Investigation of reciprocating internal combustion engine structure response and vibration transmission using a hydraulic simulation technique

Abstract

The success of theoretical engine noise prediction, using such techniques as finite element modelling, is highly dependent upon the understanding of the various mechanisms of noise generation in the running engine. The forcing mechanism of an operating engine is complicated as it involves a large number of forces taking place simultaneously or in rapid succession at various positions in the structure. A number of test methods have been developed to simulate individually each one of these loading mechanisms on non-running engines. These test methods are reviewed in this thesis with an objective judgement on their representation of the actual forcing on the running engine. A new hydraulic test method has been successfully developed to simulate various forms of loading mechanisms on the non-running engine like the gas force on top of the piston, main bearings axial and vertical loading, and piston slap force. This test method is shown to be more comprehensive, realistic, practical and representative than previous simulation techniques, especially with respect to the level of forcing which corresponds well with that of the running engine. The structural sensitivity of the engine has been evaluated for different representative loading, the most sensitive input being that of the axial force at the main bearings. The damping characteristics of a large six cylinder diesel engine block has been calculated and the crank shaft is shown to affect it as well as affecting the wave propagation through the structure. The simulation technique has also allowed the detailed study of the various forms of wave propagation in a diesel engine load carrying structure and this has shown the importance of both bending and longitudinal travelling waves in noise radiation. It is shown that bending waves provide the maximum amplitudes of vibration, whereas longitudinal wave propagation allows for the fast transfer of energy through the structure which can then convert into bending waves with high noise radiation potential. Analysis of the results has shown that some doubt must be placed on the normal mode method for predicting the response in the lower frequency range, and an alternative model based on highly damped travelling bending waves is visualized to be suitable to model the crank case wall but not the stiff upper.

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