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ANALYZIS BY SIMULATION OF IN-CYLINDER TUMBLE MOTION

Abstract: In internal combustion engines coherent and stable tumble motion generation is still considered an effective mean in order to both reduce engine emissions and promote higher levels of combustion efficiency. In this research field the engine modeling and flow simulation are of great importance because the application field of the experimental measurements is very narrow, time-consuming and expensive. This paper briefly presents the strokes of the four-stroke engine and the various types of in-cylinder motion during the intake event. The papers purpose is to evaluate current means for tumble motion generation and flow simulation by presenting existent simulation software, such as AVL Fire, SolidWorks Flow Simulation and others.Key words: Four-Stroke Engine, Flow Patterns, Tumble Motion Generation, Engine Design, Flow Simulation.

1. INTRODUCTION

Among the many design goals of combustion engines, the mixing process of fuel and oxygen occupies an important place. If a good mixture can be achieved, the resulting combustion is both clean and efficient, with all the fuel burned and minimal exhaust remaining. In turn, the mixing process strongly depends on the inflow of the fuel and air components into the combustion chamber or cylinder. If the inlet flow generates sufficient kinetic energy during this valve cycle, the resulting turbulence distributes fuel and air optimally in the combustion chamber. For common types of engines, near-optimal flow patterns are actually known and include, among others, so-called swirl and tumble motions. To analyze these in-cylinder flows good visualizations are necessary. With the general progress of CFD (Computational Fluid Dynamics) simulations, the discipline of engine design is made accessible to both numerical simulation and visualization of the resulting datasets, allowing for rapid testing of engine designs.2. FOUR-STROKE ENGINE

The four-stroke engine was first demonstrated by Eugen Langen and Nikolaus August Otto in 1866. It is also known as the Otto cycle, however, the technically correct term is actually four-stroke cycle. The four strokes of the cycle are intake, compression, power, and exhaust. Each corresponds to one full stroke of the piston; therefore, the complete cycle requires two revolutions (720) of the crankshaft to complete. The four-stroke cycle engine is the most common type of small engine.

2.1. Intake Process [5]

The intake event is when the air-fuel mixture is introduced to fill the combustion chamber. The intake event occurs when the piston moves from TDC (Top Dead Center) to BDC (Bottom Dead Center) and the intake valve is open. The movement of the piston toward BDC creates a low pressure in the cylinder. Ambient atmospheric pressure forces the air-fuel mixture through the open intake valve into the cylinder to fill the low pressure area created by the piston movement. The cylinder continues to fill slightly past BDC as the air-fuel mixture continues to flow by its own inertia while the piston begins to change direction. The intake valve remains open a few degrees of crankshaft rotation after BDC, depending on engine design. The intake valve then closes and the air-fuel mixture is sealed inside the cylinder.

2.2. Compression Process [5]

The compression stroke is when the trapped air-fuel mixture is compressed inside the cylinder. The charge is the volume of compressed air-fuel mixture trapped inside the combustion chamber ready for ignition. Compressing the air-fuel mixture allows more energy to be released when the charge is ignited. Intake and exhaust valves must be closed to ensure that the cylinder is sealed to provide compression.

When the piston compresses the charge, an increase in compressive force supplied by work being done by the piston, causes heat to be generated. The compression and heating of the air-fuel vapor in the charge results in an increase in charge temperature and an increase in fuel vaporization. The increase in charge temperature occurs uniformly throughout the combustion chamber to produce faster combustion (fuel oxidation) after ignition. The more the charge vapor molecules are compressed, the more energy obtained from the combustion process.

The energy needed to compress the charge is substantially less than the gain in force produced during the combustion process. For example, in a typical small engine, energy required to compress the charge is only one-fourth the amount of energy produced during combustion.

2.3. Ignition Process [5]

The ignition (combustion) event occurs when the charge is ignited and rapidly oxidized through a chemical reaction to release heat energy. Combustion is the rapid, oxidizing chemical reaction in which a fuel chemically combines with oxygen in the atmosphere and releases energy in the form of heat. Proper combustion involves a short but finite time to spread a flame throughout the combustion chamber. The spark at the spark plug initiates combustion at approximately 20 of crankshaft rotation before TDC. The atmospheric oxygen and fuel vapor are consumed by a progressing flame front. A flame front is the boundary wall that separates the charge from the combustion by-products, which usually progresses across the combustion chamber until the entire charge has burned.

2.4. Power Stroke [5]

The power stroke is an engine operation stroke in which hot expanding gases force the piston head away from the cylinder head. Piston force and subsequent motion are transferred through the connecting rod to apply torque to the crankshaft. The torque applied initiates crankshaft rotation. The amount of torque produced is determined by the pressure on the piston, the size of the piston, and the throw of the engine. During the power stroke, both valves are closed.2.5. Exhaust Process [5]

The exhaust stroke occurs when spent gases are expelled from the combustion chamber and released to the atmosphere. As the piston reaches BDC during the power stroke combustion is complete and the cylinder is filled with exhaust gases. The exhaust valve opens, and inertia of the flywheel and other moving parts push the piston back to TDC, forcing the exhaust gases out through the open exhaust valve. At the end of the exhaust stroke, the piston is at TDC and one operating cycle has been completed.

3. IN-CYLINDER FLOW PATTERNS [2]

From a simplified point-of-view, there are two types of ideal flow patterns in an engine cylinder: swirl motion (with the cylinder axis as the axis of rotation and the flow entering tangentially through the intake ports) and tumble motion (orthogonal to the cylinder axis, the axis of motion moves as the cylinder expands and stays halfway between the top cylinder wall and the piston head at the bottom). Both are rotational motions, however, the axis of rotation is different in each case. Depending on the type of engine, one of these patterns is considered optimal because it maximizes mixing of injected fuel and air, resulting in homogeneous combustion. The basic geometries of the respective motion patterns (henceforth termed swirl motion and tumble motion) are presented in Figure 1.

Figure 1. (Top) Stable, circulating swirl motion.(Bottom) Transient tumble motion. [2]3.1. Phases of the flow [3]

The flow in the cylinder can be divided into several distinct phases. The flow into the cylinder through the inlet valve or valves (forming a jet) does two things: first, the geometrical configuration of the inlet ports and the valves, and their opening schedule creates swirl and tumble motions in the cylinder; second, the jet itself is turbulent, and in addition much of the directed (nonturbulent) energy in the jet is converted to turbulence, resulting in a very high turbulence level during the inlet stroke. During the second half of the inlet stroke much of this turbulence decays, which is to say that the intensity decreases markedly, both because the source (the jet) is coming to an end, and due to the effects of viscosity.

During the compression stroke, the increase of density and the changes in length scales (due to the change in geometry of the charge as it is compressed) have the effect of amplifying the turbulence which remained from the inlet jet, although the viscous decay and turbulent transport continue. In addition, the swirl and tumble are affected by the same phenomenon.

If the piston and combustion chamber have been designed to produce squish (that is: if the piston crown approaches very close to some part of the combustion chamber roof), then this will have two effects: first, the fluid squeezed out of the squish clearance volume will produce organized motions, most of which will break up into turbulence; and second, the change in geometry due to the squish will have dynamical effects on the organized tumble and swirl.

Near TDC, some of the organized motions may find they have insufficient room to maintain their form, and they will break up into turbulence, increasing the turbulence level. By this time conditions in the cylinder have become crudely homogeneous, due to the transporting effect of the turbulence and the organized motions.

During combustion the turbulence level rises somewhat. Then, during the power stroke, the geometrical changes result in a strong attenuation of the turbulence, and any organized motion that has survived. This, combined with the viscous decay, results in the turbulence being sharply suppressed, so that by the time the exhaust valve opens, there is virtually nothing left. Very little turbulence is generated during the exhaust stroke.

3.2. Inducing swirl and tumble [3]

By the way in which the valves and ports are arranged, and the schedule of valve opening, mean flows can be induced in the cylinder. Motions like this are often called coherent, meaning that they are organization buried in the disorganized turbulence. Swirl and tumble, or a combination of the two, represent the most general motion that can be induced at the scale of the cylinder. In fact, it is essentially impossible to generate swirl without inducing some tumble, so that the two are always associated.

It is possible to generate tumble without swirl. However, tumble is always associated with other secondary motions, since it is generated by flow through two valves. This secondary motion, schematically presented in Figure 2, has the effect of isolating the fluid entering through the y > 0 valve from the fluid entering through the y < 0 valve. The size of this secondary motion is approximately one-half of the bore (b/2), and the intensity is also about one-half of the intensity of the main tumbling motion.

Figure 2. Schematic of secondary motion induced in tumble [3]

Tumble appears always to break down to turbulence, because as the piston approaches TDC, there is no room between the piston crown and the cylinder head for a vortex with a diameter of the order of b; only motions with scales of the order of the clearance height can survive, so the vortex breaks up into turbulence of this scale.

In the case of swirl, if the combustion chamber is pancake shaped, the swirl can survive through the burn. However, if the combustion chamber is a penta head, with squish providing the transition from cylinder to head, and/or with the piston crown protruding into the head, the swirl must accommodate itself to the changing shape of the space available to it (from circular to rectangular, and increasingly narrow), and will also break up into turbulence. It is now clear from computational fluid dynamics and flow visualization that even in engines that were designed for high swirl, is principally the associated, unavoidable, tumble that yields the turbulence just before TDC.

4. GENERATING TUMBLE

For most of the modern stratified charge and direct injection engines, tumble flows are more crucial than swirl flows. A tumble structured vortex having the dimensions comparable with the piston stroke promotes the formation of a high turbulence level close to the spark plug at the end of the compression stroke, which speeds up the combustion and allows the adoption of lean burn strategies, that otherwise are characterized by high cyclic variability because of the combustion instabilities.

The parameters which are believed to be the most significant in affecting the tumble vortex formation, stabilization, breakdown and of consequence the final level of turbulence at the end of the compression stroke are [1]:

The intake duct shape and most of all the duct angle entering the head;

The compression ratio;

The piston shape;

The squish area.

4.1. Tumble Flaps [7]

Tumble flaps, also called charge-motion flaps, are used in petrol direct injection engines (e.g. FSI engines) to generate a stratified charge. This is achieved by dividing the air intake channel into two separate channels, one of which can be closed by the tumble flap (Figure 3).

At higher engine speeds and torques, the tumble flap is opened to achieve a better filling level. During this so-called homogeneous operation, the engine functions like a conventional fuel injection engine, but with higher efficiency due to the higher compression.

This enables a reduction of fuel consumption in the low engine speed range, without sustaining losses of power or torque at higher engine speeds. The flaps are driven with either electric or pneumatic power, depending on their design.

Figure 3. Tumble flap. (Left) Stratified charge operation. (Right) Homogeneous operation. [7]4.2. Tumble Generating Intake

Both the intake duct shape and the duct angle affect in a great measure the formation of turbulence during the intake event. A specific design that generates strong tumble motion, by means of an inclined intake port, is presented in Figures 4 and 5.

Figure 4. Inclined intake port [4]

Figure 5. Tumble generated by the intake duct [9]5. CFD Simulation

Computational fluid dynamics, usually abbreviated as CFD, is a branch of fluid mechanics that uses numerical methods and algorithms to solve and analyze problems that involve fluid flows.

CFD modeling of direct injection engines is used to simulate 3D flows, mixture formation, burning and pollutant formation. It also makes it possible to analyze the interaction between the fuel and the motion of the intake air inside the combustion chamber. The main advantage of CFD modeling is that it minimizes prototyping time. CFD software is capable of making numerous studies in a relatively short time, based on the computational power [4].

The main interest in the visualization of the in-cylinder flow is the extraction and visual analysis of the swirl and tumble motion patterns. The motion of the intake jet within the cylinder simulated in CFD software is shown in Figure 6, where regions with different flow velocity appear in different colors (red represents the highest value of flow speed).

For the use in design analysis, the constructed visualizations need to be objective and reproducible, meaning that the quality of the visualization result must not depend on vital parameters to be supplied by the user. This results in comparable visualizations for different simulation results of the same prototype or possibly even among different design prototypes [2].

Figure 6. Visualization of tumble motion using CFD simulation [4]

In order to analyze tumble flows in CFD software, a static mesh is required. This can be created using graphical design programs. After having a static and a moving mesh, initial data is required regarding atmospheric and in-cylinder conditions [4].The simulation results are given in the form of attributes defined in the interior of the respective cylinder geometries. As is quite common in CFD simulations, the flow is required to vanish on the domain boundary (no-slip condition) in order to correctly model fluid-boundary friction. Nevertheless, values on the boundary of the domain are easily inferred by e.g. extrapolation of volume values next to the boundary. It is also notable that in classical engineering analysis, visualization is widely performed on two-dimensional slices [2].

Overall, the level of information that can be provided by a visualization technique increases with the dimension of the data it treats. At the same time, the visualization result need not necessarily improve due to perceptual issues such as cluttering [2]. 6. FLOW VISUALIZATION METHODS6.1. Integration-based Methods [2]

Integration-based methods are well suited to the analysis of time-dependent flows. Despite their simplistic nature, particle visualizations can provide valuable insight into the overall structure of a flow dataset. This is especially true for time-dependent data. While the basic principle is similar to that of streamlines or pathlines, an animation of moving massless particles manages to convey the dynamic nature of the flow much better than static imagery alone. In the general case, integral methods suffer from seeding issues, although strategies have been proposed to circumvent this. However, none of these approaches are concerned with time-varying data. Fortunately, engine geometries offer the inlet pipe as a natural choice of a seeding region. Integration of pathlines in time-dependent 3D flows is straightforward through the application of standard numerical integration algorithms that only require the integrand at a sparse set of points.

Figure 7 depicts a frame from an animation of massless particles moving with the flow during the early stage of the valve cycle, seeded at positions in the intake pipe. The particles are of uniform size and color-coded according to flow velocity magnitude.

Figure 7. A hybrid visualization of particles and singularity paths [6]6.2. Topology-based Methods [2]

Topological methods provide efficient means for the visualization of essential structures in steady flows. As opposed to the integral methods, they offer a fully automatic way to gain insight from vector datasets. The topological technique is typically applied in the visualization of planar flows for which it yields synthetic graph representations. It consists of critical points (vector field zeros) and connecting separatrices.

For viscous flows, the information conveyed by boundary topology can be enhanced naturally by showing the strength of flow separation and attachment along separatrices. Flow separation occurs when the flow surrounding an embedded body interrupts its tangential motion along the object's boundary and abruptly moves away from it. The opposite phenomenon is called flow attachment. Direct visualization of the boundary topology produces images such as Figure 8. Critical points are colored by type, and separatrix color varies with separation/attachment behavior from dark blue (weak) to cyan (strong). Separatrices indicate the separation between neighboring vortices on the boundary.

Figure 8. Visualization of swirl motion using boundary topology [2]

A moving cutting plane (traversing the volume of the dataset) on which the vector field is resampled and projected at regular intervals can be a powerful tool in the analysis of 3D datasets. The projection of the vector field on the plane effectively manages to discard structures orthogonal to the plane, but preserves plane-parallel flow patterns. If assumptions on the orientation of features are given, this property can be exploited. Cutting-planes are hence well suited for the qualitative analysis of swirl or tumble motion, since its axis of rotation is known.

Figure 9 shows frames from an animation of a tumble dataset, where the moving cutting planes have been applied orthogonal to the tumble axis and are color coded by their distance to the back wall of the combustion chamber for increased visual clarity. Although the visualization is not exact, the prevalent tumble structure is captured well in spite of its overall weakness and instability. The color of separatrices varies from blue to red on successive cutting planes. Tumble-like flow structures emerge clearly from the otherwise incoherent lines. The paths of critical points over the cutting plane continuum are displayed in green.

Figure 9. Visualization of flow field structures using cutting plane topology [2]7. CFD SIMULATION SOLUTIONS

Due to an intense world request, the number of simulation models developed using CFD code has considerably increased. The continuous request for answers regarding direct injection engines and the necessity of developing more efficient calculation systems led to the apparition of numerous CFD simulation programs and interfaces.

The commercially available AVL Fire is a world leader in the CFD simulation solutions and a powerful multi-purpose thermo-fluid dynamics software with a particular focus on handling fluid flow applications related to internal combustion engines and powertrains. Tailored to meet the requirements of automotive research and development teams, AVL Fire proved fast and easy to use with adjustable modeling depth and can always be upgraded by user generated code [11].

Fully embedded with SOLIDWORKS 3D CAD, SOLIDWORKS Flow Simulation intuitive CFD tool is used to simulate liquid and gas flow in real world conditions, run what if scenarios, and efficiently analyze the effects of fluid flow, heat transfer, and related forces on immersed or surrounding components. SOLIDWORKS Flow Simulation solutions include: Computational fluid dynamics; Thermal comfort factors; Simulation Visualization; Fluid flow analysis; Thermal fluid analysis [10].

ANSYS CFD simulation software predicts the impact of fluid flows on any product, throughout design and manufacturing as well as during end use. The software can be used to design and optimize new equipment and to troubleshoot already existing installations. ANSYS CFD solutions are fully integrated into the ANSYS Workbench platform which integrates pre-processing, simulation and post-processing, as well as multi-physics functionality [8]. 8. CONCLUSIONS

Due to the necessity of step-by-step improvements for optimizing the engine cycle, engine modeling and CFD simulations are of great importance, providing advantages and opportunities that the application field of experimental measurements cannot. Hence these techniques are widely used and widely recognized instruments in the design process, with the main goal of aiding designers with the most important guidelines and fundamentals.REFERENCES[1] Falfari, S., et al., 3D CFD analysis of the influence of some geometrical engine parameters on small PFI engine performances the effects on the tumble motion and the mean turbulent intensity distribution, Energy Procedia, Volume 45, pages 701710, 2014.[2] Garth, C., et al., Extraction and Visualization of Swirl and Tumble Motion from Engine Simulation Data, In Topology-based Methods in Visualization, Mathematics and Visualization, Springer Berlin Heidelberg, pages 121-135, 2007.[3] Lumley, J., Engines an introduction, Cambridge University Press, 1999.

[4] Moldovanu, D., Burnete, N., Studies regarding the tumble motion of the air inside the combustion chamber, PRODOC Conference, 2011.

[5] ***http://courses.washington.edu/engr100/Section_Wei/engine/UofWindsorManual/Four%20Stroke%20Cycle%20Engines.htm accessed 04.05.2015.

[6] ***http://cs.swan.ac.uk/~csbob/research/MotionExtracted/ accessed 04.05.2015.[7] ***http://pmmonline.co.uk/technical/function-tumble-flaps-and-swirl-flaps accessed 04.05.2015.

[8] ***http://www.ansys.com/Products/Simulation+Technology/Fluid+Dynamics accessed 04.05.2015.[9] ***http://www.car-engineer.com/toyota-develops-new-gasoline-engines-atkinson-combustion-cycle/ accessed 04.05.2015.[10] ***http://www.solidworks.com/sw/products/simulation/flow-simulation.htm accessed 04.05.2015.

[11] ***https://www.avl.com/web/ast/fire accessed 04.05.2015.