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The two main noise sources of an engine are the combustion process and mechanical impacts. Of the latter, generally the most important is piston slap, which is produced by impacts between the piston and cylinder wall. These impacts cause the engine outer surfaces to vibrate and consequently radiate noise. In this paper the time history of a piston slap impact force model was developed. This model takes into account the influence of the oil film on the impact behaviour, which was found to be an important factor. Test rigs were built to study these effects on the impact phenomenon and extensive tests were carried out.

An assessment model has been developed to determine the piston slap impact forces. This model uses Reynolds' theory for fluid film squeezing to obtain the time history of this impact force. The oil is treated as a damper and the air bubbles entrapped into the oil as a spring. An error of up to one order is found when the stiffness due to the entrapped air is not considered. In this case, the Reynolds theory over predicts the impact forces, compared with experimentally obtained impact forces. The results of this development are used to determine the piston slap in a motored internal combustion engine.

A model for piston slap force level prediction is being developed. The development project is scheduled to be carried out in three distinct steps, which are discussed here. Step one and part of two have already shown important features of the role of the oil film on the impact behavior. The Reynolds theory has been applied and a preliminary prediction model proposed. The piston-cylinder, wall-engine block structure is treated as 7 degree freedom system.