Numerical Analysis of In-cylinder Tumble Flow Structures – Parametric 0D Model Development

Abstract Both in the automotive and in the motorcycle fields the requirement of step-by-step improvements for optimizing the engine cycle is still present. In particular the focus of the optimization process is to reduce the raw emissions and at the same time to not penalize the engine performance. In this research field the engine modeling is of great importance because the application field of the experimental measurements is very narrow, time-consuming and expensive. Hence the modeling technique is a wide used and a wide recognized instrument for helping in the design process. Another important function of the modeling is to provide the engine designers with the most important guidelines. The main focus is to fast provide designers with some fundamentals during the first designing stage which, if not the conclusive, is close to the final project. The present paper deals with the development of a theoretic-interpretative 0D model which could highlight the most significant parameters in the engine design process and in particular in the determination of: • The tumble velocity at IVC and its residual value at TDC; • The squish velocity at TDC; • Their mutual interaction. These parameters are well recognized to be especially meaningful because they determine, at different times of the combustion process, the combustion velocity. The faster the combustion velocity, the lower the engine cycle-by-cycle variability

Tumble Motion Generation in Small Gasoline Engines: A New Methodological Approach for the Analysis of the Influence of the Intake Duct Geometrical Parameters

For motorbike and motor scooter applications, the optimization of the tumble generation is considered an effective way to improve the combustion system efficiency and to lower the emissions, considering also that the two-wheels layout represents an obstacle in adopting the advanced post-treatment concepts designed for the automotive applications. During the last years the deep re-examination of the engine design for lowering the engine emissions involved the two-wheel vehicles too. The IC-engine overall efficiency plays a fundamental role in determining the final raw emissions. From this point of view, the optimization of the in-cylinder flow organization is mandatory. In detail, in SI-engines the generation of a coherent tumble vortex having dimensions comparable to the engine stroke could be of primary importance to extend the engines' ignition limits toward the field of the dilute/lean mixtures. The aim of the paper is to introduce a new analysis approach for a deep insight of the 3D-CFD results performed to assess the intake duct geometry influence on the tumble motion generation during both the intake and the compression strokes. All the CFD simulations presented in the paper were performed by the AVL-FIRE v. 2010 CFD code on a SI 4 valve engine characterized by an unit displacement of 250 cm3. The tumble structure was changed during the analysis by changing the angle set defining the intake port shape. The stroke-to-bore engine ratio was kept constant to 0.7. The effects of the tumble variations were evaluated in terms of the tumble ratio, the turbulent kinetic energy and the vortex characterization at IVC. © 2013 The Authors

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

In scooter/motorbike 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. The promotion of a stable and coherent tumble structure is largely believed in literature to enhance in-cylinder turbulence accelerating combustion process. In small PFI engine layout and weight constraints limit the adoption of more advanced concepts. In previous technical papers the authors demonstrated the influence of head shape and squish area on tumble vortex formation, development, breakdown and on final value of turbulence close to spark plug for small PFI engines. The main result of the this research was that the combustion chamber having the less squish area resulted to have the highest level of turbulence close to spark plug at ignition time. The geometry under analysis in the current paper is a 3-valves pent-roof motorcycle engine. 3D CFD simulations were ran at 6500 rpm with AVL FIRE code. The chosen engine geometry was the geometry found to be the best set-up in terms of turbulence and combustion performances in the previous paper. In the present paper the head shape and the squish area were kept constant and the following engine parameters were varied: the intake duct angle (the angle of the intake duct entering the head was reduced of 6%. i.e. it was more directed toward the exhaust side of the chamber), the piston shape, and finally the compression ratio (it was reduced of 9%). The main goal of the current analysis is to understand which of these parameters is predominant in accelerating combustion for directing engine design toward the best set-up. © 2013 The Authors

Influence of the Diesel Injector Hole Geometry on the Flow Conditions Emerging from the Nozzle

Engine performances are correlated to the overall fluid dynamic characteristics of the injection system that, in turns, are strictly correlated to the fluid dynamic performance of the injector geometry. It is particularly true for actual GDI and Diesel engines where micro-orifice configurations are associated to very high injection pressure. In relation to the Common Rail Diesel engines, over the last decade different injector hole shapes have been tested. Actually, the most used configurations are: cylindrical, k, and ks. In this paper, the performance of all these three injector hole shapes are evaluated in order to find out the influence of orifice conicity and hydro-grinding level on the main fluid dynamic characteristics like cavitation evolution inside the injector as well as the flow properties at nozzle exit. The fluid dynamic behavior of each considered hole is evaluated over the injection time by performing a fully transient CFD multiphase simulation (i.e. the needle motion is reproduced during the simulation). By the proposed simulation methodology, the evaluation of the cavitating flow evolution inside the injector is performed not only from the point of view of the overall spray characteristics emerging from the injector holes but also from the cavitation erosion risk over needle, nozzle, and hole internal surfaces. © 2013 The Authors

Comparison between cooling strategies for power electronic devices: fractal mini-channels and arrays of impinging submerged jets

Power electronic devices like Insulated Gate Bipolar Transistors (IGBTs) and diodes are often characterized by power densities and dimensions that could result in very high heat flux densities. In order to guarantee the expected performance and lifetime for these components, dedicated active cooling devices are usually adopted. In the present paper, the comparison between two different cooling strategies for power electronics is presented: fractal channel design and submerged impinging jets. Each cooling strategy is tested on two different geometrical configurations. Water is used as coolant in all cases. Assessment of the considered cooling methods is done through application of the selected configurations in a simplified system composed by a rectangular chip (heat source) separated from the coolant by a solid block. Three-dimensional conjugated heat transfer simulations are performed by using RANS solver implemented in OpenFOAM and two-equations turbulence models, resolving also the viscous sublayer. Numerical results allow to compare the cooling strategies in terms of maximum chip temperature, overall chip-to-coolant thermal resistance, and pumping power required. In summary, the fractal-channel design shows limitations in guaranteeing low chip temperatures at an affordable pumping power. The submerged impinging jets approach shows very high local heat transfer coefficient by which it is possible to tailor the cooling expect on specific hot spots

Definition of a CFD Methodology to Evaluate the Cylinder Temperature Distribution in Two-Stroke Air Cooled Engines☆

On the basis of the operating cooling fluid, internal combustion engine cooling systems can be classified in two macro areas: aircooling system and liquid-cooling system. In four-stroke engines, liquid-cooling system is generally preferred to the air-cooling system because of its efficiency in the engine heat dissipation. However, thanks to its simplicity, today the engine air-cooling system is still widely used in the engine market, especially on two-stroke engine applications like small motorbike, light aircraft, and handheld products. To assure the necessary heat waste in air-cooled engines, the key point is the optimization of the air flow over the cylinder external surface. Air flow separation from cylinder external surface can result in high temperature gradients inside the cylinder volume causing destructive heat problem for the engine. It can be avoided only by a fine optimization of the cylinder fin design placed externally to the cylinder surface. To fulfil this need, the definition of specific methodology to evaluate the air-cooling effect on the engine is mandatory. In the present paper, the authors present a 3D-CFD simulation methodology designed to perform a detailed evaluation of twostroke air-cooled engines. The methodology was applied on two different engines equipping handheld brush-cutter machines. The optimization of the air-cooling system of such a machine is a very challenging task because the machine design must be very compact forcing all the engine parts to remain quite close each other. The simulation results are compared to experimental evidences in order to verify the validity of the proposed approach. © 2013 The Authors. Published by Elsevier Ltd

numerical evaluation of the applicability of steady test bench swirl ratios to diesel engine dynamic conditions

Engine coherent flow structures such as swirl and tumble motions are key factors for the combustion process due to their capability to rise turbulence levels and enhance mixing which, in turns, severely influence both fuel efficiency and pollutant emissions. Automotive industry has therefore put great efforts over the last decades in evaluating air flow during induction stroke and air flow within the cylinder. Nowadays swirl and tumble motion characterizing a specific cylinder head are evaluated experimentally at design stage mainly using stationary flow benches. Such tests allow characterizing each head prototype using non-dimensional parameters like swirl and tumble ratios and, finally, to compare the different designs. In the present work the authors focused their attention on the swirl ratio characterization, firstly reviewing the two main methodologies for evaluating such parameter and more precisely the AVL and the Ricardo ones. A numerical method is then proposed in order to reproduce the stationary test bench with the final goal to develop a fast and accurate virtual test bench for cylinder head design. Simulations have been carried out on different VM Motori engine heads for which experimental data were available. The comparison between computational and experimental swirl ratios allowed to evaluate the suitability of using a virtual test bench as alternative or complementary to experiments. These results widened the understanding of the swirl fluid-dynamics and suggested that care must be taken when comparing duct designs having no geometrical similarity. Finally dynamic simulations have been performed for the head prototypes in order to compute the engine swirl in realistic conditions and to compare it with the steady bench results. This allowed evaluating the capability of the two different "static" swirl ratio definition (AVL/Ricardo) in correctly estimating real engine swirl. © 2015 The Authors. Published by Elsevier Ltd

Bridge deck flutter derivatives: efficient numerical evaluation exploiting their interdependence

Increasing the efficiency in the process to numerically compute the flutter derivatives of bridge deck sections is desirable to advance the application of CFD based aerodynamic design in industrial projects. In this article, a 2D unsteady Reynolds-averaged Navier-Stokes (URANS) approach adopting Menter׳s SST k-ω turbulence model is employed for computing the flutter derivatives and the static aerodynamic characteristics of two well known examples: a rectangular cylinder showing a completely reattached flow and the generic G1 section representative of streamlined deck sections. The analytical relationships between flutter derivatives reported in the literature are applied with the purpose of halving the number of required numerical simulations for computing the flutter derivatives. The solver of choice has been the open source code OpenFOAM. It has been found that the proposed methodology offers results which agree well with the experimental data and the accuracy of the estimated flutter derivatives is similar to the results reported in the literature where the complete set of numerical simulations has been performed for both heave and pitch degrees of freedom