Systematic Model Decoupling Through Assessment of Power-Conserving Constraints: An Engine Dynamics Case Study

Simplified models for predicting engine mount forces have traditionally been developed based on the assumption that for a well-balanced low-speed engine, the reciprocating dynamics can be decoupled from the three-dimensional motion of the engine block. In this paper the simplification is done systematically, using a technique previously developed by the authors to search for decoupling within a model, and to partition models in which decoupling is found. Beginning with a fully-coupled bond graph model of a balanced in-line six-cylinder engine, bonds representing negligible constraint terms are found based on aggregate power flow, and are converted to modulated sources. Separate bond graphs joined by modulating signals result. The “driving” bond graph represents the reciprocating dynamics, and the “driven” bond graph represents motion of the block on its mounts. The partitions are smaller than the original model and are simulated individually to accurately predict the dominant third-order mount forces with significant computational savings. The decoupling is found without the modeler relying on traditional assumed forms of the one-way coupled model, and can be quantitatively tracked as the system parameters and inputs change.