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Achieve accurate clean engine design

Posted: 09 Nov 2012     Print Version  Bookmark and Share

Keywords:chemical processes  Computational Fluid Dynamics  CAD 

The mesh generation process begins with the definition of the rectangular computational domain. Three sets of planes orthogonal to the Cartesian coordinate system are then defined inside it. The intersection of these planes defines the set of rectangular cells (cuboids) that form the background Cartesian mesh. At the next stage of the process, specified refinement levels are honoured for areas near specific boundaries or geometric elements, such as spark plugs or valve seats, or for volumetric regions defined by spheres or "tubes" of interest. These refinement levels can be specified initially or can be imposed at some specified time windows during the simulation. The refinement is imposed, such that difference in refinement levels of neighbour cells is never higher than 1; this is necessary to minimise the approximation error. The refinement level of the cell is the number of times it has been refined relative to the initial cell in the background Cartesian mesh.

When a cell is refined, it will be evenly split into eight smaller cubes. Engine geometries inevitably contain small structures, such as the crevice between valve seats and the cylinder head. Such small fluid passages have to be resolved using very small cells, and it is impractical to use such small cells throughout the computational domain. This makes mesh refinement a necessary capability. In FORTÉ, mesh refinement can be applied on boundary surfaces or in specified volumetric regions.

FORTÉ does not rely on "adaptive" cell refinement to maintain accuracy of the combustion solution. Especially for combustion, where sudden large gradients in the solution occur, adaptive refinement can be highly error-prone, since the solution will shift as the refinement is initiated. Instead, FORTÉ relies on careful use of mesh-independent models to resolve flame-propagation and spray structures. However, it is important to note that these features relieve the mesh-generation process from constantly needing large numbers of small cells in regions that are not known a priori in a simulation. Instead, FORTÉ can rely on simple refinement around known fixed boundaries, or low-level global refinement for time periods of known interest.

Save time and effort
The FORTÉ CFD Package includes a visualiser that generates graphical representations of simulation results in a form engine designers need.

No sepa�rate tools are needed to gather, collate and view results. This visualiser provides quick creation of animations for any 3-D view, 3-D contour plots with easy control of both the cut-planes and the viewing angles; import of experimental data, automatic unit conversions, and quick comparisons of parameter-study results; automated creation of Phi-T maps to show where and when emissions are produced, including animations and last but not least export to third-party tools, including Field view, Insight, Tecplot and Excel.

Figure 2: Forté user interface and 3D visualisation example.

Recently new capabilities were added to FORTÉ: the ability to accurately simulate soot particle size as well as total particulate-matter emissions. The new technology was inspired by the accomplishments of the Model Fuels Consortium (MFC). Founded in 2005 by Reaction Design, the award-winning MFC is a highly regarded, industry-led project for the development of cleaner burning, higher-mileage vehicles. More than 20 members include Volkswagen, Peugeot (PSA) as well as US and Japanese automotive OEMs. The MFC's work focuses on creating software models and tools that enable engine designers to better determine the fuel effects on efficiency and emissions. When coupled with an accurate soot chemistry mechanism, such as the one developed by the MFC, the FORTÉ soot model provides engine designers with a physics-based approach to the prediction of soot size and mass emissions trends with unprecedented accuracy.

Soot formation occurs when fuels don't fully combust. While in the past decade, engine exhaust regulations focused on limiting the total amount of soot, and NOx emissions, future regulations will include particulate matter as well. The Euro 5+ and Euro 6 standards and proposed U.S. Tier 3+ and Tier 4 regulations limit both mass and number of particles. Various medical studies have shown that soot is unhealthy, especially particulate sizes of less than 2.5 microns—the human body cannot eliminate such small particles. Designing an engine that minimises the amount of soot produced is much more cost-effective than after treatment

The MFC contracted with the University of Southern California to obtain needed fundamental measurements that were used to validate the soot model parameters. The model accurately predicts engine-out trends, including total mass emissions and particle size distribution. This allows engine designers to compare disparate engine designs or combustion schemes relative to one another. The MFC and Reaction Design have been working proactively to develop soot modelling approaches that can help reduce these harmful emissions, as we anticipate that more regulations will follow with increasingly strict limits on particle size and number. The ability to predict soot particle sizes and track their progress from formation through agglomeration and oxidation in an engine is of significant benefit for engine developers, who will not only be able to better address air quality regulations, but also help shave time from their design cycle.

About the author
Bernie Rosenthal is CEO of Reaction Design.

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