CFD Analysis of 2-Stroke Engines

Abstract1d and 3d CFD simulation is a standard engineering practice on 4-stroke engines. However, the application to the 2-Stroke cycle is not so trivial, in particular the transition from detailed CFD-3d analyses to simplified 1d engine simulations. A first critical issue is the correlation between the instantaneous cylinder gas composition and the composition of the exhaust flow: such a correlation, required by the 1d software, is very difficult to be determined by experiments, so that CFD-3D analyses remain the only reliable source of information. These CFD analyses must be carefully designed, in order to be representative of the actual operating conditions of the engine, and to provide reliable results.Another problem is the characterization of the ports discharge properties. Conventional experiments at a steady flow bench are affected by several approximations: first of all, flow patterns within the cylinder are quite different from the actual ones; moreover, the real flow is fully transient.Finally, a 3-d analysis on a single whole cycle is not sufficient to accurately predict the evolution of the thermodynamic quantities, as the conditions of the charge at exhaust port opening changes from cycle to cycle, affecting all the internal processes. A robust and cost-effective multi-cycle simulation methodology is therefore required.The paper describes a methodology to perform reliable CFD analyses on 2-Stroke engines, with the support of a conventional steady flow bench. This methodology is applied to a 2-Stroke engine prototype, for which a comprehensive set of experimental data is available.The good agreement between simulation and experiments demonstrates the soundness of the proposed approach