The demand for quieter vehicles is increasing with not only the advent of stringent Noise, Vibration and Harshness (NVH) norms but also because of customer expectations. In motorcycles, tactile point vibration - handle bar, rider footrest, passenger footrest, and pillion handle - is a metric that characterizes the vibrational behavior. As engine is the most predominant source of vibration and noise, it is crucial to understand the dynamics of the engine and the transfer of engine forces from the engine to the structure supporting it.
This paper discusses a method of modeling the engine to predict mount forces, which are correlated with the experimentally measured data. The engine under study is a four-stroke single cylinder engine. Multi-body dynamics approach is used to predict the crank bearing loads and mount forces. A fully coupled engine model is built to capture the interaction between the piston-crank mechanism and the engine casing. The effect of modeling the crank bearing stiffness on the bearing loads is investigated and compared against those obtained using a simplistic assumption of a rigid crankshaft mounted on rigid crank bearings. The effect of engine casing flexibility on the bearing loads and mount forces is also investigated.
An engine-mounting fixture with multi-axis load cells has been built to measure the mount forces in a dynamometer test cell. Typically, accelerations are measured on the engine and the mount stiffnesses are required to calculate the forces transmitted to the supporting structure. In this particular exercise, forces transmitted from engine to supporting structure are directly measured using the load cells. These measured engine mount forces are in reasonable agreement with the analytical predictions. The influence of the flexibility of the engine casing and flexibility of the mounts on the frame side (engine isolation) is also investigated for a range of engine RPM.