Clamping Force Mechanism of Electro-Mechanical Continuously Variable Transmission for Automotive

Transcription

1 ISSN: Australian Journal of Basic and Applied Sciences Journal home page: Clamping Force Mechanism of Electro-Mechanical Continuously Variable Transmission for Automotive Izhari Izmi Mazali, Kamarul Baharin Tawi, Bambang Supriyo, Mohd Sabri Che Kob, Nurulakmar Abu Husain, Mohd Salman Che Kob Faculty of Mechanical Engineering, University Technology Malaysia, Johor Bahru, Johor, MALAYSIA A R T I C L E I N F O Article history: Received 28 January 2015 Accepted 25 February 2015 Available online 6 March 2015 Keywords: clamping force, continuously variable transmission, automotive, Belleville spring, electro-mechanical A B S T R A C T Pulley-based continuously variable transmission (CVT) is a type of transmission that is widely used currently for automotive application. In a pulley-based CVT, torque is transferred from engine to wheels by using the friction force between the V-belt and the pulleys without using any discrete gears. Therefore, providing sufficient clamping force on the V-belt is very important in order to allow the torque to be transferred from the engine to the wheels. However, due to application of electro-hydro-mechanical (EHM) actuation system in the existing pulley-based CVT, the process to control sufficient clamping force becomes quite complicated. This leads to the inefficiency of the transmission, which eventually increase the fuel consumption of the car. In this paper, the working principle of the pulley-based CVT is briefly described. Consequently, the proposed clamping force mechanism for electro-mechanical (EM) CVT is explained and its benefits are discussed AENSI Publisher All rights reserved. To Cite This Article: Izhari Izmi Mazali, Kamarul Baharin Tawi, Bambang Supriyo, Mohd Sabri Che Kob, Nurulakmar Abu Husain, Mohd Salman Che Kob, Clamping Force Mechanism of Electro-Mechanical Continuously Variable Transmission for Automotive. Aust. J. Basic & Appl. Sci., 9(9): 31-36, 2015 INTRODUCTION Producing an efficient transmission for applications in automotive is one of the biggest challenges currently faced by automotive original equipment manufacturers (OEMs). An efficient transmission is vital for improving the efficiency of car s powertrain system which eventually leads to the improvement in car s fuel economy. There are many types of automotive transmissions used presently by the automotive OEMs and pulley-based continuously variable transmission (CVT) is one of them. Pulleybased CVT, unlike conventional stepped transmissions, uses no discrete gears. Instead, it applies pulleys and metal pushing V-belt (V-belt) to continuously change its ratio. As a result, it offers continuous ratio change, wide ratio coverage and also potential in weight reduction due to the no application of discrete gears. All these factors contribute in making the car s powertrain system more efficient. In a pulley-based CVT, the process to change CVT ratio from low ratio to overdrive ratio is achieved through the axial movement of the primary pulley and the secondary pulley simultaneously shown in Fig. 1 (Klaassen, 2007). In this figure, the light gray pulley and the dark gray pulley represent the primary pulley (connected to the engine) and the secondary pulley (connected to the wheels) respectively. The axial movement of these pulleys results in the change of the V-belt radius on both pulleys; hence the CVT ratio is changed accordingly. Finally, sufficient clamping force is exerted from the pulleys to the V-belt in order to avoid the V-belt from slipping, so that the torque can be transferred successfully from the primary pulley to the secondary pulley. Nevertheless, the existing pulley-based CVT in the global markets still poses the issues of power loss between pulleys, high power consumption from the internal combustion engine (ICE) and also V-belt s misalignment (Akehurst, 2001; Bonsen et al., 2003; Faye, 2009; Kirchner, 2007; Klaassen, 2007; Tawi, 1997; Tawi et al., 2013). The important factors that cause these issues are the electro-hydro-mechanical (EHM) system to actuate pulley s movement and the high clamping force on the V-belt. Corresponding Author: Izhari Izmi Mazali, Drivetrain Research Group, Faculty of Mechanical Engineering, University Technology Malaysia, Johor Bahru, Johor, Malaysia. izmi86@yahoo.com

2 32 Izhari Izmi Mazali et al, 2015 Fig. 1: CVT ratio in a pulley-based CVT from low ratio (A) to medium ratio (B) and overdrive ratio (C). Clamping force in a pulley-based cvt : The objective of an automotive transmission is to transfer torque from the car s engine to the wheels. Conventionally, this role is fulfilled through a set of discrete gears. In a pulley-based CVT, however, the torque is transferred from the primary pulley to the secondary pulley through the V-belt. In order to achieve this, sufficient clamping force on the V-belt is required. The sufficient clamping force, Fc, produces normal force, FN, which results in the traction force, Ff, between a V-belt s element and the pulley (Fig. 2). The relationship between Fc, FN and Ff are shown in mathematical equations (1) and (2). Ff then transmits the torque from the primary pulley to the secondary pulley through the V-belt. The relationship between Ff and the torque transferred from the primary pulley (Tin) to the secondary pulley (Tout) are shown in mathematical equations (3) and (4). In addition, μ represents traction coefficient between the pulley and the V-belt s element and α represents the angle of the pulley. Rp and Rs, on the other hand, are the radii of the V-belt on the primary pulley and the secondary pulley respectively. Fig. 2: Clamping force, Fc, exerted on a V-belt s element from the pulley and the resulted normal force, FN, on the element. (1) (2) (3) (4) Currently, most of the pulley-based CVTs in the market use EHM system to provide sufficient clamping force on the V-belt. In this system, the clamping force on the V-belt is provided through the hydraulic pressure that is produced in the EHM system. However, this system still has some limitations that hinder the real potentials of the pulley-based CVT. According to Bonsen (2005) and Klaassen (2007), the process to control sufficient clamping force on the V-belt through EHM clamping force mechanism is quite complicated due to the compressibility of the hydraulic oil. Furthermore, in order to effectively control the clamping force, an additional hydraulic control system is also required in the EHM clamping force mechanism (Tawi et al., 2013). Because of these factors, for safety purpose, most car and transmission manufacturers set the clamping force at the minimum 30% more than required in the normal operation condition (Bonsen et al., 2005). This, unfortunately, results in the high

3 33 Izhari Izmi Mazali et al, 2015 power consumption from ICE, which is detrimental to the fuel economy of the car. Besides, with the EHM system, the process to maintain the required clamping force also leads to the high power consumption from ICE. This is because the clamping force on the V-belt is exerted through the hydraulic pressure from the EHM system. In order to maintain the clamping force, continuous hydraulic pressure is required, thus ICE must provide continuous power to produce the required hydraulic pressure continuously. This process also results in very high power consumption from ICE, making the powertrain system of the car less efficient. Proposed clamping force mechanism of electromechanical cvt: The proposed clamping force mechanism of electro-mechanical (EM) CVT is developed as a continuation of Electro-Mechanical Dual-Acting Pulley CVT (EMDAP CVT) developed earlier by the researchers from Universiti Teknologi Malaysia (Tawi et al., 2013; Supriyo et al., 2012; Supriyo et al., 2013). The schematic diagram of this proposed mechanism is illustrated in Fig. 3 and it essentially uses 2 Belleville springs and a DC motor (the secondary DC motor) on the secondary pulley to provide clamping force on the V-belt. Fig. 3: The proposed clamping force mechanism of EM CVT. During the process of changing the CVT ratio, the Belleville springs are responsible to provide continuous clamping force on the V-belt to avoid excessive V-belt s slipping. The role of the secondary DC motor is to adjust the clamping force whenever required. To adjust the clamping force, sufficient torque from the secondary DC motor is delivered to the moving power screw through the clamping gears (Fig. 3). Then, the moving power screw will adjust the compression of the Belleville spring accordingly, hence results in the changing of the effective clamping force on the V-belt. Fig. 4 illustrates the process of adjusting the compression of the Belleville in order to increase or decrease the effective clamping force on the V-belt. The actual prototype of this proposal is shown in Fig. 5. Fig. 4: The process of increasing (A) and decreasing (B) the effective clamping force on the V-belt.

4 34 Izhari Izmi Mazali et al, 2015 Fig. 5: The actual prototype of the proposed clamping force mechanism of EM CVT. Specifications of the belleville springs: According to Supriyo (2011), the required clamping force on the V-belt for 1.6L gasoline engine is in the range of 8kN to 10kN. Hence, the selected Belleville springs for application in this proposed clamping force mechanism must be able to handle the compression force of up to 11kN for safety purpose. The selected Belleville spring is the Belleville spring (disc spring) DIN 2093-C112 and it has an outer diameter of 112mm, inner diameter of 57mm, thickness of 3mm and a maximum deflection (compression) of 3.9mm. According to the specifications provided by Wittel et al., 2011, with a compression of 75%, this spring will have a compression force of 10.5kN and it will have a compression force of around 11.5kN when fully flat (compression 100%). The graph showing the relationship between the compression of the spring and the resulted compression force is illustrated in Fig. 6 and the disc spring s actual picture is shown in Fig. 7. Fig. 6: Relationship between the compression of spring and the resulted compression force of the Belleville spring DIN 2093-C112.

5 35 Izhari Izmi Mazali et al, 2015 Fig. 7: Belleville spring DIN 2093-C112 for the proposed clamping force mechanism. Potential benefits of the proposed clamping force mechanism: There are three significant benefits that can be reaped from the proposed clamping force mechanism of EM CVT, namely no power consumption from ICE to maintain clamping force on the V-belt, simple process to provide the sufficient clamping force and also minimum misalignment of the V-belt. Firstly, the proposed clamping mechanism allows the clamping force on the V-belt to be adjusted through the compression condition of the Belleville spring. After the adjustment, the compression of the Belleville spring can be retained through the thread design between the moving power screw and the clamping gear. As a result, no extra power required from ICE in order to maintain the clamping force on the V-belt. Since no extra and continuous power is required from ICE to maintain the clamping force, thus more power can be channeled to the car s wheels. This will definitely make the powertrain system of the car more efficient, hence significantly improves the fuel economy of the car. In addition, there is a potential to make the process of providing the sufficient clamping force simpler and more effectively. This is because of the application of the Belleville spring as a mean to provide the clamping force instead of the hydraulic oil. In the proposed mechanism, the clamping force can be increased or decreased by just compressing or releasing the Belleville spring accordingly. Moreover, by eliminating the hydraulic oil from the mechanism, the issue of its compressibility and the possible leakage, which are the important weaknesses of the hydraulic system, can be dismissed (Klaassen, 2007; Kirchner, 2007). Hence, the process to provide and to control sufficient clamping force becomes simpler and more effective. With the simple and more effective process to control the sufficient clamping force, the excessively high clamping force on the V-belt can potentially be avoided. A study by Vogelaar (2010) suggests that by avoiding the excessively high clamping force on the V-belt through an effective and efficient control of the clamping force, the efficiency of the CVT is improved significantly. As a result, it offers a possibility to notably reduce the fuel consumption of the car. The other potential benefit from the proposed clamping force mechanism is the minimum effect of the V-belt s misalignment, which is achieved through the dual-acting pulley mechanism. With this mechanism, the clamping force can be increased consistently from both sheaves of the secondary pulley, therefore minimizing the effect of the V- belt s misalignment. As suggested by Tawi (1997) and Faye (2009), the misalignment of the V-belt reduces the lifespan of the V-belt and pulleys while also decreases the efficiency of the CVT. For that reason, by minimizing the effect of the V-belt s misalignment, there is a potential to improve the efficiency of the CVT even further, while at the same time increasing the durability of the CVT s components especially the V-belt and pulleys. Conclusion: With the proposed clamping force mechanism for EM CVT discussed in this paper, the issues of power loss, high power consumption from ICE and also V-belt s misalignment can be significantly minimized, hence improves the efficiency of the CVT. Eventually, with the more efficient CVT in the car s powertrain system, the fuel economy of the car can be improved. For the future works, a clamping force testing on the prototype will be performed. The objective of this testing is to evaluate the actual

6 36 Izhari Izmi Mazali et al, 2015 compression capability of the selected Belleville spring. After that, the CVT ratio testing on the prototype will be carried out to investigate the workability of the prototype. These tests are very crucial to prepare the prototype for a potential onthe-road testing in the future. ACKNOWLEDGEMENT The authors would like to thank all parties that have involved in the project especially Universiti Teknologi Malaysia (UTM) and Malaysia s Ministry of Science, Technology and Innovation (MOSTI) for providing the necessary supports especially in term of funding (MOSTI s Technofund under reference number TF0609D131 and UTM s GUP Tier 2 Vot No. 06J85). REFERENCES Akehurst, S., An Investigation into the Loss Mechanisms Associated with a Pushing Metal V-belt Continuously Variable Transmission. Ph.D. Thesis, University of Bath, United Kingdom. Bonsen, B., Efficiency optimization of the push-belt CVT by variator slip control. PhD. Thesis, Technische Universiteit Eindhoven, Netherlands. Bonsen, B., R.J. Pulles, S.W.H. Simons, M. Steinbuch and P.A. Veenhuizen, Implementation of a slip controlled CVT in a production vehicle IEEE Conference on Control Applications, Toronto, Canada: pp: Bonsen, B., T.W.G.L. Klaassen, K.G.O.v.d. Meerakker, M. Steinbuch and P.A. Veenhuizen., Analysis of slip in a continuously variable transmission. Proceedings Dynamic Systems and Control, ASME International Mechanical Engineering Congress and Exposition (IMECE), Washington, USA, 1-2: Faye, Z., Study on Electro-Hydraulic Control System for CVT Metal Belt Axial Misalignment IEEE International Conference on Mechatronics and Automation, Changchun, China, pp: Kirchner, E., Leistungsübertragung in Fahrzeuggetrieben; Grundlagen der Auslegung, Entwicklung und Validierung von Fahrzeuggetrieben und deren Komponente. Springer, Heidelberg, Germany. Klaassen, T.W.G.L., The Empact CVT; Dynamics and Control of an Electromechanically Actuated CVT. PhD. Thesis, Technische Universiteit Eindhoven, Netherlands. Nishizawa, H., H. Yamaguchi and H. Suzuki, Friction Characteristics Analysis for Clamping Force Setup in Metal V-belt Type CVTs. Technical Journal/R&D Review of Toyota CRDL, Special Issue: Basic Analysis Towards Further Development of Continuously Variable Transmissions, 40(3): Supriyo, B., K.B. Tawi, H. Jamaluddin, A. Budianto and I.I. Mazali, Shifting Performance Fuzzy-PID Ratio Controller of Electro-Mechanical Continuously Variable Transmission. Latest Trends in Circuits, Control and Signal Processing, Proceedings 3rd International Conference on Circuits, Systems, Control, Signals, Barcelona, Spain, pp: Supriyo, B., K.B. Tawi, M. Hussein, I.I. Mazali, M.S.C. Kob, M. Azwarie and Y.Z. Abidin, Ratio Calibration of Electro-Mechanical Dual Acting Pulley Continuously Variable Transmission System. Latest Trends in Circuits, Control and Signal Processing, Proceedings 13th International Conference on Instrumentation, Measurement, Circuits and Systems (IMCAS 13), Kuala Lumpur, Malaysia, pp: Tarutani, I., H. Tani and Y. Nagasawa, Analysis of the Power Transmission Characteristics of a Metal V-belt Type CVT. Technical Journal/R&D Review of Toyota CRDL, Special Issue: Basic Analysis Towards Further Development of Continuously Variable Transmissions, 40(3): Tawi, K.B., Investigation of Belt Misalignment Effect on Metal Pushing V-Belt Continuously Variable Transmission. PhD. Thesis, Cranfield University, United Kingdom. Tawi, K.B., I.I. Mazali, B. Supriyo and M.S.C. Kob, Electro-Mechanical Continuously Variable Transmissions for Automotive Applications. Proc. of International Conference on Innovative Technologies IN-TECH 2012, Rijeka, Croatia, pp: Tawi, K.B., I.I. Mazali, B. Supriyo, N.A. Husain, M. Hussein, M.S.C. Kob and Y.Z. Abidin, Independent Clamping Actuator for Electro- Mechanical Continuously Variable Transmission. Latest Trends in Circuits, Control and Signal Processing, Proceedings 13th International Conference on Instrumentation, Measurement, Circuits and Systems (IMCAS 13), Kuala Lumpur, Malaysia, pp: Vogelaar, G., VT2+; Further improving the fuel economy of the VT2 transmission. 6th International Conference Continuously Variable Hybrid Transmissions CVT2010, Netherlands, pp: Wittel, H., D. Muhs, D. Jannasch and J. Vossiek, Roloff/Matek Maschinenelemente; Normung, Berechnung, Gestaltung; 20. Auflage. Viewer + Teubner Verlag, Wiesbaden, Germany.