The identification of the inertial characteristics of an internal combustion engine through steady state vibration measurements

Recent product design and development efforts have been focused on improving engine-mounting technology to achieve better vibration isolation, smooth vehicle movement, and noise reduction. Understanding the origins and parametric dependencies of the vehicle engine noise and vibration is the first step in any abatement or reduction strategy.; The decoupling and coupling of engine modes from the engine's reciprocating inertia forces and all external excitations, such as road input, requires accurate understanding of the engine inertial characteristics, such as the center of gravity location and mass moments of inertia. The precise identification of these properties becomes more critical when the engine mounting system is coupled with the suspension and chassis systems. These inertial characteristics play a crucial part in determining the system modal map, and the appropriate locations and stiffness characteristics of the engine mounts.; This work documents an experimental and analytical investigation in identifying the inertial properties of an internal combustion (IC) automotive engine such as mass moments of inertia and the location of the engine center of gravity, based on the steady-state vibration measurements.; The study was conducted on a four-stroke, four-cylinder engine. The engine was idealized as a rigid mass attached to a rigid foundation (engine cart) by viscoelastic springs. The engine was allowed six degrees of freedom, these being translational along, and rotational about three mutually perpendicular axes. The unbalanced second order reciprocating inertia forces and inertia rolling moment were considered as the excitation functions.; Based on this model the six differential equations were transformed to obtain a system of six algebraic equations for the inertial characteristics as a function of the response amplitudes. Actual engine measurements were used to obtain the amplitudes of the steady state response. The decoupling of these amplitudes required the knowledge of two coordinates of the center of gravity. The problem was simplified by initially obtaining values for two unknowns (the location of center of gravity in the axial and lateral directions), by placing the engine in a stable position on three scales (load cells). The vertical coordinate of the center of gravity was acquired by coupling the equations of motion with the equations of the operating deflection shapes. These equations describe the motion of the engine supported on three viscoelastic springs subjected to the engine's inertial reciprocating forces and moments. The mass moments of inertia were obtained by substituting the decoupled experimental vibration amplitudes into the rigid body equations.; This newly developed method yields a solution domain for the coordinates of the center of gravity and mass moments of inertia of the engine. The results account for the continuous variation of the engine geometry with the crank angle. This new method could be applied to any engine, including large engines. It avoids the use of complicated structures required by the pendulum method or the sensitive free-free suspension of powerplant necessary for the modal method.