Simulation Based Approach for Optimization of Intake Manifold 2011-26-0074 Published on 19th-21st January 2011 SIAT, India Devananda B Pai, Hari Shankar Singh and P V Fayaz Muhammed Maruti Suzuki India Limited, India Copyright © 2011 SAE International and Copyright © 2011 SIAT, India ABSTRACT INTRODUCTION Change is only thing which is permanent in today’s world. In a highly volatile market, one can only survive by being fast, flexible and first mover. This requires analyzing market situation, conceiving new models and developing them in the minimum possible time. Simulation is a very powerful tool for squeezing model development cycle. It significantly reduces time required for optimization and also gives the designer flexibility of going for wide variety of design options, which is not feasible otherwise through experimentation. Simulation also helps in detailed understanding of the physical phenomenon of the respective system. The Intake system of an engine regulates the air-charge inside the combustion chamber. For a gasoline engine, the five main parts of an intake system, apart from sensors, are: Ducts, resonators, air cleaner, the intake manifold and throttle body. The torque output obtained from an engine is determined by its volumetric efficiency. The volumetric efficiency of the engine is dependent upon the quantity of air charge present inside the combustion chamber which in-turn is controlled by the intake System. Detailed Analysis was done using 1-D CFD code to optimize various parameters of the Intake Manifold at the initial design stage. Detailed virtual DOE was done to achieve the required level of engine performance. The numbers of concepts were limited for final engine bench testing resulting in reduction of the testing time. Strong co-relation was achieved between the simulation and actual testing data. The design considerations that affects yield from the intake system are: Intake Manifold Runner length Intake Manifold Runner diameter Intake Manifold Plenum volume Air Cleaner volume and For further fine tuning 1D and 3D CFD codes were coupled to enable better optimization of the intake manifold geometry. This also led to better prediction of the engine performance. Clean-Side Duct length Dirty Side Duct This paper describes the effective use of advanced simulation tools for design optimization of intake manifold. The air cleaner volume and dirty/clean side ducts help in getting maximum air at optimum temperature. These parts also play a vital role in the noise characteristics of the engine intake system. System designer typically uses intake manifold to tune the torque output as per the requirement. Separate simulation was carried out for capturing the 3D effects of intake manifold geometry on 3D CFD code using standard boundary conditions. Keywords : Simulation, Intake Manifold, 1D-CFD, DOE, 3D-CFD, Optimization, Brake Torque 1 Symposium on International Automotive Technology 2011 The optimal design of the intake manifold has the objective of getting minimum flow resistance, good air distribution between cylinders and runners that take advantage of the ram and tuning effects, trying to achieve the maximum cylinder charge for a selected operating range of the Engine. Initially, simulation base file of a 3 cylinder Spark Ignition Engine was created in GT Power. This virtual engine consists of the entire engine components starting from air Suction pipe to exhaust tail pipe. The geometrical parameters including section diameters and flow length of different components was obtained from CAD drawings. The other parameters required for conducting analysis are pressure drop across engine valves and a definition of combustion inside the cylinder. IMPORTANCE OF SIMULATION IN INTAKE DESIGN Traditionally intake manifolds are designed by experimental approach, in which various prototype intake manifolds would be built and tested on the engine. Intake manifold performance for each build would be assessed from the evaluation of experimentally determined data. This “cut and try” approach of intake manifold development can be very costly and time consuming, and it may lead to an acceptable but not necessarily optimal solution. Complete Intake system simulation, on the other hand, can often be an extremely rapid, cost effective and insightful method of getting a peak solution and understanding of physical phenomena behind it. SCOPE OF INTAKE TUNING The Pressure drop across engine valves was represented with measured coefficient of discharge from the flow bench and the combustion was defined by measured in-cylinder pressure across engine cycle (P-θ) using a combustion analyzer. The subsequent step was to simulate the base file using GT Power and match the result (performance results) with the actual bench testing data. The measured and predicted engine torque and volumetric efficiency are shown in Fig. 1 and 2 respectively. The predicted torque and volumetric efficiency show good correlation to data throughout the speed range. The maximum error in torque and volumetric efficiency was about 4%. MANIFOLD The customers of overseas markets are quite different from their Indian counterparts. In European countries and Japan, the cars tend to travel at speeds greater than 100 km/h quite frequently and the engine revs more than 4000 speed to achieve this speed. The torque output of the engine needs to be adequate at this speed range in order to provide a comfortable drive. So engines are normally designed by providing a healthy torque in higher speed band. On the contrary, the Indian traffic condition rarely allows driver such a luxury. The Engine’s operating speed in Indian road condition hardly crosses 4000. In India, the customers have to stop and accelerate very frequently. Consequently a good low-end torque turns out to be handful. For a naturally aspirated engine, the torque distribution between lower and higher speeds is a trade-off. Considering the growing Indian market and regular operating speed range in India, it was reasonable to think of re-tuning engine to suit the Indian road conditions, giving a better low end torque. Figure 1. Simulation and Test Data Comparison of Brake Torque DESIGN OF EXPERIMENTS After the matching of virtual output to actual bench test data, the focus now shifted to the design of experiments and associated iterations in order to achieve an improved low–end torque. Thus the functional target was set - improving the low-end torque. SIMULATION METHODOLOGY The effect of varying critical parameters on Engine output were assessed as explained below The simulation of the Engine discussed herein was conducted using GT Power simulation code. The GT Power code contains comprehensive engine performance models built on top of a 1D Computational Fluid Dynamics (CFD) code, thus allowing the prediction of both engine performance quantities as well as the characterization of intake system dynamics. Manifold Runner Length As the intake valve opens, a rarefaction zone is created behind the intake valve and this low-pressure region starts 2 Symposium on International Automotive Technology 2011 Table 1. Iterations Details with respect to Runner Length Figure 2. Simulation and Test Data Comparison of Volumetric Efficiency to move away from the cylinder through the runner. When this zone reaches the plenum - all contiguous air rushes to this low-pressure zone and it forms a compression zone. This compression wave moves back through the runner towards the cylinder. As it reaches the cylinder it creates a ram effect and maximum air enters into the cylinder. This wave travels with a velocity of sound. At the same time piston moves down and reaches its maximum velocity at 850 ATDC simultaneously create maximum vacuum inside cylinder. For a perfectly tuned manifold the ram air has to reach the cylinder at the same time when there is maximum vacuum inside the cylinder (at 85 degree crank-angle after TDC) so that maximum air enters the cylinder. But as the engine speed varies these two incidents do not happen at the same time. Therefore peak volumetric efficiency at one particular speed which represents a peak point in the engine torque curve. As the intake manifold runner length changes the distance which compression and rarefaction waves travel also changes, and the peak torque point shifts away from its earlier peak. Figure 3. Variation of Brake Torque with respect to Intake Manifold Runner Length Manifold Runner Diameter At lower speeds a smaller runner diameter would increase the air-flow velocity and therefore facilitate entry of more air-charge inside the cylinder. On the contrary, at higher operating speeds, a lower runner diameter would result in choking effect or turbulence and consequently lower volumetric efficiency. After conducting several simulations it was found that a diameter reduction of 5mm gives an improvement of output torque in the lower speeds without much concession in the higher speeds. Table 2 shows the iterations details with respect to runner diameter. Fig. 4 shows the variation of brake torque with respect to intake manifold runner diameter. When the runner length increases, it delays the arrival of compression wave at the intake valve and this tends to shift the peak torque point to a lower speed. In simpler terms – If an engine is tuned for a given speed and if the runner length is increased, the arrival of compression wave at intake valve happens after 850 for the corresponding rated speed. As the runner length increases peak torque point shifts to a lower speed. So iterations done with different runner length and observed the variation in torque and power. A runner length increase of 120mm found to be good option by considering lay out and performance. Table 1 shows the iterations details with respect to runner length. Fig. 3 shows the variation of brake torque with respect to intake manifold runner length. Table 2. Iterations Details with respect to Runner Diameter Plenum Volume At higher speeds a large Plenum volume caters to the higher mass flow requirements of the Engine giving a minor improvement in output. On the contrary, the experimental data shows that the throttle response for higher plenum volumes is poor. After conducting several simulations, it was found that a plenum volume increase of 0.2 liter gives good performance. This factor is practically constrained by the space available in the engine room. 3 Symposium on International Automotive Technology 2011 with respect to air cleaner volume. Fig. 6 shows the variation of brake torque with respect to air cleaner volume. Table 4. Iterations Details with respect to Air Cleaner Volume Figure 4. Variation of Brake Torque with respect to Manifold Runner Diameter Table 3 shows the iterations details with respect to plenum volume. Fig. 5 shows the variation of brake torque with respect to intake manifold plenum volume. Table 3. Iterations Details with respect to Plenum Volume Figure 6. Variation of Brake Torque with respect to Air Cleaner Volume Air Cleaner Hose Length The functioning of an intake system can be considered as being analogous to a two-spring - two-mass system with two separate natural frequencies. These two frequencies represent the two peaks in torque curve. The increase or decrease in hose-length corresponds to a change in mass of the system resulting in a change in distance between the two peaks in the torque curve. An increase in the hose length would increase the distance between the two torque peaks. In test engine, an increase in hose length results in a decrease in the peak torque value. But as already mentioned it would cause an improvement in the lower and upper range torques. A hose length increase of 60 mm was found to be optimal in terms of lay out and performance. Table 5 shows the iterations details with respect to air cleaner hose length. Fig. 7 shows the variation of brake torque with respect to air cleaner hose length. Figure 5. Variation of Brake Torque with respect to Plenum Volume Air Cleaner Volume Table 5. Iterations Details with respect to Air Cleaner Hose Length The air cleaner volume also enhances the torque at lower speeds but only up to a limited increase in Air Cleaner volume. Any further increase only helps in improving the acoustics performance as it reduces low frequency noises. A large air cleaner avoids the usage of a resonator. Based on constraints and targets, a volume increase of 1.5 liter worked out to be a good solution. Table 4 shows the iterations details 4 Symposium on International Automotive Technology 2011 Figure 7. Variation of Brake Torque with Respect to Air Cleaner Hose Length Combined Results Figure 9. Combined Simulation: Predicted Vol Efficiency Output vs. Speed The optimal value for each parameter was chosen from the above study and the results were calculated in a combined simulation exercise. The present and suggested values of these parameters have been shown in Table 6. Fig. 8 and 9 show the combined results for brake torque and volumetric efficiency respectively. After arriving at this optimized solution, a proto part was developed and actual bench testing was conducted on the physical part. The following results were obtained for the bench test: Table 6. Present and Suggested Values of Design Parameters The result shows an overall improvement in torque values in line with the prediction. There is a deviation observed between actual and simulated torque curve in the speed range between 1000 and 3000 rpm. This is primarily accounted due to the influence of idle air control valve, which was not taken into consideration in the simulation because of its complexity. Also changed knock characteristics in lower speed in the new engine with respect to the present engine reduced the accuracy of simulation. Fig. 10 and 11 show the actual testing results for brake torque and volumetric efficiency respectively. Figure 8. Combined Simulation: Predicted Brake Torque Output vs. Speed Figure 10. Actual Testing results: Brake Torque vs. Speed 5 Symposium on International Automotive Technology 2011 0.2 Nm of torque increase was found due to the presence of bell mouth. Fig. 14 shows the variation of brake torque with respect to bell mouth. Figure 11. Actual Testing Results: Vol Efficiency vs. Speed Figure 12. Intake Manifold Without Bell Mouth The better performance at the higher speed were majorly contributed by the flow resistance. This was improved in the new design by changing the material of manifold from aluminium to plastic for experimentation. T H E O P T I M I S AT I O N O F F L O W JUNCTIONS The 1D software GT POWER is used to decide the basic parameters like runner length, runner diameter and the plenum volume and its effect on engine performance parameters like torque and power. Since GT POWER is 1D software it is not possible to capture the effects of complex 3D geometry of the intake manifold on the engine performance. Standalone analysis is done in 3D CFD software STAR CD to calculate the air flow resistance of the Intake Manifold and to find the charge distribution in each cylinder. Standard boundary conditions are used for the simulation against the actual engine boundary conditions and also it is difficult to correlate the results of 3D analysis on engine performance. Figure 13. Intake Manifold With Bell Mouth Coupling the 2 software’s: GT POWER and STAR CD, with complex 3D Geometry represented in STAR CD and engine boundary conditions from GT POWER eliminates the disadvantages of both the software’s and also helps in detailed understanding of the flow inside the intake manifold. This coupled analysis is used to find out effect of different intake manifold geometry on engine performance. Study was done on intake manifold with different bell mouth configurations to optimize flow junctions in order to get a reduced pressure drop. Figs. 12 and 13 show the intake manifold without and with bell mouth respectively. Simulation was carried out at the rated speed (6000 rpm, the speed at which maximum air-flow happens) and nearly Figure 14. Simulation Results: Brake Torque vs. Speed 6 Symposium on International Automotive Technology 2011 CONCLUSIONS Some guidelines for the design of Intake system of 3 Cylinder naturally aspirated spark-ignition engine has been reported. A 1-D analysis was utilized for a more refined definition of its geometrical characteristics, which also take in to account constraints imposed by layout of Engine bay .A further refinement is done with help of 3-D CFD software to get exact shape of flow components. The comparison with experimental data has confirmed the robustness of the whole design procedure. 3. John J Silvestri, Thomas and Michael Costallo, “Study of Intake System wave Dynamics and Accoustics by Simulation and Experiment”, SAE Paper No. 940206, 1994 4. John B Haywood, “Internal Combustion Engine Fundamentals”, McGRAW HILL INTERNATIONAL EDITIONS Automotive Technology Series 5. GT-Power, user’s manual and tutorial, GT-SUITETM Version 6.0, March 2003, Gamma Technologies ACKNOWLEDGMENTS CONTACT To Mr. Piyush Agrawal and Mr. Prasenjit Khan for the consistent support and encouragement received from them. P V Fayaz Muhammed Assistant Manager, EN1D, ERD1 Maruti Suzuki India Ltd Email : MuhammedFayaz.PV@maruti.co.in REFERENCES 1. Carl H Wolgemuth and Donald R Alson., “Study of Engine Breathing Charecteristics”, Pensylvania State University. SAE Paper No. 650448, 1965 2. Rafel Royo, Jose Corberan and Antonio Perez, “Optimal Design of the Intake System”, Universidad Politechnica de Valencia. SAE Paper No. 940210, 1994 The Technical Paper Review Committee (TPRC) SIAT 2011 has approved this paper for publication. This paper is reviewed by a minimum of three (3) subject experts and follows SAE guidelines. Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SIAT 2011. The author is solely responsible for the content of the paper. SAE Customer Service: Tel: 877-606-7323 (inside USA and Canada) Tel: 724-776-4970 (outside USA) Fax: 724-776-0790 Email: CustomerService@sae.org SAE Web Address: http://www.sae.org Printed in USA All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form by any means, electronic, mechanical photocopying, recording, or otherwise, without the prior written permission of SIAT 2011. 7