Bond graph modeling and simulation of the thermodynamic and mechanical engine

Over the past few years, the improvement of engine performance has obtained considerable attention. Numerical models have been developed for potential guidance and optimization. However, the dynamic behavior of the integrated engine system is usually quite complex to analyze. The primary obstacle lies in the close coupling of different energy domains, especially the thermodynamic and the mechanical. Traditional modeling techniques ordinarily focus only on single-domain systems. As a rule, algebraic equations of all these energy domains must be enumerated to arrive at a possible solution with the necessary help of transitional variables. Unfortunately, such solutions usually encounter limitary difficulties due to the practical complexity of systematic analysis. The work presented in this paper comes up with a helpful modeling methodology on the basis of bond graph. Bond graph is considered to be ideally suitable for multiple energy domains, noticeably incorporating them in a unified modeling framework. Bond graph is consistent with the principle of energy conservation in nature. As viewed from direct energy conversion, three different forms of energy exchange are practically determined in the thermodynamic system, i.e. mechanical work, heat transfer and mass flow. Accordingly, the cylinder inside the engine is analogically modeled as a C-field with three power-flow ports. The C symbol is a storage element in bond graph notation, stores some kinds of free energy, just like a capacitor or spring. In succession, the model of mechanical system is also figured out for the crank and connecting rod mechanism, similarly but comparatively easier. Main energy effect is embodied by corresponding mechanical work, comprises back-and-forth movement and circular movement. Convenient connection of models, i.e. the thermodynamic and the mechanical, presents a systematic description of the whole working process. Based on the operating principle of bond graph, the normal state equations are expediently deduced for the engine under discussion. Finally, systematic dynamic simulation is conducted with the structure parameters of an actual diesel engine. Favorable results demonstrate the validity of the bond graph methodology.