An Investigation of the Damping Influences in Engine Torsional Oscillation

This paper describes an investigation of the damping influences occurring in multi-crank engines when in a state of torsional oscillation. The major part of the experimental work is carried out on a four-cylinder petrol engine. The crankshaft is of the four-throw-two-bearing type and Its torsional flexibility is examined analytically for the condition of no constraint and complete constraint at the bearings. Torsion experiments are carried out with the crankshaft in place, and the results are in close agreement with analysis. In the vibration experiments a Geiger torsiograph is used and resonance conditions are examined. Extreme methods of lubrication are adopted and the outstanding influence of lubrication in the control of amplitude at resonance is demonstrated. For the engine dealt with the principal source of damping is traced to the main bearing. The dissipation of energy in an oil film, due to journal vibration is examined analytically and expressed in terms of journal displacement. From an investigation of the journal displacements caused by crankshaft oscillation, it is found that practically all the input energy of vibration could be accounted for in this manner, thus confirming the experimental observations. The results are finally reduced to an overall non-dimensional factor which is practically constant for the range of frequencies Investigated. Results from a number of multi-crank engines on which further experiments had been carried out were available and these, together with other published data, are also reduced to this simple non-dimensional form. The value of the factor is not constant for the different engine systems, but appears to be correlated to the elastic curve form at the engine, and an average curve taken on this basis is presented as a guide in design