A Quasi-3D Model for the Simulation of the Unsteady Flows in I.C. Engine Pipe Systems

Increasing demands on the capabilities of engine simulation and the ability to accurately predict both performance and acoustics has lead to the development of several numerical tools to help engine manufacturers during the prototyping stage. The aid of CFD tools (3D and 1D) can remarkably reduce the duration and the costs of this stage. The need of achieving good accuracy, along with acceptable computational runtime, has given the spur to the development of a geometry based quasi-3D approach. This is designed to model the acoustics and the fluid dynamics of both intake and exhaust system components used in internal combustion engines. Models of components are built using a network of quasi-3D cells based primarily on the geometry of the system. The solution procedure is based on an explicitly time marching staggered grid approach making use of a flux limiter to prevent numerical instabilities. The equations of mass and energy are solved at cell centers whereas the momentum equation at cell connections (ports). The quasi-3D approach has been fully integrated into a 1D research code in order to study the behavior of complex shape devices under real engine pulsating flow conditions. To this purouse, specific submodels have been developed in order to model elements such as air filters and catalysts, which are commonly integrated inside airboxes or silencers. The numerical method was firstly validated resorting to the shock tube test case, in order to evaluate the stability of the method when sharp gradients need to be resolved. A second step focused on the study of the acoustic perfomance of a cylindrical expansion chamber with the aim of investigating the dissipation introduced by the flux limiter and the importance of the shape reconstruction for capturing higher order modes. The final validation was carried out on a single-cylinder engine for motorbike application, where the components exhibiting a high degree of complexity, namely the airbox and the silencer, were modeled by means of the quasi-3D. following a pure geometrical shape recontruction criterion. Measured data of volumetric efficiency have been compared to calculated data, highlighting a good the capability in capturing dynamic effects with computational runtime much lower than the one required by the integration of fully 3D models with the 1D.