Diesel engine-radiated noise as function of engine type, intake and exhaust restriction, and operating load-speed schedule

Abstract

The diesel engine itself is an important source of diesel-powered vehicle noise, and becomes dominant after proper treatment of intake/exhaust and cooling system noise at vehicle speeds below 50 miles per hour. An investigation was conducted to quantify the effects of intake and exhaust restrictions, and load-speed scheduling on the radiated noise from four diesel engines, i.e., four-cycle naturally aspirated, four-cycle turbocharged w/wo precombustion chamber, and two-cycle blower scavenged. All engines were inline six-cylinder and rated approximately 250 hp at 2100 rpm. Each engine was asymmetrically positioned in a reverberant chamber, and sound pressure measurements were obtained 3 ft from the engine. The noise associated with intake, exhaust, cooling, and their respective ducting systems was appropriately abated to permit quantification of engine-radiated noise. Dampers were placed in the intake and exhaust ducting systems for pressure regulation, and engine loads were applied with an electric dynamometer. Exhaust emission data including temperature were also monitored. The engine-radiated noise was not significantly affected by intake pressure restrictions up to 60 in H2O or exhaust restrictions up to 90 in. H2O, except with naturally aspirated engines. The precombustion chambered engine exhibited low sound output with respect to the mechanical power available under various load-speed conditions. The engine noise efficiency, defined as engine-radiated sound power/engine brake horsepower, has a minimum value for air-to-fuel ratios in the range 15:1 to 19:1, and its dependence on engine speed is most pronounced for turbocharged engines. The latter finding suggests the presence of noncombustion or mechanical-combustion related noise sources in turbocharged engines. Engine sound pressure level calculations, projected to 50 ft, were made from the engine noise data with the resulting A-weighted noise levels ranging from 74 to 85 dBA at rated engine speed.