Experimental investigations of in-cylinder flows of Engine with Intake Shrouded Valve

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1 Experimental investigations of in-cylinder flows of Engine with Intake Shrouded Valve * Department of Mechanical Engineering, Sri Sivani College of Engineering, Srikakulam, AP , India ** Internal Combustion Engine Laboratory, Department of Mechanical Engineering Indian Institute of Technology Madras, Chennai , India ABSTRACT This paper deals with experimental study during suction and compression using particle image velocimetry of internal combustion engine at 1000 rev/min with shrouded intake valve. The results were compared with base intake valve. To characterize tumble flow, tumble ratio (TR) and average turbulent kinetic energy (TKE) parameters are used. At end of compression stroke (330CAD), shrouded intake valve irrespective of its position in assembly of cylinder head showed lower TR and higher TKE compared to base intake valve. The present study will be useful in understanding the effect of intake valve geometry on nature of in-cylinder tumble flows in modern spark ignition engines under realistic conditions. Keywords: Engine, Tumble, Shroud valve, Position and CAD. 1. INTRODUCTION Now-a-days, modern spark ignition (SI) engines like stratified charged and gasoline direct injection (GDI) engines are becoming very popular because of their low fuel consumption and exhaust emissions. Generally, charge stratification is achieved by well guided in-cylinder rotating air flows. Therefore, for optimization of modern IC engines, it is very much essential to understand in-cylinder air flow behavior under different operating conditions and configurations of IC engine. Today, an optical tool like Particle Image Velocimetry (PIV) is extensively used for in-cylinder flow measurements due to its many advantages over other contemporaries. Previously, Kern et al reported that formation and decay mechanism of tumble and its effect on turbulence. They found that, turbulence intensity became twice at end of compression stroke and turbulence field became more homogeneous from their measurements using LDV in a small four-valve engine [1]. Intake system of engine was modified to produce a strong tumble flow which was measured by PIV system and a two-tracer planar laser induced fluorescence (PLIF) system was developed to visualize fuel stratification in the cylinder of a three-valve twin-spark ignition engine [2]. In cylinder flow field and turbulence scale at ignition timing play an important role in enhancing propagation speed of initial flame and engine combustion. Thus, flow analysis in a gasoline engine is important for optimum design of combustion chamber and intake manifold system. Improvement in air swirl or tumble flow in the cylinder has become one of most promising concepts for reducing HC and CO emissions during engine operation [3]. The development of a high efficiency gasoline engine, optimization of intake port shape for a five-valve engine design guidelines were established [4] Khalighi et al., investigated in-cylinder flow structures generated by different intake configurations in a pancake-shaped combustion chamber engine during both intake and compression strokes using Computational Fluid Dynamics (CFD) code. And found that standard intake valve configuration (no shroud) generates a well-defined tumbling flow structure at BDC which is sustained and amplified by the compression process and, in turn, causes generation of a high turbulence level before TDC [5]. In lean burn engines, cycle-by-cycle variations are more due to lean combustion. In these engines, practical approach to improve the combustion stability is to shorten combustion duration by enhanced mean in-cylinder flows and turbulence [6]. Generating a significant vortex flow (swirl and tumble) inside the IC engine cylinder during intake stroke generates high turbulence during later stage of compression stroke leading to fast burning rates [7]. The quantified in-cylinder cold flows of a four-valve, single-cylinder, pentroof chamber, optical SI engine by LDV are correlated with combustion process [8]. The laser doppler velocimetry (LDV) measurements in a diesel engine found that linear dependence of mean fluid motion and turbulence on engine speed in the range rev/min [9]. The rotating flows can significantly increase turbulence during combustion period 85

2 leading to reduced burning time and increased thermal efficiency in premixed SI engines [10]. The studies on effect of in-cylinder flows on flame initiation and propagation in modern four-valve SI engines reported that spark plug orientation relative to mean flow using shadowgraphy technique. They reported that high mean flow velocities and turbulence levels can shorten combustion duration in lean mixtures [11]. Engine incylinder flow studies using Particle Tracking Velocimetry (PTV) and reported that combustion control can be achieved through turbulence in a premixed lean burn engine and control of air-fuel mixing in a GDI engine [12]. The study on effect of curved piston configurations on tumble flows in a four-valve engine using LDV technique reported that main tumble is initially located under inlet valve/s and moves towards exhaust valve/s during compression [13]. Instantaneous velocity of motion measurements from LDV experiments at different engine speeds, results shown that almost linear dependence of mean motion and turbulence on engine speed in the range investigated [14]. The various PIV experimental measurements on various types of IC engines reported that flow structure changes substantially along cylinder length due to geometry of intake port and occurrence of tumble motion during intake stroke [15-17]. In-cylinder flow measurements using PIV at end of intake stroke on a plane between intake valves of a single-cylinder optical engine at engine speeds of 750, 2000 and 3500 rpm. They reported that tumble ratio significantly changed for engine speeds in between 2000 and 3500 rev/min [18]. Experimental investigations using Laser Doppler Anemometer (LDA) reported that fluid flow structure during intake stroke was very much affected by intake valve lift and also observed that a strong flow reversal just below intake valve at time of intake valve closure [19,20]. Engine in-cylinder flows using PTV reported that swirl and tumble flows should be optimized for achieving good combustion. [21] By using digital PIV technique on in-cylinder flows in a single-cylinder engine one can estimate parameters like turbulent length scale, vorticity and strain rate distribution, etc. [22]. The above discussion clearly depicts that a good understanding of engine in-cylinder flow structure in an IC engine is very much vital for optimization of combustion chamber. 2. EXPERIMENTAL SETUP AND EXPERIMENTS A single-cylinder, vertical, two-valve, four stroke, air-cooled engine (Table1) was coupled to an induction motor of 3.7 kw through an electronic speed controller. In this study, motor along with engine was run at a speed of 1000 rev/min. In order to facilitate PIV measurements, an extension of cylinder liner was made using a transparent cylinder ring and fitted to engine without altering compression ratio. It facilitated field of view (FOV) of size 87.5x35 mm. Intake manifold of engine was connected to a plenum to mix and supply the air and seeding particles with uniform distribution among them. The schematic view of experimental setup is shown in Fig.1. Figure 2 shows optical access made in engine for PIV measurements. Objective of this work is to study effect of shrouded (shroud plate with arc of 180 0, thickness of 3 mm and height of 8 mm) intake valve position (Fig.3b, right side position, left side position, rare side position and front side position is represented by P1, P2, P3 and P4 respectively). Table 1. Engine specifications Engine type No. of cylinders 86 4-stroke, 2-valve, motored one Bore x stroke (mm) 87.5 x 110 Compression ratio 10:1 Rated engine speed (rpm) 1500 Maximum valve lift (mm) 7.6 Intake/exhaust port diameter (mm) 28.5 Intake valve opening (CAD btdc) 4.5 Intake valve closing (CAD abdc) 35 Exhaust valve opening (CAD bbdc) 35 Exhaust valve closing (CAD atdc) 4.5

3 The PIV system consists of a double pulsed ND-YAG laser with 200 mj/pulse energy at 532 nm wave length, a CCD camera of resolution 2048x2048 pixels with a frame rate of 14 per second and a set of laser and camera controllers, and a data acquisition system and a software (Fig.1). The triggering signals for laser and camera were generated by a crank angle encoder mounted on engine crankshaft with a resolution of one CAD. These signals were supplied to controllers via a signal modulator. A master signal of crank angle encoder was set to occur at suction top dead center (TDC) of engine (considered as a zero CAD). The triggering signals for laser and camera at required CAD can be set within software. A seeding unit was used to generate fine particles of one micron size with Di-Ethyl-Hexyl-Sebacat (C 26 H 50 O 4 ) as a seeding material. The seeding density was controlled very accurately by varying the amount of pressurized air supplied to seeding unit. The laser sheet of 0.5 mm thickness was aligned with and camera was placed to view FOV which was set on a vertical plane passing through axis of cylinder. 1. Engine, 2. Motor, 3. Encoder, 4. Test bench, 5. Speed controller, 6. Intake plenum, 7. Air compressor, 8. Seeder, 9. Camera, 10. ND-YAG Laser, 11. Laser sheet, 12. Signal modulator, 13. Data acquisition system Fig.1. Schematic diagram of experimental setup Fig.2. Transparent cylinder liner for optical access In this study, in-cylinder tumble flow measurements have been carried out during suction (30 to 180 CADs) and compression (210 to 330 CADs) strokes in steps of 30 CAD and at every measuring point, 500 image pairs were recorded and stored. The time interval (Δt) between the two images of an image pair was evaluated (6 µs for suction and 8 µs for compression strokes) based on pixel shift (< 5 pixels), FOV, maximum expected velocity of flow in FOV and resolution of the camera [23, 24].To minimize light reflections, a band-pass filter with central wavelength of 532 nm was mounted on camera. During PIV recording, camera focus was set to 3 (f = 3) to obtain sharp particle images. LaVision DAVIS (Data acquisition and visualization software) was used for image acquisition and data post-processing. 87

4 a). Shroud valve b). Position (P) of the shroud Fig.3. Shroud Valve and Positions During post-processing, interrogation window size of 32x32 pixels with multi-pass cross-correlation algorithm has been used [25]. Ensemble average velocity vectors were computed from 500 raw image pairs at required CAD. The typical raw image at 30 CAD is shown in Fig.6. It was observed that, at an engine speed of 1000 rev/min, maximum amplitude of vibration of cylinder liner at point of measurement was equal to microns during Δt, separation time between images, whose effect on error of measurement can be neglected. Before conducting experiments, calibration of in-cylinder environment as a whole was done along with Plexiglass liner. The calibration procedure was repeated until a root mean square (rms) fit error was less than 1.5 [23] (LaVision, 2006). During the experiments, it was also ensured that clear images were captured every time. 3. RESULTS AND DISCUSSION The tumble motion must be evaluated under transient conditions due to significant effect of piston shape and its motion on it unlike swirl. Swirl ratio can be obtained through steady flow experiments [21]. It is very much clear that, vortex and air flow patterns occurring at 330 CAD is very much required [24]. This study deals with in-cylinder tumble flows under transient engine conditions. Figure 4 show ensemble average velocity vectors of in-cylinder tumble flows with superimposed streamline patterns at 330 CAD. It is good to have more air movement at 330 CAD for better air-fuel mixing and combustion. Good charge stratification is crucial and it is achieved with strong tumble flow [28]. Even though, present study, deals only with incylinder air flow pattern, however it almost depicts mixture flow pattern as well, because mixtures are very lean in these SI engines. Therefore, study of tumble air flow pattern at 330 CAD is very much useful especially for stratified charged and direct injection SI engines. Figure 4 (a) illustrates in-cylinder flow structure for baseline configuration using unmodified base intake valve. Figure 4 (b) illustrates in-cylinder flow structure for modified intake valve for different shroud positions. These velocity fields indicate presence of a flow structure comprised of CW rotating vortex within combustion space. From Fig.4 (a) and (b), it is observed that there is a clear large vortex flow structure of the air flow with base valve in comparison with different shroud positions. For better diagnosis and quantification, tumble ratios and average turbulent kinetic energies were evaluated for entire cycle in the following sections. 88

5 Tumble ratio International Journal of Engineering Technology, Management and Applied Sciences a). Base intake valve (b). Shrouded intake valve Fig.4. Ensemble average velocity vectors with different intake valves at 330 CAD Variation of Tumble Ratio In this study, in-cylinder tumble flows are characterized by tumble ratio (TR) calculated as per Huang et al. [27]. Here, the TR is defined as ratio of mean angular velocity of vortices on target plane to average angular velocity of crank. Irrespective of formation of vortex and air flow patterns during early stages, study of air flow pattern at 330 CAD is very much required. For this, a bar chart (Fig.5) comparison between tumble ratios at 330 CAD before TDC indicated that in-cylinder tumble was highest for standard intake valve and lower for shrouded intake valve irrespective of its position. There is no apparent well-defined tumbling flow structure at any CAD with shrouded valve (not shown here) indicated a general lack of dominant or clear tumble flow structure. The reason for this is that the jet flow emerging from the intake valve does not interact directly with the cylinder walls to produce the tumbling flow structure due to newly attached shroud plate which disturbed the flow in entry level itself. Stronger the tumble motion during suction stroke; more turbulent kinetic energy released by its breakdown during late compression stroke towards TDC. It guarantees higher turbulence levels at the time of ignition [6]. It is found that, standard intake valve have resulted in highest TR compared to shrouded valve. Shroud valve with positions P1, P2, P3 and P4 shown TR of 0.067, 0.054, and where as base valve shown TR of 0.11 at end of compression (330 CAD). The highest TR produced shroud valve with positions P1 showed about % decrease in comparison with base valve operation at end of compression (330 CAD) Standard Shroud_P1 Shroud_P2 Shroud_P3 Shroud_P4 Shroud position Fig.5. Tumble Ratio at 330 CAD 89

6 Average turbulent kinetic energy, (m/s)^2 International Journal of Engineering Technology, Management and Applied Sciences 3.2. Variation of Average Turbulent Kinetic Energy (TKE) The TKE of flow indicates its strength as a whole. Figure 6 shows the bar chart of TKE at 330 CAD for different valves. The TKE is more for shrouded compared to base. This may be due to that in the cases of shrouded valve since beginning of suction the entire tumble flow pattern disturbed randomly. In summary, even though there is higher TKE with shrouded compared to base valve during suction and compression, but at 330 CAD this kind of flow is not suitable for stratified situations, it may be suitable for premixed homogeneous situations. The shroud valve with positions P1, P2, P3 and P4 showed higher TKE of 14.45, 13.19, and 6.03 m 2 /s 2 where as base valve shown TKE of 3.73 m 2 /s 2 at the end of compression (330 CAD). The highest TKE is produced by shroud valve with positions P1 about 6.37 times higher than base valve at the end of compression (330 CAD) Standard Shroud_P1 Shroud_P2 Shroud_P3 Shroud_P4 Shroud position Fig.6. TKE for 330CAD 4. CONCLUSIONS Irrespective of position of shroud plate, shrouded valve providing more TKE and poor TR in compression with base valve. The base intake valve generated a well-defined tumbling flow structure at BDC which is sustained towards end of compression process. The shrouded valve does not produce any defined tumble at any CAD irrespective of its position during suction and compression. The highest TR produced shroud valve with positions P1 has shown about % decrease in compression with base valve operation at end of compression (330 CAD). The highest TKE is produced by shroud valve with positions P1 about 6.37 times higher than base valve at end of compression (330 CAD). In summary, even though there is higher TKE with shrouded compared to base valve during suction and compression, but at 330 CAD this kind of flow is not suitable for stratified situations, it may be suitable for premixed homogeneous situations. REFERENCES 1. Kern Y. Kang, Je H. Baek. Tumble Flow and Turbulence Characteristics in a Small Four-Valve Engine, SAE Li. Y, H Zhao, B Leach, and T Ma. Development of a Fuel Stratification Spark Ignition Engine, Proc. IMechE Vol. 219 Part D: J. Automobile Engineering, Lee. K H and C S Lee. Effects Of Tumble And Swirl Flow On Turbulence Scale Near Top Dead Centre In A Four- Valve Spark Ignition Engine., Proc. Instn Mech. Engrs Vol. 217 Part D: J. Automobile Engineering, Lee. K, C Lee and Y Joo. Optimization Of The Intake Port Shape For A Five-Valve Gasoline Engine, Proc Instn Mech Engrs Vol 215 Part D, Khalighi.B, Daniel C. Haworth, and Mark S. Huebler. Multidimensional Port andin-cylinder Flow Calculations and Flow Visualization Study in an Internal Combustion Engine with Different Intake Configurations, SAE Paper No Arocoumanis, C., Hu, Z., Vafidis, C. and J. H. Whitelaw. Tumbling Motion: A Mechanism for Turbulence Enhancement in Spark-Ignition Engines, 1990, SAE Paper No

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