Study of Structural Noise of a Multi-Cylinder Diesel Engine

The world over has resulted in severe pollution problems.They are classified as air and noise pollution.Air pollution is caused by to dispersion of emittents from engine exhaust to the atmosphere at different concentration levels. Similarly,the emission of unwanted sound from engine structure, intake and exhaust is the principal source of noise pollution. In diesel engines structurally, radiated noises have numerous origins.The complexity arises from the fact that the whole engine structure is simultaneously excited by several forces of widely different characteristics.Primary exciting force which is a gas force in the cylinder resulting from the combustion.Secondary exciting forces of considerably different characteristics are generated by the operation slider crank mechanism but related to some primary gas force in some non-linear manner resulting in piston impact, impacts in bearing,impacts in timing gears etc.Force produced in accessories such as valve gear system, fuel injection system etc. In the present study, the structure-born noises are studied by isolating exhaust noise from measurement areas at different moderate loads and speed conditions to see the impact of noise generation of a heavy-duty diesel engine on the test bed.The engine considered was an 80KW multi-cylinder diesel engine on a test bed coupled with a hydraulic dynamometer. The engine was manufactured by Tata Motors Limited. The noise was measured at a distance of 1m and 2m at 10 different locations at different load and speed conditions.With this noise contours are plotted to see the behavior of noise emission from engine structure with variation of load and speed. The piston slap phenomenon was also evaluated in the study. The overall frequency vs sound pressure level (SPL) graphs were plotted and it was found that two peaks were observed in the spectrum. One peak at low frequency of 250Hz at low engine speed and another peak at 1000Hz to 2000Hz.As the speed of the engine increases, the low-frequency peaks are reduced and mid-frequency peaks increased which is the indication of rough combustion at low frequency at low engine speed and as speed increases, the combustion becomes smooth. As the speed increases the structure-induced vibration increases in mid frequency range. It is observed from the resultant engine noise, spectra remain nearly unaltered at the different speed and load conditions. It is also observed that the rate of increase of noise is 40dB(A)/tenfold increase of speed.