THE TORSIONAL EXCITATION OF VARIABLE INERTIA EFFECTS IN A RECIPROCATING ENGINE

Reciprocating machinery has a cyclically varying inertia function and as a consequence exhibits non-linear frequency coupling between rotational speed and the systemʹs torsional natural frequencies. Although in theory the type of frequency coupling which takes place has been well explored, it has been difficult to investigate the effect experimentally and hence to verify the theory. It is the purpose of this paper to present the findings of a recent experimental investigation, which uses an a.c. servo motor to excite torsionally variable inertia effects in a single cylinder engine. The measurements confirm the theory. As anticipated, frequency coupling was observed experimentally as modulation of torsional resonant frequencies by the rotating frequencies and their harmonics. This effect was reciprocal, with excitation of natural frequencies causing secondary resonance sidebands, and excitation at secondary resonance sidebands generating a significant resonant response.