sound pressure field of two in-line turbocharged four-stroke diesel engines contained a high degree of asymmetry whereas the sound pressure field of a two stroke vee configuration engine appeared representative of a finite monopole acoustic radiating source. Major external engine surface acoustic radiators, common to all engines tested, included engine front, block sides, and valve covers. Other major external sources were after-coolers and exhaust manifolds for specific engine designs. Oil pan noise, although a lesser contributor to overall engine noise, depends on factors which include the design, material and construction. Finally, comparisons are made of noise data acquired for the acoustic facility and data acquired for the same engine installed in a typical highway truck chassis located out of doors.