A Multidimensional Interface for the Predictive CFD Simulation of the 2-Stroke Engine 2006-32-0059

The two stroke engine has a wide range of application, especially in the field of recreational vehicles, handheld products and small two-wheelers. This is due to the advantages of the two stroke working principle: high power density, low weight, and low costs. In order to reduce the system-inherent disadvantages of the loop-scavenged two stroke engine developments using latest methods are necessary. One of these methods is the CFD simulation of the scavenging process, the high pressure cycle and the injection process. Reliable predictive simulation in the early development phase of a new engine is required to shorten the development time and to reduce prototype and test bench costs.

In previous investigations (1) [SAE 2005-32-0099; JSAE 20056552] the strategies for the simulation and the requirements for a predictive simulation were discussed. Finally a new methodology which bases on the combination of 3-dimensional (3D) and 0/1-dimensional (0/1D) CFD simulation was presented. For this methodology a new coupling interface is required.

In the current paper this newly developed interface and the implementation of the interface in a commercially available CFD-code is presented. The special feature of this coupling is the capability of being placed on any position in the 3D CFD mesh. Positioned in the opening port area between cylinder and exhaust port for instance, the coupling allows differing flow areas on both sides even with a non-conformal grid. The special treatment of the non-conformal respectively only partly overlapping flow cells also improves the convergence for the critical process of the exhaust port opening.

The multidimensional coupling interface is able to handle both 3D/3D and 3D/1D or 3D/0D connections. The 3D/1D and 3D/0D connections allow the replacement of regions with typically high numbers of cells by fast-calculating 0/1D models. Thus it is possible to replace the 3D mesh of the exhaust by a 1D exhaust model, respectively to connect the 3D code with 0D models for the crankcase and/or the reed-valve (2, 3). With this method a reduction of 3D cells and therefore computational time can be achieved. Therefore, the coupling interface is the basis for the creation of a tool box with “exchangeable parts”. This allows adjusting the model in the development process according to the required flow details, accuracy and available simulation time.

This paper includes the detailed discussion of the coupling interface and its validation based on test cases. Also the models which are necessary for the tool box are presented. The impact on the “every day” development work and an outlook on further steps conclude the paper.