Tumble Ratio Research Articles

Numerical study on dissimilar guide vane design with SCC piston for air and emulsified biofuel mixing improvement

Crude palm oil (CPO) is one of the most potential biofuels that can be applied in the conventional diesel engines, where the chemical properties of CPO are comparable to diesel fuel. However, its higher viscosity and heavier molecules can contributes to several engine problems such as low atomization during injection, carbon deposit formation, injector clogging, low mixing with air and lower combustion efficiency. An emulsification of biofuel and modifications of few engine critical components have been identified to mitigate the issues. This paper presents the effects of dissimilar guide vane design (GVD) in terms of height variation of 0.25 R , 0.3 R and 0.35 R at the intake manifold with shallow depth re-entrance combustion chamber (SCC) piston application to the incylinder air flow characteristics improvement. The simulation results show that the intake manifold with GVD improved the performance of the air flow characteristic particularly swirl, tumble and cross tumble ratios from the intake manifold to the engine. The GVD with the height of 0.3 R was found to be the optimum design with respect to the overall improvement of the air flow characteristic. The improvement of the air flow characteristic with the application of GVD and SCC piston in the engine was expected to contribute to a better air fuel mixing, fuel atomization and combustion efficiency of the engine using emulsified biofuel as a source of fuel.

Parametric analysis of a 4-stroke GDI engine using CFD

The present study focuses on the evaluation of the effect of engine parameters on the characteristics of a GDI engine by CFD analysis. The analysis was carried out at three engine speeds (2000, 3000, and 4000rev/min), at three compression ratios (10, 11, and 12) and at three fuel injection pressures (200, 300, and 400bar). The overall equivalence ratio of the in-cylinder mixture was maintained at 0.75 in all the above cases. Finally, it is observed that, the turbulent kinetic energy and tumble ratio were more sensitive to the engine speed than to other parameters. The fuel injection pressure was found to play a vital role in obtaining combustible mixture near the spark plug at the time of spark. In addition, a low heat release rate occurred at the engine speed of 4000rev/min compression ratio of 10 and fuel injection pressure of 200bar.

Investigation of the Effect of the In-Cylinder Tumble Motion on Cycle-to-Cycle Variations in a Direct Injection Spark Ignition (DISI) Engine Using Large Eddy Simulation (LES)

Cycle-to-cycle variations (CCVs) limit the extension of the operating range by inducing load variations and even misfire and/or knock for direct injection spark ignition (DISI) engines and hence need to be controlled. One of the effective and flexible ways to reduce CCV is to employ a charge motion control valve. This study is aimed to analyze the flow characteristics and CCV using large eddy simulation (LES) and fast Fourier transform (FFT) in a non-reacting, DISI engine equipped with a tumble flap (i.e., a specific type of charge motion control valve) inside the intake port. The in-cylinder flow characteristics are analyzed in detail, and the possible effects of multi-scale structures of the fluid field on the subsequent ignition and combustion processes are also discussed. Computational results indicate that closing the tumble flap helps enhance the intensity of the coherent structures and increase the total integral length scale (ILS) while decreasing the Kolmogorov scale and stabilizing the flow field by suppressing the CCV of tumble ratio and tumble center. Furthermore, based on a newly developed FFT triple decomposition, each instantaneous flow field is decomposed into three subfields, termed ensemble mean part and low- and high-spatial frequency parts, respectively. It is found that switching the tumble flap position greatly affects the first two subfields, but it has negligible effect on the last part. With the closed tumble flap, the energy portion of the mean part increases, the rate of energy decay reduces, and the CCV of the low- and high-spatial frequency parts decreases.

Influences of intake ports and pent-roof structures on the flow characteristics of a poppet-valved two-stroke gasoline engine

In order to minimize the short-circuiting of the intake charge in a poppet-valved two-stroke engine, a “step structure” on the intake side of the pent-roof was employed to shroud the bulk flow at the upper periphery of the intake valves at low valve lifts. Several intake-port geometries and pent-roof angles were evaluated on their charging and reversed tumble generating abilities using a steady flow test rig built in computational fluid dynamics, among which top-entry ports and relatively small pent-roof angle were selected. The scavenging processes of the two-stroke engine at high load and 3000 r/min condition with different intake valve seat heights were simulated and evaluated. The local flow patterns and composition distribution were also studied. The results show that lowering the intake valve seat height achieves higher delivery ratio and charging efficiency, but marginally better scavenging efficiency and slightly lower trapping efficiency. The scavenging process experiences a slow displacement period due to low flow momentum for the high valve seat cases and flow interaction at the exhaust exits for the low valve seat cases. Reversed tumble is the dominant flow pattern during the scavenging process. The lower the valve seat, the higher the tumble ratio in the early stage and at the end of the scavenging period. For all valve seat configurations, the least scavenged regions are located near the engine head wall and between the intake and the exhaust valves, while the regions rich in fresh charge are located along the major path of the reversed tumble. Low intake valve seat height is effective in increasing the overall in-cylinder air proportion. Further reducing it can result in higher fresh charge proportion below the exhaust valves and near the head wall.

Effect of engine parameters on in-cylinder flows in a two-stroke gasoline direct injection engine

This paper deals with the in-cylinder flow field analysis in a two-stroke engine under motoring conditions by particle image velocimetry (PIV) and computational fluid dynamics (CFD). The main objective is to analyze the effect of engine parameters viz., engine speed, compression ratio (CR) and port orientation on the in-cylinder flows in a loop-scavenged two-stroke gasoline direct injection (GDI) engine, with an aim to help researchers to design fuel efficient and less polluting two-stroke engines. In this study, a single-cylinder 70cm3 two-stroke engine which is very commonly used for the two-wheeler application, is considered. The engine cylinder is modified to provide optical access into the in-cylinder region. The PIV experiments are conducted at various engine speeds viz., 500, 1000 and 1500rev/min, and the plane averaged velocity vector fields obtained, are analyzed to understand the in-cylinder flow behavior. The CFD study is also carried out using the commercial CFD code, STARCD, to study and compare the in-cylinder flow parameters at various engine operating conditions. The CFD results are compared with the experimental results to the extent possible. The CFD predictions are found to be in good agreement with the experimental results. Therefore, the CFD analysis has been extended further to understand the effect of various engine parameters on the in-cylinder flows. We found that the turbulent kinetic energy and tumble ratio increased by about 25% and 20% respectively, when the engine speed was increased from 1000 to 1500rev/min. Also, we found that the turbulent kinetic energy and tumble ratio decreased by about 13% and 26% when the compression ratio was increased from 7 to 8. In addition, we found that the port orientation, rather than port areas had a greater influence on the in-cylinder flow parameters.

Effect of the piston top contour on the tumble flow and combustion features of a GDI engine with a CMCV: a CFD study

ABSTRACTIn spite of much progress in the development of gasoline direct injection (GDI) engines, choosing an appropriate piston top contour to obtain desirable combustion efficiency is still an arduous process for engineers. This study investigates the combined effects of piston bowl geometry and a charge motion control valve (CMCV) on tumble flow and combustion features in GDI engines. Based on the model validation, the processes of intake, spray, mixture formation and combustion at different engine speeds are simulated and analyzed for different piston shapes for the two cases of opening and closing the CMCV. The results show that the bowl on the top of piston is beneficial for the formation and development of tumble flow. The flat top piston with the CMCV closed is able to achieve acceptable combustion pressure. However, with the increase of engine speed and load, the advantages of the flat top pistons gradually disappear; the dual offset bowl piston has a minimum tumble ratio and turbulence kinetic energy (TKE) at the end of the compression stroke because of the projection in the middle of the piston top surface which leads to a lower pressure rise rate and a reduced flame propagation speed at high load. The closed CMCV contributes to a faster evaporation rate and a more uniform mixture at lower speeds. It is not recommended for use at high speeds due to lower intake air mass and reduced combustion pressure. The research provides an effective way for engineers to choose an appropriate piston top contour combined with a CMCV to obtain desirable combustion efficiency.

Effects of Variable Valve Lift on In-Cylinder Air Motion

An investigation into in-cylinder swirl and tumble flow characteristics with reduced maximum valve lifts (MVL) is presented. The experimental work was conducted in the modified four-valve optical spark-ignition (SI) test engine with three different MVL. Particle image velocimetry (PIV) was employed for measuring in-cylinder air motion and measurement results were analyzed for examining flow field, swirl and tumble ratio variation and fluctuating kinetic energy distribution. Results of ensemble-averaged flow fields show that reduced MVL could produce strong swirl flow velocity, then resulted in very regular swirl motion in the late stage of the intake process. The strong swirl flow can maintain very well until the late compression stage. The reduction of MVL can also increase both high-frequency and low-frequency swirl flow fluctuating kinetic energy remarkably. Regarding tumble flow, results demonstrate that lower MVLs result in more horizontal intake flow velocity vectors which can be easily detected under the valve seat area. Although the result of lower MVLs show a higher tumble ratio when the piston is close to the bottom dead centre (BDC), higher MVLs substantially produce higher tumble ratios which can be confirmed when most cylinder area lies in the measuring range.

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.

Numerical Analysis of a Torch-Ignition System for an Internal Combustion Engine

Torch ignition systems in spark-ignition engines represents an interesting option in the efforts to reduce pollutants emission and specific fuel consumption. Based on this idea, this paper presents a 3D model of a prechamber created for a spark-ignition engine and focuses on the numerical analysis of the fluid flow inside the modified chamber. This kind of analysis is very important once it allowed evaluating aspects like turbulence parameters, pressure inside the chamber and prechamber, fluid recirculation and a possible prechamber’s geometry for the engine. The studies were done in a four valve Single Cylinder Research Engine – SCRE. For the numerical modeling and fluid flow investigation was used STAR-CD software. Results show higher values of tumble ratio and kinetic energy with the prechamber.

The Interactions of In-Cylinder Flow and Fuel Spray in a Gasoline Direct Injection Engine With Variable Tumble

Particle image velocimetry (PIV) system was used to measure the tumble structure of the in-cylinder airflow in a four-valve optical gasoline direct injection (GDI) engine. The tumble ratio was controlled by a flap in the manifold and a baffle in the intake port. With proper orthogonal decomposition (POD) method, the velocity field was decomposed into four parts, i.e., the mean, coherent, transitional, and turbulent. The effect of tumble motion on the cycle-to-cycle variation (CCV) of airflow and spray was investigated by calculating the shear strain vorticity. The results indicate that the flow structure can be effectively changed through the combination of flap and baffle by forming a single large-scale tumble flow with the tumble ratio three times higher than the original one. According to POD analysis, it is revealed that the large-scale strong tumble motion leads to the energy occupation ratio of the mean part greatly increase by up to 30%, while the energy transferred to the coherent part is reduced. The above process also decreases the CCV of the coherent part by 50%; thus, the CCV of the whole airflow in the cylinder can be suppressed. A single large-scale tumble increases the maximum shear strain rate up to 2400 s−1. Meanwhile, the maximum vorticity increases to about 6000 s−1 by rolling up of the airflow. The contact area between spray droplets and air becomes larger, and the momentum exchanges between them contribute to wider sprays cone angle and shorter penetration distance when the flap is closed. The statistics of the measurements illustrate that a single large-scale tumble can promote the formation of homogeneous mixture and reduce the fluctuation between multicycles.

Effects of Scavenging System Configuration on In-Cylinder Air Flow Organization of an Opposed-Piston Two-Stroke Engine

In-cylinder air flow is very important from the point of view of mixture formation and combustion. In this direction, intake chamber structure and piston crown shape play a very crucial role for in-cylinder air pattern of opposed-piston two-stroke (OP2S) engines. This study is concerned with the three-dimensional (3D) computational fluid dynamics (CFD) analysis of in-cylinder air motion coupled with the comparison of predicted results with the zero-dimensional (0D) parametric model. Three configurations viz., a flat piston uniform scavenging chamber, a flat piston non-uniform scavenging chamber and a pit piston non-uniform scavenging chamber have been studied. 0D model analysis of in-cylinder air flow is consistent with 3D CFD simulation. It is concluded that a pit piston non-uniform scavenging chamber is the best design from the point of view of tumble ratio, turbulent kinetic energy and turbulent intensity, which play very important roles in imparting proper air motion. Meanwhile a flat piston uniform scavenging chamber can organize a higher swirl ratio and lower tumble ratio which is important to improve the scavenging process.

Effect of piston shape on in-cylinder flows and air–fuel interaction in a direct injection spark ignition engine – A CFD analysis

DISI (Direct injection spark ignition engines) are very popular today because of their low fuel consumption and exhaust emissions due to their operation at lean stratified mixture. Their performance depends on the level of mixture stratification, which in turn depends on in-cylinder flows and air-fuel interaction at the time of fuel injection. However, these are very much affected by combustion chamber configuration, which in turn is mainly dependent upon the piston shape. In this study, a CFD (computational fluid dynamics) analysis is carried out to analyze in-cylinder air flows and air-fuel interaction created by different piston shapes viz., flat, flat-with-center-bowl, inclined and inclined-with-center-bowl pistons in a four-stroke engine at an engine speed of 1500 rev/min. Here, a start of fuel injection is considered at end of the suction stroke with a multi-hole injector operating at an injection pressure of 500 bar. Engine specifications are selected from the literature for which experimental data is available for comparison. Finally, it is found that flat-with-center-bowl piston results in about 51% higher tumble ratio and about 21% higher turbulent kinetic energy of in-cylinder flows, better mixture stratification along with 33% higher evaporation rate and 33% higher percentage of fuel evaporation as compared to those of flat piston.

An Investigation into In-Cylinder Tumble Flow Characteristics with Variable Valve Lift in a Gasoline Engine

In this paper, the investigation into in-cylinder tumble flow characteristics with reduced Maximum Valve Lifts (MVL) is presented. The experimental work was conducted in a modified four-valve Spark-Ignition (SI) test engine, with optical accesses for measuring in-cylinder air motion in the vertical direction. Three different MVL of 6.8 mm, 4.0 mm and 1.7 mm were tested and Particle Image Velocimetry (PIV) was employed for those measurements. Measurement results were analysed by examining the tumble flow field, the tumble ratio variation and the fluctuating kinetic energy distribution. Meanwhile, a numerical analysis method for detecting the vortex centre was developed. From results of the vortex centre distribution, the cyclic variation of the in-cylinder flow was explored. The phase-averaged flow fields show that higher MVLs could produce stronger vertical flows which turn more toward to the piston top and finally are possible to form big scale tumble flow structure. Although lower MVLs create a higher tumble ratio when the piston is close to the Bottom Dead Centre (BDC), higher MVLs substantially produce higher tumble ratios when the piston is moving close to the Top Dead Centre (TDC). In terms of kinetic energy, lower MVLs result in higher values including higher total kinetic energy and higher fluctuating energy. Finally, the vortex centres results demonstrate lower MVLs could enhance cycle-to-cycle variation due to the weakened tumble vortex.

Numerical Analysis of Tumble, Cross Tumble and Swirl Ratios in a Single Cylinder Research Engine

In-cylinder flow such as tumble and swirl has an important role on the engine combustion efficiencies and emission formations. In particular, the tumble flow whichis dominant in current high performance gasoline engines has an important effect on the fuel consumption and exhaust emissions under part load conditions. In this research a numerical analysis on a single cylinder engine using computational fluid dynamics (CFD) is presented. From the results of the commercial code, the tumble, cross-tumble and swirl ratios are calculated for evaluating the flow development through intake and compression strokes. The reliability of the results is accessed through a validation process, which consists of comparing numerical and experimental results for in-cylinder pressure, along with a grid independence study. The results for the in-cylinder pressure show good agreement between numerical and experimental results, and the grid independence is rapidly achieved. The results for rotational ratios around X and Z axis show lower and unstable values, and indicate that the vortex on those plane change its rotation direction. On the other hand, the ratio around Y axis has the greater values, and also keeps its rotation direction.

Effect of Piston Shape on Scavenging in a Two-Stroke Engine - A CFD Analysis

This study focuses on effect of piston shape on in-cylinder flows in a conventional loop scavenged two-stroke engine using CFD. Three piston-shapes viz., flat, flat with center-bowl and flat with center-dome are analyzed along with standard piston. The CFD solution has been obtained by using commercial code STAR-CD. First the CFD results are compared with those of experimental values obtained from particle image velocimetry (PIV). For comparison among various piston shapes, various parameters viz., velocity vector plots, tumble ratio (TR), turbulent kinetic energy (TKE) at various crank angles are used. Scavenging and trapping efficiencies are also calculated for each piston configuration. From the results, it is found that flat with center-bowl piston is the best in terms of in-cylinder flow characteristics. It gives higher TR and TKE with very little effect on efficiencies.

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 250cm3. 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.

Effects of tumble combined with EGR (exhaust gas recirculation) on the combustion and emissions in a spark ignition engine at part loads

The effects of tumble combined with EGR (exhaust gas recirculation) on the combustion and emissions were experimentally investigated in a spark ignition engine in this study. A low pressure loop EGR system and a variable tumble valve in intake ports were employed, by which wide range of EGR ratio and tumble ratio could be achieved. All the tests were performed at part loads, and the equivalence ratio was kept constant at 1.0. The results show that the combination of EGR and enhanced tumble motion produces a significant improvement in fuel economy (13.1–19.5%) and a reduction in the coefficient of variations of indicated mean effective pressure (CoVIMEP) due to the significantly shortened combustion duration. By optimizing the combustion process with EGR and enhanced tumble, NOx (nitrogen oxides) emissions drastically decrease, while HC (hydrocarbons) emissions slightly rise. For CO (carbon monoxide) emissions, the enhanced tumble takes a positive effect.

Optimization of intake port design for SI engine

It is well known that in-cylinder flow is very important factor for the performance of SI engine. An appropriate in-cylinder flow pattern can enhance the turbulence intensity at spark time, therefore increasing the stability of combustion, reducing emission and improving fuel economy. In this paper, the effect of intake port design on in-cylinder flow is studied. It is found a vortex existed at the upper side of intake port of a production SI engine used in the study, during the intake stroke, which will reduce both tumble ratio and volumetric efficiency. A minor modification on intake port is made to eliminate the vortex and increase tumble ratio while keeping volumetric efficiency at the same level. It is demonstrated that the increase in tumble in the new design results in a 20 per cent increase in the fuel vaporization. In this study, both KIVA and STAR-CD are used to simulate the engine cold flow, as well as ICEM CFD and es-ice used as pre-processor respectively due to the complexity of engine geometry. Simulation results from KIVA and STAR-CD are compared and analyzed.

Energy efficient piston configuration for effective air motion – A CFD study

Air motion inside the cylinder is very important from the point of view of energy efficiency. In this direction, piston configuration plays a very crucial role. This study is concerned with the CFD analysis of in-cylinder air motion coupled with the comparison of predicted results with the experimental results available in the literature. Four configurations viz., flat, inclined, centre bowl and inclined offset bowl pistons have been studied. For numerical analysis STAR-CD CFD software has been used. Experimental results available in the literature for comparison are obtained by PIV measurements. From this study, it is concluded that a centre bowl on flat piston is found to be the best from the point of view of tumble ratio, turbulent kinetic energy, turbulent intensity and turbulent length scale which play very important role in imparting proper air motion, there by increasing the energy efficiency of the engine.

Cycle-to-cycle variation analysis of in-cylinder flow in a gasoline engine with variable valve lift

In spark ignition engines, cycle-to-cycle variation (CCV) limits the expansion of the operating range because it induces the load variations and the occurrence of misfire and/or knock. Variable valve actuation (VVA) or variable valve lift (VVL) has been widely used in SI engines to improve the volumetric efficiency or to reduce the pumping losses. It is necessary to investigate the CCV of in-cylinder gas motion and mixing processes in SI engines with VVA/VVL system. This study is aimed to analyze the CCV of the tumble flow in a gasoline direct injection (GDI) engine when VVL is employed. Cycle-resolved digital particle image velocimetry (CRD-PIV) data were acquired for the in-cylinder flow field of a motored four-stroke multi-valve GDI optical engine. The CCV of in-cylinder gas motion with a series of valve profiles and different maximum valve lift (MVL) was analyzed, including cyclic variation characteristics of bulk flow (tumble centre and tumble ratio), large- and small-scale fluctuation, total kinetic energy, and circulation. The results show that the CCV of the in-cylinder flow is increased with reduced MVL. With lower MVLs, stable tumble flow cannot be formed in the cylinder, and the ensemble-averaged tumble ratio decreases to zero before the end of the compression stroke due to violent variation. In addition, the evolution of the circulation shows larger variation with lower MVLs that indicates the ‘spin’ of the small-scale eddy in the flow field presents violent fluctuation from one cycle to another, especially at the end of the compression stroke. Moreover, the analyze of the kinetic energy indicates the total energy of the flow field with lower MVLs increases significantly comparing with higher MVL conditions due to the intake flow jet at the intake valve seat in the intake stroke. However, the CCV of the in-cylinder flow becomes more violent under lower MVL conditions, especially for the low-frequency fluctuation kinetic energy. Thus, present strong tumble flow can lower the CCV of the air motion. It is necessary to manage strong tumble or other bulk flow (such as swirl flow) in order to improve the stability of ignition and combustion for GDI engines with VVL, especially at the lower MVL conditions.