Critical analysis of analogous mechanical models used to describe hydraulic engine mounts

Passive hydraulic engine mounts differ from mechanical isolators in several ways. Complex poles and zeros suggest that the hydraulic mount has at least one mass (inertia) element even at low frequencies. This would suggest thus that the constraint forces at its input and output ends are not identical; but they are indeed, like any conventional spring. Accordingly, analogous mechanical models of hydraulic mounts, consisting of spring, dashpot and mass elements, could lead to incorrect results in the context of system analysis. Examples clarify the competing model paradigms and their interpretation.