RUHR-UNIVERSITÄT BOCHUM
FAKULTÄT FÜR MASCHINENBAU Institut: Product and Service Engineering
Schriftenreihe Heft 15.2
Peng Dong
Optimized Shift Control in Automatic Transmissions with respect to Spontaneity, Comfort, and Shift Loads
Lehrstuhl für Industrie- und Fahrzeugantriebstechnik Prof. Dr.-Ing. Peter Tenberge
RUHR-UNIVERSITÄT BOCHUM
FAKULTÄT FÜR MASCHINENBAU Institut: Product and Service Engineering
Schriftenreihe Heft 15.2
Peng Dong
Optimized Shift Control in Automatic Transmissions with respect to Spontaneity, Comfort, and Shift Loads
Lehrstuhl für Industrie- und Fahrzeugantriebstechnik Prof. Dr.-Ing. Peter Tenberge
Optimized Shift Control in Automatic Transmissions with respect to Spontaneity, Comfort, and Shift Loads
Dissertation
zur Erlangung des Grades Doktor-Ingenieur
der Fakultät für Maschinenbau der Ruhr-Universität Bochum
von
Peng Dong, M.Sc.
aus Yantai, China
Bochum 201η
Herausgeber:
Lehrstuhl für Industrie- und Fahrzeugantriebstechnik Institut: Product and Service Engineering Fakultät für Maschinenbau Ruhr-Universität Bochum, 44780 Bochum
Dissertation:
Referent: Prof. Dr.-Ing. Peter Tenberge Korreferent: Prof. Dr.-Ing. Xiangyang Xu
Tag der Einreichung: 06. Januar 2015 Tag der mündlichen Prüfung: 06. März 2015
© 201η Institut: Product and Service Engineering Ruhr-Universität Bochum Alle Rechte vorbehalten
ISBN 3-89194-216-8 Acknowledgements
Acknowledgements
This dissertation was written during my work as a Ph.D. student at the Chair of Industrial and Automotive Drivetrains of Ruhr-University Bochum. I truly enjoy the time I spent here with all the colleagues and friends. It is a big honor for me to work in such a great team in the past three years.
I am deeply thankful to my Ph.D. father, Prof. Dr.-Ing. Peter Tenberge, who provides me with this interesting research topic. His excellent supervision, consistent support and encouragement, and many invaluable suggestions greatly help me throughout the research work. I am also grateful to Prof. Dr.-Ing. Xiangyang Xu, who carefully took review of my dissertation and contributed to it with a lot of good suggestions. I would like to thank Prof. Dr.-Ing. Alexander Hartmaier for his interest in my research and for taking time to serve as the chairman of my oral examination.
I would also like to thank all my colleagues from the Chair of Industrial and Automotive Drivetrains and the colleagues from Chemnitz University of Technology. It’s my pleasure to work with them in such a pleasant working atmosphere. The good communication and discussion benefit me a lot in the creation of this dissertation.
I would like to express my gratitude to the industry partner Shengrui Transmission Co., Ltd. President Xiangwu Liu and his team launched the 8AT successfully into the Chinese market in 2014. It is a remarkable achievement. I would like to thank all the engineers for good communication and discussion. Especially, I will give my special thanks to Dr. Wei Guo for his support of the test data.
I also thank the China Scholarship Council for providing me with the financial support during my study in Germany.
Finally, I am deeply indebted to my parents and my girlfriend. Without their selfless support, deep love, and great patience, I cannot finish this dissertation.
Bochum, March 2015 Peng Dong
Acknowledgements
Kurzfassung
Kurzfassung
Automatikgetriebe sind komplexe und hochintegrierte Elemente des Fahrzeugantriebsstrangs. Mit dem Ziel die Kraftstoffeffizienz heutiger Fahrzeuge zu steigern, nimmt die Zahl der Gänge und die Gesamtspreizung der Automatikgetriebe in den letzten Jahren stark zu. Das hat eine Häufung der Schaltvorgänge im Betrieb zur Folge. Gleichzeitig steigt die Anforderung der Kunden sowohl an den Komfort als auch die Spontanität der Schaltungen. Darüber hinaus muss die Schaltqualität auch im Serienprodukt und über dessen gesamte Lebensdauer sichergestellt werden. Die Verbesserung der Schaltqualität trotz der vorhandenen Fertigungstoleranzen, Verschleiß und Alterung ist also eine der Hauptaufgaben der Steuerungssoftwareentwicklung für automatisch schaltende Getriebe.
Um das dynamische Verhalten eines Automatikgetriebes während des Schaltvorgangs untersuchen zu können wird ein Simulationsmodell eines Fahrzeugantriebsstrangs entwickelt. Um die damit durchgeführten Schaltungen bewerten zu können, werden fünf objektive Kriterien eingeführt. Diese sind quantitative Größen, die einen Vergleich verschiedener Simulationen ermöglichen. Zwei verschiedene Regelstrategien werden untersucht: Eine Steuerung mit offener Wirkungskette und eine Regelung mit Rückkopplung. Mit diesen Regelstrategien werden Schaltvorgänge eines 8-Gang Automatikgetriebes simuliert, und die Kernursachen identifiziert, die zu einer schlechten Schaltung führen. Gegenüber der Steuerung weißt die Regelung weniger einzustellende Parameter auf und ist robuster gegenüber Störungen.
Eine gute Regelstrategie ist nicht ausreichend um eine hohe Schaltqualität sicherzustellen, da die eingesetzten Schaltelemente empfindlich auf Fertigungstoleranzen, Alterung und Verschleiß reagieren. Daher wird deren Einfluss genauer untersucht und eine adaptive Regelung, die einige dieser Einflüsse kompensiert, entwickelt. Auf Basis der Drehzahlverläufe über der Zeit während eines Schaltvorgangs wird ein Adaptionsalgorithmus für den Befüllvorgang des Hydraulikzylinders des zu schließenden Schaltelementes entworfen. In Fahrversuchen wird gezeigt, dass mithilfe des entwickelten Adaptionsalgorithmus ein starker Drehzahlabfall, ein Hochdrehen des Motors oder ein Verspannen des Getriebes, welches durch eine schlechte Parametrierung hervorgerufen wird, beseitigt werden kann. Solche Maßnahmen können also dazu beitragen die Schaltqualität über der Lebensdauer auf einem hohen Niveau zu bewahren.
Abstract
Abstract
Automatic transmissions are complicated and highly integrated gearboxes in the vehicle powertrain. In order to improve fuel economy of the vehicle, automatic transmissions tend to have more speeds and a big total ratio range in recent years. The number of shift operations increases in the normal daily driving. People want to have a smoother and faster shift feeling than before. In addition, shift quality is required to be consistent in mass production and with mileage accumulation. Therefore, how to improve shift quality and how to cover the influence of build-to-build variations and life-cycle variations are the key issues in the development of transmission control software.
In this dissertation, a simulation model of the vehicle powertrain is developed. The dynamic behaviour of the automatic transmission in the shifting process can be simulated through this model. In order to evaluate the shift quality through simulation, five objective criteria are proposed. They are quantitative indicators that can be calculated and compared between different simulations. Two different kinds of control strategies, namely the open-loop control strategy and the closed-loop control strategy are developed in the simulation model. The shifting process of an 8-speed automatic transmission is simulated under the control of the developed strategies. According to the simulation results, the key points which result in a bad shift quality in both control strategies are identified and described. Compared with the open- loop control strategy, the closed-loop control strategy has fewer calibration parameters and improves the control robustness.
Only a good control strategy is not enough for the control of the shifting process. Shift quality is easily affected by build-to-build variations in mass production and life-cycle variations with mileage accumulation. The main influencing factors in build-to-build variations and life-cycle variations are investigated in this dissertation. It is necessary to have the adaptive control in the software to compensate for the tolerances of different transmission builds, the system variations throughout the transmission service life, and the disturbances from the transmission inside and outside. Some adaptive control methods which make use of the speed information and the time information thus are proposed in this thesis. Based on these adaptive control methods, an adaptive control strategy for the filling of the on-coming clutch is developed for the 8-speed automatic transmission. Vehicle tests verify that this adaptive control strategy can effectively eliminate the sharp speed drop, the engine flare, and the clutch tie-up in the shifting process of power on upshift. Shift quality thus can be improved by the adaptive control.
Table of Contents I
Table of Contents
1 Introduction ...... 1 1.1 Background and Motivation ...... 2 1.2 Objectives of the Research ...... 4 1.3 Overview of the Dissertation...... 5 2 Basics ...... 9 2.1 Vehicle Transmissions ...... 9 2.1.1 Manual Transmissions (MT) ...... 9 2.1.2 Automated Manual Transmissions (AMT) ...... 10 2.1.3 Automatic Transmissions (AT) ...... 11 2.1.4 Dual Clutch Transmissions (DCT) ...... 14 2.1.η Continuously Variable Transmissions (CVT) ...... 15 2.1.θ Hybrid Transmissions ...... 16 2.2 Multi-gear Systems ...... 19 2.2.1 Transfer Gear Sets ...... 19 2.2.2 Planetary Gear Sets ...... 20 2.2.3 Calculation of Kinematics ...... 21 2.2.4 Calculation of Kinetics ...... 23 2.3 Shifting Elements ...... 27 2.3.1 Dog Clutches ...... 27 2.3.2 One-way Clutches ...... 28 2.3.3 Brake Bands ...... 29 2.3.4 Multi-plate Clutches and Brakes ...... 30 2.3.4.1 Clutch Pack ...... 31 2.3.4.2 Plate Carrier ...... 32 2.3.4.3 Piston, Chamber, and Seals ...... 32 2.3.4.4 Return Springs ...... 33 2.4 Electro-hydraulic Control Systems ...... 35 2.4.1 Electronic Control System ...... 35 2.4.2 Hydraulic Actuation System ...... 37 3 State of the Art ...... 41 3.1 Review of Powertrain Modeling and Simulation ...... 41 3.2 Review of Shift Control in AT...... 45 4 Modeling of the Vehicle Powertrain ...... 51 4.1 Modeling of the Combustion Engine ...... 52 II Table of Contents
4.2 Modeling of the Hydrodynamic Torque Converter ...... 53 4.3 Modeling of the Shifting Elements ...... 57 4.4 Modeling of the Transmission Kinetics and Kinematics ...... 60 4.η Modeling of the Proportional Solenoid Valve ...... 66 4.θ Modeling of the Driving Resistance ...... 67 5 Shifting Process of the Clutch to Clutch Shifting ...... 71 η.1 Power on Upshift ...... 74 η.2 Power on Downshift ...... 77 η.3 Power off Upshift ...... 80 η.4 Power off Downshift ...... 82 η.η Pressure Profile of Different Shifting Types ...... 85 6 Evaluation of the Shift Quality ...... 95 θ.1 Transmission Output Torque and Longitudinal Vehicle Acceleration ...... 98 θ.2 Vehicle Jerk ...... 100 θ.3 Shifting Time ...... 102 θ.4 Power Loss ...... 103 θ.η Friction Energy ...... 104 7 The Open-loop Control Strategy ...... 107 7.1 The Open-loop Control Strategy for Power on Upshift ...... 109 7.1.1 Phase 1-2 ...... 110 7.1.2 Phase 2-3 ...... 114 7.1.3 Phase 3-4 ...... 122 7.1.4 Phase 4-5 ...... 127 7.1.η Phase 5-6 ...... 132 7.2 The Open-loop Control Strategy for Power off Downshift ...... 134 7.3 The Open-loop Control Strategy for Power on Downshift ...... 142 7.3.1 Phase 1-2 ...... 144 7.3.2 Phase 2-3 ...... 150 7.3.3 Phase 3-4 ...... 154 7.3.4 Phase 4-5 ...... 160 7.4 The Open-loop Control Strategy for Power off Upshift ...... 162 8 The Closed-loop Control Strategy ...... 169 8.1 The Closed-loop Control Strategy for Power on Upshift ...... 172 8.1.1 Phase 2-3 ...... 173 8.1.2 Phase 3-4 ...... 178 8.2 The Closed-loop Control Strategy for Power off Downshift ...... 184 Table of Contents III
8.3 The Closed-loop Control Strategy for Power on Downshift ...... 186 8.3.1 Phase 1-2 ...... 187 8.3.2 Phase 2-3 ...... 191 8.4 The Closed-loop Control Strategy for Power off Upshift ...... 194 9 Influence of Build-to-Build and Life-Cycle Variations ...... 201 λ.1 Friction Coefficient ...... 204 λ.2 Kiss-point Pressure ...... 210 λ.3 Tolerance of Solenoid ...... 212 λ.4 Pressure Loss and Response Delay ...... 218 λ.η Sampling Time ...... 221 10 Adaptive Control of the Gear Shifting ...... 223 10.1 Adaptive Control for the Torque Phase ...... 226 10.2 Adaptive Control for the Inertia Phase ...... 231 10.3 Test Verification of the Developed Adaptive Control Strategy ...... 237 11 Conclusions and Outlooks ...... 245 11.1 Conclusions of the Dissertation...... 245 11.2 Outlooks of Future Research Activities ...... 248 12 Notation ...... 251 13 List of Abbreviations ...... 259 14 References ...... 261
IV Table of Contents
1 Introduction 1
1 Introduction
Vehicle transmissions play an important role in the automotive industry. Each vehicle needs a transmission to match the engine speed and the engine torque with the vehicle speed and the vehicle load. Figure 1.1 shows a big different characteristic between the wheel torque demand and the internal combustion engine. From this figure it is easy to know why a transmission is necessary for a vehicle.
(1) 4000 vmax Friction limit kph
3000 (2)
2000 Wheel torque [Nm] Wheel torque [Nm] torque Wheel Engine power [kW] power Engine Engine torque [Nm] torque Engine 1000 (3) Max. speed (4)
0 0 70 140 210
(a) VehicleVehicle speed speed [km/h] [km/h] (b) Engine speed [rpm]
Figure 1.1: (a) Demand of the maximum wheel torque; (b) Engine map with specific fuel consumption [N1]
As shown in figure 1.1, the maximum engine torque is much smaller than the wheel torque demand. On the contrary, the maximum engine speed is much bigger than the wheel rolling speed (maximum 1700rpm for a passenger car with maximum driving speed 200km/h and tire radius 0.316m). The transmission just links these so many different characteristics and makes them to match each other: (1) The transmission converts the maximum engine torque to the wheel torque using big speed ratios for the requirements of the vehicle acceleration and climbing ability. (2) The transmission keeps the engine operating points along the line of the minimum fuel consumption when the vehicle is cruising on the highway. (3) The vehicle achieves its maximum driving speed at the operating point of maximum engine power through transmission. (4) The vehicle can launch smoothly and run slowly in crowded traffic through transmission.
2 1 Introduction
In addition, the right graphic in figure 1.1 shows that the same power requirement of the vehicle can be satisfied at different engine operating points (point and point in the engine map). The available acceleration torque decreases with the arrow from the point to the point . However, the point has a higher specific fuel consumption than the point . Accordingly, the driver can achieve a high driving dynamic or a high driving economic at different engine operating points by using transmission.
1.1 Background and Motivation
A trend towards increasing the number of transmission speed becomes apparent in recent years. The first reason is the issue of global energy crisis and environment protection. It requires more reduction in vehicle fuel consumption and exhaust emissions. The second reason is the increasing demands for driving pleasure. Figure 1.2(a) shows that the output torque of internal combustion engine can’t meet the demand of wheel torque without the transmission. The shaded area takes up most parts of the traction hyperbola area and can’t be used by the vehicle. Figure 1.2(b) shows that the proportion of the shaded area, i.e. the proportion of the impossible driving states, is significantly smaller when a 4-speed transmission is used. The high efficiency region of the engine can be better applied. In addition, increasing the number of speed as much as possible gives a correspondingly better approximation to the traction hyperbola [N1]. The driver can make use of the full engine power at any vehicle speed as he wishes.
In manual transmissions, more speeds mean more shift operations of the driver. However, transmissions with the function of automatic shifting can solve this issue and relax the driver in a heavy traffic. The most common transmission type having such a function in passenger cars is the automatic transmission. Automatic transmission is known as power shift transmission which has no power interruption during the gear shifting. The key issue for the application of automatic transmission is its shift quality. Drivers and passengers often complain about the shift quality of automatic transmission because its shifting impact is more perceptible to customers than the shifting impact of manual transmission. The reason can be interpreted from the human psychology. When people drive in a vehicle with a manual transmission, the gear shifting is carried out by the driver. People know when shift starts from the actions of the driver and then have mental and body preparations for the upcoming shifting impact in their subconscious. However, the sensors of human body are replaced by electronic units in automatic transmissions. Driver and passengers don’t know when gear shifting happens. As a result they will feel the shifting impact without mental and body preparations and subjectively think it is an uncomfortable shift with a bad shift quality. Therefore, the 1 Introduction 3 control of the shifting process in automatic transmissions must be more precise to satisfy the customers and reduce their complaints about the shift quality. And this topic is always concerned by both manufactures and customers of automatic transmissions. Wheel torqueWheel
Available traction torque of internal combustion engine
(a) Vehicle speed
Available traction torque in the 2nd gear
Driving resistance line Traction torque required for 0% gradient Wheel torque Wheel
(b) Vehicle speed
Figure 1.2: (a) Comparison between the traction torque supply and the wheel torque demand without a transmission; (b) Comparison between the traction torque supply and the wheel torque demand with a 4-speed transmission [N1]
With the development of electronic and hydraulic control system, shift quality in automatic transmissions has been much improved today. In a good control of the shifting process, drivers and passengers can hardly feel any shifting impact. Even they don’t know the vehicle is shifting. Meanwhile the shifting time is shortened to give the driver a good response for the 4 1 Introduction power requirement. The driver now can experience the same driving pleasure as in the vehicle with a manual transmission.
However, different builds of the same type of automatic transmission have different tolerances in mass production; the wear of components will increase throughout the transmission life; the vehicle will run in various environment conditions. These aspects greatly affect the control of the shifting process and cause a big deviation of shift quality in different builds. As a consequence, together with the increasing requirements for the shift quality, how to cover the tolerances and how to keep the shift quality consistent throughout the transmission service life are currently the key issues for the manufacturers of automatic transmissions.
1.2 Objectives of the Research
One objective of this dissertation is to develop a simulation model of the vehicle powertrain for the dynamic simulation of the shifting process in automatic transmissions. This model includes an internal combustion engine which outputs the torque to the transmission depending on the rotational speed and the throttle opening degree. The detailed transmission structure is defined in an Excel file which can be read into the simulation model. A hydrodynamic torque converter with torsion damper system is considered in the simulation model. At the output side of the transmission, a driving resistance model including the rolling resistance, air resistance, climbing resistance and acceleration resistance is built up to simulate the vehicle load during the gear shifting. For the control of the shifting process the pressure to current characteristic of a proportional solenoid valve is used in the model. In order to investigate the influence of different friction characteristics on the shift quality, the dynamic friction coefficient as a function of the slipping speed of shifting element is introduced in the powertrain model.
The focus of this thesis is the optimized control of the shifting process in automatic transmissions. Hence the other objective of this thesis is to develop detailed control strategies for different shifting types. The powertrain model is detailed enough to serve this investigation and it permits to integrate specific control strategies for the shifting process. The key points which affect the shift quality thus can be easily found through the simulation.
Two different control strategies are developed in this thesis. One is an open-loop control with a lot of control parameters. The other uses a closed-loop control method which reduces the number of parameters meanwhile improves the robustness of the control strategy. By adjusting relative control parameters in the control strategies, the shifting process can be optimized with respect to different shift modes (sport, normal, comfort). This adjustment can be used to guide 1 Introduction 5 the transmission calibration work. Such a simulation model with detailed control strategies is very helpful and achieves a good effect for the improvement of the shift quality.
This thesis also aims to investigate the influences of the tolerances and the mileage accumulation on the shift quality. The shifting elements are electro-hydraulic actuated. Therefore, the characteristics of proportional solenoid valves must be considered. In order to eliminate the gap to clamp the clutch pack, the clutch pressure needs to overcome the friction force of seals and return springs. The pressure response of a hydraulic system is greatly influenced by the temperature. The sensors have measurement errors and the TCU has a sampling time limited by its power. Furthermore the torque to pressure characteristic of shifting elements greatly depends on the friction coefficient. All above factors influence the shifting process and vary greatly in different builds of mass production. Even in the same build these factors are always changing because of mileage accumulation and different driving environment. An investigation for the main influencing factors is carried out in this thesis. Obviously, in order to get a good shift quality and keep it consistent, the transmission control software must have the adaptive control. Therefore, some adaptive control methods are proposed in this thesis to compensate for the influence of the factors mentioned above. Based on these adaptive control methods, an adaptive control strategy for the filling of the on-coming clutch is developed for an 8-speed automatic transmission. Vehicle tests verify that this adaptive control strategy can effectively eliminate the sharp speed drop, the engine flare, and the clutch tie-up in the shifting process of power on upshift.
1.3 Overview of the Dissertation
Chapter 1 gives a general introduction of the research objectives and the research contents in this thesis.
In chapter 2, corresponding basics of this research are introduced. This chapter starts with a classification of the vehicle transmissions. Then the planetary gear sets and the transfer gear sets are introduced in this chapter, including the kinematic and kinetic relationships of each element in both gear systems. They are basics for a mathematical modeling of automatic transmissions. The dynamic description of the shifting process is also based on them. Moreover, shifting elements used in automatic transmissions, especially multi-plate clutches and brakes are detailed explained. Since multi-plate clutches and brakes are electro-hydraulic actuated, the “brain” and the “blood vessel” of automatic transmissions, namely the electronic control system and the hydraulic actuation system, are also described in this chapter. These two systems control the actions of automatic transmissions and determine the shift quality.
6 1 Introduction
Many researchers have studied the clutch to clutch shifting and have proposed different control methods to improve the shift quality. Modeling and simulation are an efficient way and have contributed a lot in the development of shift control strategies. A review of literatures is given in chapter 3. This review includes the dynamic modeling of vehicle powertrains. In addition, a summary of findings about the control of the shifting process in automatic transmissions is also introduced in this chapter.
The powertrain model is discussed in chapter 4. It is a mathematical model developed in the software Mathcad. Since the software Mathcad can read data from the Excel file, the transmission structure and some characteristics are predefined in the Excel file as inputs. This provides a possibility for the shifting simulation of different kinds of transmissions with only few modifications to the simulation model. In addition to the structure definition, the modeling of the combustion engine, the hydrodynamic torque converter, the shifting elements, and so on are also described in this chapter.
There are four main shifting types in automatic transmissions, namely power on upshift, power on downshift, power off upshift, and power off downshift. The basic theory and the differences of the four shifting types are demonstrated in chapter 5. The shifting process of a clutch to clutch shifting without one-way clutch makes use of a micro-slip control method. The speed, torque, and pressure profiles of this micro-slip control method are introduced in this chapter. The clutch to clutch shifting can mimic the operation of one-way clutch to achieve the same good shift quality by applying this method.
Chapter 6 mainly talks about the evaluation of the shift quality. In order to assess a specific gear shifting is good or bad, there should be some subjective and objective criteria. These criteria will be discussed in this chapter.
Chapter 7 discusses the developed open-loop control strategies for the four basic shifting types. The issues in the shifting process and how the open-loop control strategies deal with them are explained based on the dynamic simulation results. From the simulation results, the key points affecting the shift quality can be easily found in the control of the shifting process.
In chapter 8, the developed closed-loop control strategies for the four basic shifting types are introduced. The control effect is also verified by the dynamic simulation results. The closed- loop control strategy makes use of the clutch slip as the feedback information. Compared with the open-loop control strategy, the closed-loop control strategy reduces the amount of the calibration work and improves the control robustness of the shifting process. 1 Introduction 7
In mass production, build-to-build variations and life-cycle variations have a big influence on shift quality. The main factors are introduced in chapter 9. Through some test and simulation results it can be seen the uncertainty of physical characteristics is the main reason for the difficulties of keeping a consistent good shift quality.
Adaptive control can compensate for the influence of build-to-build variations and life-cycle variations on shift quality. Some shift adaptations are introduced in chapter 10. These methods evaluate shift quality according to the speed information and the time information. Meanwhile, an adaptive control strategy for the filling of the on-coming clutch is developed for an 8-speed automatic transmission. Vehicle tests verify that this adaptive control strategy can effectively eliminate the sharp speed drop, the engine flare, and the clutch tie-up in power on upshift. With the help of end-of-line test, the adaptive control could provide the shift quality of automatic transmissions with a good consistency.
Finally, the work of this thesis and the outlooks of future research activities are summarized in chapter 11.
8 1 Introduction
2 Basics 9
2 Basics
2.1 Vehicle Transmissions
Transmissions for passenger cars and commercial vehicles are mainly divided into the following six types: (1) Manual transmissions (MT)ν (2) Automated manual transmissions (AMT)ν (3) Automatic transmissions (AT)ν (4) Dual clutch transmissions (DCT)ν (η) Continuously variable transmissions (CVT)ν (θ) Hybrid transmissions.
This section will give a general overview of the six types of transmissions. The main characteristics are distinguished and compared. Due to the research focus of this thesis, special attention is paid to the shifting process in the comparison.
2.1.1 Manual Transmissions (MT)
Manual transmissions are composed of transfer gear sets, shift folks, synchronizers, and a friction clutch. Different transfer gear sets realize different speed ratios. The shift forks and synchronizers are used to change the power flow path through different transfer gear sets. The friction clutch is for the power interruption between engine and transmission when launching and shifting.
A large proportion of vehicles are equipped with manual transmissions. In particular manual transmissions occupy most of the transmission markets in Europe. Manual transmissions have lower price and lower maintenance cost. However, the shifting process in manual transmissions has power interruption. And all shift operations need to be executed by the driver. The complex shift operations make the driving boring especially in heavy city traffic. The speed number of manual transmissions is limited in passenger cars because nobody wants to shift all the time during driving.
Today manual transmissions also tend to have more speeds and a big total ratio range because it can facilitate slow moving of vehicles in a traffic jam, provide a good acceleration performance, and reduce fuel consumption at high cruising speed. The manual transmissions of passenger cars usually have either five or six forward speeds. Figure 2.1 shows a 6-speed manual transmission from Mercedes-Benz. It is designed for front transverse application. ZF 10 2 Basics develops the first 7-speed manual transmission for passenger cars in the world as shown in figure 2.2.
Figure 2.1: Mercedes-Benz 6-speed manual transmission [W1]
Figure 2.2: ZF 7-speed manual transmission [P1]
2.1.2 Automated Manual Transmissions (AMT)
Automated manual transmissions are just manual transmissions which the gear shifting is automated. Automated manual transmissions also contain transfer gear sets, synchronizers and 2 Basics 11 a friction clutch. Instead of driver actions, the engagement and disengagement of the friction clutch, the operations of gear shifting are executed by actuators automatically. The foot- activated clutch pedal and the manual activation of a shift lever are replaced and now controlled by the transmission control unit. This simplifies complex shift operations and thus relieves driver stress. However like manual transmissions, automated manual transmissions also have power interruption in the shifting process according to the principle of the design. As a result driver and passengers will feel less comfortable during the gear shifting compared with automatic transmissions.
Automated manual transmissions are currently most common in commercial vehicles such as heavy duty trucks because they have lower requirements for the shift quality but more speeds for different driving conditions. In addition, automated manual transmissions have low cost and high efficiency in the application of commercial vehicles compared with other type of transmissions. Figure 2.3 shows an automated manual transmission of maximum 16 speeds developed by ZF for heavy duty trucks.
Figure 2.3: ZF AS Tronic transmission for heavy trucks [R1]
2.1.3 Automatic Transmissions (AT)
Automatic transmissions mainly consist of a hydrodynamic torque converter, multi-gear systems (planetary gear sets or transfer gear sets), shifting elements (multi-plate clutches and brakes, brake bands, one-way clutches, dog clutches), and electro-hydraulic control system. 12 2 Basics
The hydrodynamic torque converter provides comfortable and safety vehicle launching in all driving conditions. Due to power splitting in planetary gear sets and transfer gear sets, automatic transmissions have a high torque capacity and a compact design. Through the engagement of different shifting elements, automatic transmissions achieve different speed ratios. The electro-hydraulic control system is just responsible for the ratio change and some other functions like diagnostics, lubrication, etc.
Conventional automatic transmissions only use several coupled planetary gear sets in their multi-gear systems. It is especially suitable for standard inline application. Figure 2.4 shows such a type of transmission from Mercedes-Benz. It is a 9-speed automatic transmission in mass production from 2013. For front transverse application conventional automatic transmissions need one transfer gear set for the power flow from the input shaft to the counter shaft. The possibilities of applying more transfer gear sets in the multi-gear systems have been investigated and an excellent structure with three transfer gear sets, three planetary gear sets, and five shifting elements (1 brake and 4 clutches) has been found [T1]. There are 8 forward speeds and 1 reverse speed in this transmission. In each speed three shifting elements are locked. Only two opening shifting elements create some little dragging losses. This transmission, as shown in figure 2.5, is developed for front transverse application by Shengrui Transmission Corporate Limited and is in mass production from 2014.
Figure 2.4: Mercedes-Benz 9-speed automatic transmission [D1] 2 Basics 13
Figure 2.5: Shengrui 8-speed automatic transmission [T1]
Most space in automatic transmissions is occupied by shifting elements. Modern automatic transmissions can shift with one shifting element being engaged while another is being disengaged simultaneously, which is called “clutch to clutch shifting”. In the shifting process the power flow is not interrupted but transferred from one shifting element to another. Therefore, automatic transmissions have no power interruption during the gear shifting. Some automatic transmissions have one-way clutches to help to improve the shift quality. With the development of electro-hydraulic control, now this one-way clutch can be cancelled for a compact design. As shown in figure 2.6, ZF has developed a 9-speed automatic transmission for front transverse application in which two shifting elements are dog clutches. The dog clutch saves more space but also requires a more precise control for the shifting process.
Figure 2.6: ZF 9-speed automatic transmission [E1] 14 2 Basics
2.1.4 Dual Clutch Transmissions (DCT)
Dual clutch transmissions combine the advantages of manual transmissions (high efficiency) with those of automatic transmissions (power shifts). The principle of dual clutch transmissions is based on the idea of two independent sub-gearboxes each connected to the engine via its own friction clutch. One sub-gearbox carries the odd gears (1, 3, 5…) and the other the even gears (2, 4, 6…) [N1]. In each sub-gearbox there are also transfer gear sets, synchronizers, and shift forks like in the manual transmissions.
The shifting process of dual clutch transmissions has no power interruption. Before the shifting, the synchronizer of the target gear is pre-selected in the respective sub-gearbox which is disconnected from the engine. Then the gear shifting can be accomplished as clutch to clutch shifting by torque delivery from one sub-gearbox to another, thereby retaining full traction at the wheels [G1]. In principle, the control of the shifting process in dual clutch transmissions is similar to automatic transmissions. However, dual clutch transmissions can’t skip shift without power interruption regarding the speeds in the same sub-gearbox (i.e. from the 1st to the 3rd or from the 2nd to the 4th).
The first dual clutch transmission for passenger cars, as shown in figure 2.7, went into production in 2003. It is a 6-speed transmission from Volkswagen with wet clutches from BorgWarner. Dual clutch transmissions can be equipped with wet clutches or dry clutches according to the design. Compared with wet clutches, dry clutches are relative higher in efficiency but have a lower torque capacity.
Figure 2.7: VW 6-speed dual clutch transmission [S1] 2 Basics 15
2.1.5 Continuously Variable Transmissions (CVT)
The speed ratio in continuously variable transmissions can be continuously varied without interrupting the power flow. Therefore the shifting process in continuously variable transmissions is very smooth and comfortable. In combination with a good integrated powertrain control, the engine can always run at an ideal operating point for good fuel economy or good driving performance.
There are two kinds of continuously variable transmissions for passenger cars. One uses pulley drive, namely a chain (Figure 2.8) or a belt (Figure 2.9). The other uses friction gear such as the toroidal variator. Friction gear transmissions have a higher torque capacity than pulley transmissions. But its high cost is a barrier for friction gear transmissions to enter the market.
Figure 2.8: Audi continuously variable transmission with chain [N2]
Figure 2.9: Jatco continuously variable transmission with belt [Y1] 16 2 Basics
2.1.6 Hybrid Transmissions
Hybrid transmissions have two power sources that one is the internal combustion engine and the other is the electric motor. The torque-speed characteristic of electric motor has a good supplement to the internal combustion engine which can be seen from figure 2.10: The electric motor can reach its maximum torque in the low engine speed region thus gives the vehicle a good launching and acceleration performance. Besides, the electric motor can recover the kinetic energy during vehicle braking and coasting, stop and start the engine automatically at a traffic light, realize a pure electric driving, support the engine output under some extreme driving conditions. All these advantages provided by the combination of internal combustion engine and electric motor in hybrid transmissions lead to a better fuel economy and a better driving performance. Power [kW] Power Torque [Nm] Torque
Speed [rpm] Speed [rpm]
Figure 2.10: Characteristic curves of internal combustion engine and electric motor [N1]
According to the connection of the internal combustion engine and the electric motor, hybrid transmissions can be classified into three types: serial hybrid; parallel hybrid; power-split hybrid. In serial hybrid transmissions, the combustion engine functions only as an electricity producer. There is no mechanical connection between the engine and the wheels. Two electric machines are necessary in serial hybrid transmissions. One works as a generator and needs to have the same maximum torque and maximum speed as the engine. The other is required to be big enough to match the vehicle load. The efficiency of serial hybrid transmissions is low because all engine power flows through the inverter and the electric machines.
In parallel hybrid transmissions, both the engine and the electric motor are mechanically connected to the wheels. The wheel power may be supplied by the engine, the motor, or by 2 Basics 17 both. The parallel hybrid transmissions are easy for the design modification based on existing transmission concepts. For example, figure 2.11 shows a hybrid variant of ZF 8-speed automatic transmission. It is a parallel hybrid transmission which just uses an electric motor to replace the hydrodynamic torque converter. The shifting process in it also has no difference with automatic transmissions. In power-split hybrid transmissions, as shown in figure 2.12, the engine power is split into mechanical and electric paths. Two electric machines and some planetary gear sets are necessary for the splitting and joining of the two paths. The speed ratio can be continuously varied through the control of the two electric motors. Therefore, the power-split hybrid transmission permits a continuous torque and speed conversion without power interruption.
Figure 2.11: Hybrid variant of ZF 8-speed automatic transmission [G2]
Figure 2.12: Mercedes-Benz AHS-C-Two-Mode-Hybrid transmission [W2] 18 2 Basics
In conclusion table 2.1 gives an overview of different transmission concepts and their shifting characteristics.
Table 2.1: An overview of different transmission concepts
Time
Time
Hybrid
transmissions
Electric motor Electric
Electric motor, Electric
velocity
Vehicle
torque
Traction
planetary gear sets planetary
Time
Time
variable
friction gear friction
Continuously
transmissions
friction clutch friction
pulley drive or drive pulley
friction clutch, friction
Torque converter, Torque
velocity
Vehicle
torque
Traction
Torque converter or converter Torque
Time
Time
lutch
gear sets,
Dual clutch Dual
Two friction Two
transmissions
synchronizers
clutches, transfer clutches,
velocity
Vehicle
Traction torque
Time
Time
Yes Yes Yes Yes
Automatic
transmissions
shifting elements shifting
Torque converter, Torque
velocity
Vehicle
Traction torque
planetary gear sets, planetary
Time
Time
transmissions
synchronizers
Friction clutch, Friction
transfer gear sets,
velocity
Vehicle
torque Traction
Automated manual Automated
Time
Time
No No
Fixed Fixed Fixed Fixed Variable Variable or Fixed
Manual
Manual Automatic Automatic Automatic Automatic Automatic
transmissions
synchronizers
Friction clutchFriction clutch Friction converter Torque c Friction
Friction clutch, Friction
velocity
Vehicle
transfer gear sets,
torque Traction
element
Moving-off Qualitative
Speed ratio Power shift
and velocityand
components
profile during profile
Shift actuation Shift
vehicle traction
Key mechanical power power on upshift Transmission type 2 Basics 19
2.2 Multi-gear Systems
In order to generate different speed ratios, transmission must have multi-gear systems, namely either transfer gear sets or planetary gear sets, or their combinations. Transfer gear sets are more common in MT, AMT, and DCT. Planetary gear sets are more applied in AT and power split hybrid transmissions.
Gumpoltsberger [G3] investigated hybrid forms of AT and DCT which applies transfer gear sets and planetary gear sets together in one transmission structure. By configuring planetary gear sets on different parallel shafts, advantages can be gained especially in terms of installation length. By using different transfer gear sets as the coupling of two parallel shafts, the transmission speed ratios can be better optimized than a conventional planetary transmission. Such a type of automatic transmission [T1] integrated with transfer gear sets and planetary gear sets has been in mass production in Chinese market since 2014.
2.2.1 Transfer Gear Sets
Transfer gear sets are the simplest and most common type of gear sets. As shown in figure 2.13, there are two parallel shafts rotating in a fixed position in transfer gear sets. Each shaft connects one gear and the motion is transferred between the two shafts through the meshing of gear teeth. In transmissions these two shafts are supported on the housing by bearings. The stationary ratio of a transfer gear set is just the reciprocal of the tooth number ratio. The direction of the two shafts are opposite in transfer gear sets.
Figure 2.13: Transfer gear set [N1] 20 2 Basics
2.2.2 Planetary Gear Sets
It can be seen from figure 2.14 that a minus planetary gear set is composed of a sun gear, a carrier, a ring gear, and several planetary gears. There are three coaxial shafts among which the carrier is the axle of planetary gears. The planetary gears rotate both around the main axle and on its own axle. Torque is distributed between several planetary gears to ensure a low stress on each of them. The stationary ratio of a planetary gear set is just the tooth number ratio between the ring gear and the sun gear. The tooth number of the ring gear is assumed to be negative because it is an internal gear.
Figure 2.14: Minus planetary gear set [N1]
Compared with the transfer gear set, advantages of the planetary gear set can be concluded as following: (1) Coaxial arrangement of three shafts and compact constructionν (2) High torque capacity as a result of torque distribution between several planetary gearsν (3) Low weight and small size in comparison with the transfer gear setν (4) High efficiency in comparison with the transfer gear setν (η) Multiple options of power splitting and power joiningν (θ) Realization of different speed ratios in only one planetary gear set.
These advantages make planetary gear sets widely applied in automatic transmissions even though they have high effort of manufacturing, high production costs, and more complex construction in comparison with transfer gear sets.
2 Basics 21
In addition to the minus planetary gear set, VDI-Guideline 2157 [V1] introduces a lot of other kinds of planetary gear sets. The following calculations of kinematics and kinetics are also based on the VDI-Guideline 2157.
2.2.3 Calculation of Kinematics
A planetary gear set has two degrees of freedom. It means for the three shafts of a planetary gear set two rotational speeds must be firstly defined. Then the 3rd rotational speed can be calculated. Equation (2.1) shows the kinematic relationship of the three shafts in a planetary gear set, which is also known as Willis-equation (2.2).
nnSC i0 (2.1) nnRC nS i 0 n R (i 0 1) n C 0 (2.2)
The stationary ratio i0 is negative in a minus planetary gear set and positive in a plus planetary gear set.
For the dynamic analysis of a planetary gear set, the angular accelerations of the three shafts are required. The angular acceleration corresponds to the time derivative of the rotational speed. Thus the Willis-equation of the rotational speeds also applies to the angular accelerations in a planetary gear set (2.3).
S i 0 R (i 0 1) C 0 (2.3)
A transfer gear set has only one degree of freedom. It can be assumed to be a planetary gear set which the carrier is locked to reduce one degree of freedom. Therefore, equation (2.1) also applies to transfer gear sets and the carrier can be considered as housing with zero rotational speed (2.4).
n11 0 n i0 (2.4) n22 0 n
The angular accelerations of a transfer gear set are calculated similarly according to equation (2.5).
11 0 i0 (2.5) 22 0
Since the kinematic equations of planetary gear sets and transfer gear sets are uniform, it facilitates the computer programing for kinematic calculations of a complex transmission structure. The kinematic relationship can be described by a system of linear equations 22 2 Basics depending on different transmission structures. Such a system of linear equations can be written into a matrix format and easily be solved by a matrix method in mathematical software. Here an example of a simple transmission structure is introduced as follows.
Figure 2.15 shows a simple transmission structure which has only one minus planetary gear set. The sun gear is the input shaft. The ring gear is connected to the housing to reduce one degree of the transmission freedom. The carrier is the output shaft. Assuming that the input speed is known as nin, then the system of linear equations to describe the kinematics of this simple transmission structure is expressed by equation (2.6), (2.7), and (2.8).
Input: Output:
nin, αin, Tin nout, αout, Tout
Figure 2.15: Example of a simple transmission structure
nS i 0 n R (i 0 1) n C 0 (2.6) nnS in (2.7) n0R (2.8)
These three equations can be represented in a matrix format (2.9).
1 i0 i 0 1 n S 0 1 0 0 n n (2.9) R in 0 1 0 nC 0
The first line of the coefficient matrix is the Willis equation of this transmission structure. The other two lines are the kinematic boundary conditions, such as speed specifications of the input shaft and the housing. The unknown vector of this matrix contains the rotational speeds of the three shafts. They can be calculated and solved according to equation (2.10).
1 nS 1 i 0 i 0 1 0 n in n 1 0 0 n 0 (2.10) R in 1 nC 0 1 0 0 nin (1 i 0 ) 2 Basics 23
This matrix expression can be efficiently computed by mathematical software even for a complex transmission structure with a lot of unknown variables.
2.2.4 Calculation of Kinetics
As shown in figure 2.16, the relationship of shaft torques in a planetary gear set can be obtained from the equilibrium of forces on the planetary gears.
Force diagram Torque diagram F R TR 1 Sun gear 2 Ring gear
TC FC 3 Carrier FS Space-fixed axle 2 TS 1 3 Non-fixed axle rS rC rR
Figure 2.16: Force and torque diagram of a minus planetary gear set
The tangential force FR is equal to the force FS because of the torque balance on the planetary gear (2.11). Both forces have the same direction. The tangential force from the carrier FC is in the opposite direction. The sum of all forces has to be zero (2.12).
FFSR (2.11)
F FSRC F F 0 (2.12)
The stationary ratio is equal to the ratio of the pitch circle radius rR and rS (2.13). The torque from each element to the planetary gear is equal to the tangential force multiplied by the radius (2.14-2.16). The carrier radius is calculated based on the pitch circle radius in equation (2.17).
rR i0 (2.13) rS
TSSS F r (2.14)
TRRR F r (2.15)
TCCC F r (2.16) rr r SR (2.17) C 2
Based on the equations from (2.11) to (2.17) and without considering the torque losses, the kinetics of a planetary gear set can be expressed by equation (2.18) and (2.19). 24 2 Basics
TR T S i 0 0 (2.18)
T TSRC T T 0 (2.19)
When considering the torque losses in a planetary gear set, the calculation of torques can be modified from equation (2.18) to equation (2.20).
w TR T S i 0 0 0 (2.20)