Determining the rigid-body inertia properties of cumbersome systems: comparison of techniques in time and frequency domain

The inertia properties of an engine to be mounted on complex mechanical systems (e.g., cars, boats, etc.) are usually evaluated by considering the engine like a rigid-body (RB). These properties are mainly employed to design the geometry and the stiffness of the engine support, so as to build the dynamic model of the mechanical system the engine has to be mounted on. In this paper, two different techniques for measuring the RB inertia properties are compared: one in the time domain and the other in the frequency domain. The time-domain technique is based on the measurement of the oscillation period of a trifilar pendulum. In this context an ad hoc calibration procedure has been implemented in order to improve the quality of the results. On the other hand, the frequency-domain technique is based on the measurement of the Frequency Response Function (FRF) of the softly suspended RB. These techniques are applied to estimate the RB inertia properties both of an engine block and of a marine diesel engine. Eventually, their pros and cons are highlighted.