The paper describes an integrated methodology for the accurate characterization of the thermal behavior of internal combustion engines, with particular reference to a high performance direct injected SI engine for sport car applications. The engine is operated at full load and maximum power revving speed, which is known to be critical from the point of view of thermal stresses on the engine components.
In particular, two different sets of 3D-CFD calculations are adopted: on one side, full-cycle in-cylinder analyses are carried out to estimate the point wise thermal heat flux due to combustion on the engine components facing the combustion chamber. On the other side, full-engine multi-region CHT calculations covering the engine coolant jacket and the surrounding metal components are used to compute the point wise temperature distribution within the engine head, liner and block.
An iterative procedure is then implemented in order to exchange relevant thermal data between the two modeling frameworks: at first, local gas temperatures and heat transfer coefficients from the in-cylinder simulations are applied as thermal loads to the CHT calculations; then, the resulting surface temperature maps are re-applied to the combustion chamber walls for the subsequent in-cylinder CFD analysis. The procedure is iteratively repeated until convergence is met in terms of thermal characterization of the engine.
The procedure takes into account the non-uniform spatial distribution of both thermal loads from the combustion process and resulting temperature distribution on the combustion chamber walls, which are known to be particularly evident when considering direct injected stratified charged engines. The resulting thermal fields are compared to those deriving from a traditional approach for both the in-cylinder and CHT calculations, i.e. the application of uniform heat fluxes on the CHT side and the use of uniform wall temperatures on the in-cylinder side. The resulting differences are highlighted and critically discussed, with particular emphasis on those which could have a relevant impact on both the fatigue strength evaluation of the engine and the arising of local undesired phenomena such as surface ignition and knocking.