Torsional vibration of powertrains: an investigation of some common assumptions

The recent introduction of stringent emission requirements together with economic pressure has led to a particular focus on increasing powertrain efficiency. This has seen the incorporation of real-time measurements to predict system behaviour and engine condition. Accurate models for all components are thus important. This work investigates the consequences of some common assumptions made in low frequency torsional powertrain models, and proposes improved models where appropriate. In particular, piston-to-cylinder friction, (crank/gudgeon) offset, and tyre torsional behaviour were studied. It was found that piston-to-cylinder friction can increase the inertia variation of an engine mechanism. Offset also modifies its nonlinear behaviour. For small (gudgeon) values these effects are minimal. However, for large (crank) values, the effects should not be ignored. The low frequency torsional damping properties of a small pneumatic tyre were found to be more accurately represented as hysteretic rather than viscous. Time Domain Receptance modelling was used to extend the results to a multicylinder engine powertrain. The findings should be of interest to both industry and academia alike.