The paper presents the modelling the engine operation with a variable state of compression with the use of the Reverse Engineering method and simulations of the phenomena with the use of the Ansys Fluent software package. The analysis and selection of appropriate combustion parameters directly affects the degree of fuel consumption in petrol engines. Less fuel consumption means not only lower operating costs but also less environmental pollution. Reverse engineering, Computational fluid dynamics CFD, Variable compression ratio VCR Proceedings ITTAP'2023: 3rd International Workshop on Information Technologies: Theoretical and Applied Problems, November 22-24, Proceedings
speed appears rational. given by the formula: where ...
ηt – theoretical efficiency, ε – compression ratio, k – polytropic exponent.
Bearing in mind the current need to care for the natural environment [
Equation (1) shows that increasing the compression ratio of the engine will increase its efficiency.
Increasing the efficiency of the engine will reduce fuel consumption with the same dynamic parameters
of the engine or increase power and torque for the same amount of fuel supplied to the engine. Hence,
for many years, work has been carried out on constructions enabling dynamic change of the
compression ratio depending on the engine speed [
One of the solutions is the design of the SVC (Saab Variable Compression) engine [
2020 Copyright for this paper by its authors. CEUR
ceur-ws.org engine speed (Fig.1). For low engine speeds, the head is positioned vertically to the piston axis (Fig.1a). In this position, the minimum volume of the combustion chamber is obtained. At the same time, this results in a maximum compression ratio. As the engine speed increases, the head tilts away from the piston axis (Fig.1b). Deviation of the cylinder head de-symmetrizes the combustion chamber, increasing its volume. Increasing the volume of the combustion chamber while maintaining the same cubic capacity results in lowering the compression ratio. The appropriate mechanism is responsible for the appropriate adjustment of the tilt angle of the head. When tilting the head, the position of the crankshaft remains unchanged
A Variable Compression-Turbocharged (VCT) system was developed by a Japanese manufacturer
in cooperation with Nissan. This system was tested on a turbo-charged engine with displacement of
under 2000 cm3. This system allows for a smooth change in compression ratio in the spectrum ranging
from 14:1 to 8:1 [
.
A the Faculty of Mechanical Engineering and Aviation of Rzeszów University of Technology, a
variable compression ratio engine was constructed on the basis of a conventional 4 VD 14,5/12-1 SRW
type engine [
The presented solutions require significant interference with the construction of the engine. This is
a costly solution, as it requires appropriate adaptation of motor production lines. Therefore, in the patent
[
The subject of this work is the solution described in patent application No. P.414750 of 10-11-2015
[
Zawór upustowy Kanał upustowy
Due to the need to enter the valve opening times and opening start times into the bypass valve control system, it is necessary to determine the relationship between these times and engine revolutions. For this purpose, a numerical model of the engine operation was developed, with the structure shown in Fig. 4. A computer simulation should allow to verify the proposed design and determine optimum opening and closing angles of the valve. Moreover, the simulation was used to determine critical engine operation parameters. Those critical parameters include engine speed and compression ratio at which engine knock will begin to occur. This data, along with flow parameters of the blow-off valve will allow to properly program the Engine Control Unit. Flow analysis in engines is very important as gas flow affects the efficiency of the engine.
Numerical research was based on an existent geometry of the engine prototype. Ansys FLUENT software with SpaceClaim and ICE (Fluent) modules was used to simulate engine operation. To ensure that simulation data is in best possible agreement with reality, 3D scans of engine components were performed to depict engine geometry as accurately as possible. A finite element mesh was generated basing on obtained 3D scan data.
The paper presents the issues related to the mapping of the shape and generation of the computational mesh. Attention was drawn to the important elements of geometry that cause serious difficulties in generating a computational mesh. The paper discusses ways of introducing appropriate simplifications aimed at accelerating numerical calculations..
In the first stage, a three-dimensional numerical image of the prototype engine had to be developed. Since the basis for modeling is the existing prototype, it is necessary to reproduce its shape as faithfully as possible so that in the future the results of the numerical analysis could be used to determine the proper parameters of the prototype engine control system. In order to faithfully reproduce the shape of the existing engine elements, the Reverse Engineering method was used. This method consists in 3D scanning of existing elements, and then generating their vector equivalents on this basis.. a)
Three-dimensional representations of main engine parts were obtained through 3D scanning (fig.5). As can be seen, those representations contain many defects, lacking surface continuity. Therefore they required further 3D modeling work to obtain continuous surfaces depicting shapes that are in agreement with real shape of the engine components. Despite this, the use of 3D scanning allows to retain proportions and basic dimensions of scanned elements, facilitating further modeling. Remaining element surfaces were generated with the use of SpaceClaim software, which is part of the Ansys FLUENT package. 3.1.
In order to obtain a three-dimensional image of the basic engine components, the SpaceClaim module was used. This module has tools for processing surface objects (Mesh). This mapping consists in indicating characteristic points and creating a mesh with a given density, which brings the scanned surface (gray) closer to the parameterized surface (green), which allows to create a solid (Fig.7).
The surface generated by the software recreates the scanned object with accuracy greater than 10 m. The solid generated in this way is a matrix with several thousand characteristic points, which means that further numerical calculations would require very high computing power. The solid generated in this way is a matrix with several thousand characteristic points, which means that further numerical calculations would require very high computing power, and it would often be impossible to carry out calculations on such a complex object using the FEM method. Therefore, part geometry has to be simplified in a way that allows to retain details significant for the accuracy of numerical calculations. a) extrusion by leading curves
Figure 8 shows two ways to simplify geometry available in SpaceClaim. A much better mapping is obtained using the method of combining simple solids. Often this type of algorithm is not good enough, because in some places the surfaces generated in this way may lead to incorrect generation of computational meshes for the FEM method. In these areas, it is necessary to enter the surface manually. However, this requires some experience from the operator
In order to verify the correctness of the generated surfaces, the SpaceClaim module has a rich library of validation tools. Using the tools, it is possible to detect unnecessarily divided curves, overlapping solids, find small or missing surfaces. This type of test at the initial design stage will allow for significant time savings in subsequent modeling stages 3.2.
The next stage of modeling works is putting the individual elements into an assembly and giving them specified constraints associated with boundary conditions.
A very helpful tool is the PortFlow module that allows you to quickly verify the correctness of the operations performed. It simulates the calculations without modeling the combustion of the mixture the calculations are made only for the gas flow. This allows you to quickly check the correctness of the earlier stages, and by radically reducing the modeled phenomena, it significantly shortens the verification process
The Design Modeler module in Ansys FLUENT software was included with this purpose in mind. Working with this module is largely automated, requiring the user to specify units, import geometries and specify characteristic surfaces, solids or points. With the use of Decompose command, the software rebuilds the imported geometry for further use in accordance to prior user specification. The module deletes parts of the geometry, while simplifying or adding others, basing on imposed constraints. Therefore, obtained part geometries can differ from base ones, depending on assumed constraints.
An example of geometry for Port Flow simulation was depicted in fig 8. In comparison, a piston model intended for more complex “Combustion Simulation” analysis will be rebuilt to a more complex shape, as depicted in fig. 9. At the design stage, it is recommended to check the validity of previously developed geometry by running several different analyses, with the use of “Decompose” command, which checks key parameters of analyzed solids.
A very important and relatively difficult step is the FEM mesh generation. The Meshing Module in Ansys FLUENT is designed for this purpose. This stage of work is considered difficult, as many geometry errors become evident here. An FEM mesh generated for geometry seen in fig. 8 is depicted in fig. 10. Only at first look the mesh seems correctly generated, however after checking mesh validity with the use of „Orthogonal Quality” and „Skewness" commands, it can be seen that the mesh possessed numerous faults, which generate errors in further computations, making correct analysis impossible.
Example of mesh errors are depicted in figure 12. They are mainly associated with the near-surface layer. In analysis of flow phenomena, it is especially difficult to model sharp edges. Leaving these areas in this form will significantly prolong the calculations, and in extreme cases will lead to divergence of numerical calculations or will generate division by zero errors. However, on a macro scale, they do not have a major impact on the nature of the fluid flow. Therefore, it is recommended to round the edges wherever possible.
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Due to the time of calculations, a number of tests were carried out involving the introduction of simplifications of the geometry that do not significantly affect the quality of the numerical simulation. On this basis, it was found that sufficiently good results are obtained for a relatively simple geometry, while maintaining accuracy in mapping the volume of the combustion chamber, compression ratio, piston diameter, diameters of the intake and exhaust channels. Keeping these criteria is sufficient to achieve the goal of computer simulation, namely to determine the critical operating parameters of the internal combustion engine. To achieve this goal, it was sufficient to simplify the shape of the combustion chamber to the form shown in Figure 13.
If more accurate results are obtained, a more accurate mapping of the combustion chamber is necessary (Fig. 14a). This solution results in unnecessary density of the mesh on the sharp curves of the inlet channels. Hence, the simplification consisting in smoothing the arcs of the inlet channels (Fig. 14b) does not cause significant differences in the simulation results, and significantly speeds up the calculations.
a) 4. Podsumowanie i wyniki badań
The problems of modeling flow problems in internal combustion engines discussed in the paper were used to develop a numerical model of the engine. This model makes it possible to calculate the compression parameters in the engine chamber. On this basis, it is possible to determine the critical parameters of the engine's operation. Figure 15 shows exemplary calculation results. The results were related to the angle of rotation of the crankshaft. Figure 15a shows the cycle of movement of the piston and the intake and discharge valve. Figure 15b shows the maximum values of pressure and temperature in the cylinder for the cycle defined in this way. On this basis, it is possible to analyze changes in parameters inside the engine chamber and determine the times related to controlling the discharge valve so as not to lead to premature ignition in the engine combustion chamber
The work was financed from the funds under Priority Axis I - Innovations in the economy, Measure 1.1 Innovations in enterprises, of the Regional Operational Program for the Opolskie Voivodeship for the years 2014-2020. "R&D works on how to effectively power the internal combustion engine" decision no. RPOP.01.01.00-16-063/16-00.