Advanced engine dynamics using MBS: Application to twin-cylinder boxer engines

Engine simulation is an important issue to design mechanical components and to reduce the development cost of new vehicles. Dynamic simulations of a twin-cylinder boxer engine are carried out with rigid and flexible multibody models using the finite element approach. In a first step, simulations of the engine running at constant revolution speed (4000 rpm) are done taking advantages of the actual geometry of engine parts coming from CAD models. Forces due to the gas pressure are added in the simulation to calculate more precisely the load applied on each engine part. A mixed model is developed; pistons and connecting rods are considered as rigid bodies while the crankshaft is meshed with flexible brick elements. With this model, calculation of crankshaft stresses and displacements is possible. Then, simulations are made with engine speed variations to evaluate the effect of inertia. Due to the complex shape of the crankshaft, the CPU time and computer resources can be important. As simulations with a variable rotation speed have to be carried out on a larger number of engine revolutions, it becomes difficult to work with the fully detailed crankshaft geometry. So, simplified models (a tridimensional model with simplified geometry and a beam model) of the crankshaft are developed and compared. These models are first validated with different tests including results of constant speed flexible simulations.