An Integrated 3D CFD Simulation Methodology for the Optimization of the Mixture Preparation of 2-Stroke DI Engines 2007-32-0029

For the development of high-performance 2-stroke engines with internal mixture preparation it is essential to know about the interaction between charge motion and injection spray. With no prototypes available conceptual investigations can only render such information by using 3D CFD simulation. In this way an optimization of mixture preparation and charge motion can be achieved by varying the transfer and boost ports. To allow for the influence of these modifications on the mass balance (volumetric and trapping efficiency), the entire system of the loop-scavenged two-stroke engine has to be investigated. The state of the art calculation domain for 2-stroke 3D CFD simulation is bounded at the inlet of the crankcase (reed valve) and sometimes also at the outlet of the cylinders. The reasons lie in the so far not sufficiently reproducible components (e.g. reed valve) as well as in the reduction of calculation time. Beside the possibility of a coupled 1D and 3D simulation (SAE Paper No. 2005-32-0099 and SAE Paper No. 2006-32-0059), it is possible to apply a methodology with adaptive boundary conditions for the evaluation of the entire engine in order to overcome these restrictions.

This publication presents an integrative methodology for the simulation of two-stroke engines with adaptive boundary conditions. For the calculation domain, the boundary condition values are varied in the area of the intake by using the measured characteristic curves of the reed valves during the calculation process. Detailed measurements of the reed valves as well as 1D simulation of the entire engine using measurement data of already available SI variations of the engine serve as a basis. The combustion chamber, the crankcase, the transfer-, boost and exhaust ports as well as the exhaust system can be completely reproduced in the 3D calculation zone.

The optimization of the mixture preparation of a high-performance two-stroke engine is exemplarily demonstrated by this method. Particular consideration is given to the influence of the intake port geometry on the charge motion in the combustion chamber and therewith on the mixture formation. Finally the results of the 3D CFD simulation are contrasted with those of the experimental engine, and an outlook on the application of this calculation method is given.