1975-02-01

Affecting Diesel Engine Noise by the Piston 750799

The ever increasing motorization is entailing, apart from all its positive effects on our way of life, also some aftermaths detrimental to mankind. Air pollution and noises have become serious problems. More and more people are getting environment-minded and strongly expect that this annoyance be checked, at least kept in tolerable limits. Experts in all industrialized countries are engaged in controlling disturbing noises and there can be no doubt that internal combustion engines, automotive Diesel engines in particular, are producing no small share of this kind of nuisance. Intensive attention is therefore being paid to the silencing of noises lately.
As far as mechanical engine noise is concerned the piston is a contributing element due to is side motion in the cylinder. The impulses caused by the slapping piston lead to cylinder wall acceleration and produce bone-conducted noise which travels axially and radially through the cylinder wall until it reaches the surface of the engine, from where it is radiated as airborne noise.
Since it is not possible to measure piston noises at their origin this paper deals with the methods of affecting the piston movements and the impact of the piston, as it hits the cylinder wall, by means of measurements with pistons whose pins are not offset i.e. on which the axis of piston and pin coincide. Moreover measurements are described with positively and negatively offset pins, whereby the pin axis is slightly displaced with respect to the piston axis.
After describing various designs of pistons with controlled thermal expansion and of the articulated piston, which allows to reduce the fitting clearance as compared with one-piece full-skirt pistons, this paper reports on noise measurements with such pistons and on the influence of skirt coatings on piston noise.

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