The transmission | Schaeffler Symposium 2018

Over the next few years, the number of powertrain concept variants for motor vehicles will continue to grow. But regardless of whether they are driven by internal combustion engines, hybridized powertrains or electrical-only vehicles, as shown in Figure 1: The goal of all of these concepts is to use as little primary energy as possible for driving motor vehicles, thus reducing CO₂ emissions to the lowest amounts technology can achieve.

Figure 1 Automation trend in transmission systems and percentage of powertrain concepts in the overall market in the Schaeffler scenario for 2030

For the transmission, this results in the requirement to generate as few losses as possible when transmitting the power and converting the speed ratio and torques – in particular, this includes losses through friction as well as auxiliary energy for actuation – to supply as much mechanical energy as possible to the wheels. The trend of automating clutches and shifting operations helps implement optimized operating strategies, in particular with regard to the increasing electrification of powertrains.

Challenges

With regard to internal combustion engines, there are additional requirements for the powertrain due to engine-based measures to reduce consumption and emissions [1]. such as downsizing and cylinder deactivation, which is now also used for three-cylinder engines [2]. The resulting torsional vibrations on the crankshaft must be damped effectively and isolated from the rest of the powertrain to meet drivers’ comfort requirements.

Besides the minimization of friction and auxiliary energy losses [3], a further weight reduction of transmission components can also contribute to CO₂ savings. At the same time, lighter and smaller components can help meet the ever more challenging design space specifications, particularly in hybrid vehicles. In addition, a systematic search for CO₂ reduction potential must also consider the large number of small consumers of auxiliary energy in the powertrain. This primarily includes the hydraulic pumps and electric motors used to operate the actuators for automation that actuate clutches and other transmission components. Every single watt counts: With real power-on-demand concepts it is now possible to change the average power consumption of actuators from the three-digit watt range to the low two-digit watt range.

No matter how good technical solutions are, though: They will not prevail on the market unless they are available at marketable costs. This requires components that are designed precisely for specific requirements – which means that over-engineering must be avoided – as well as highly efficient manufacturing processes. However, cost evaluations must include all costs – not only the direct costs of the components but also the indirect costs incurred from power loss and related CO₂ emissions for which legal regulations are becoming increasingly strict worldwide.

Solutions

The following chapters show how Schaeffler develops its components, subsystems and systems for all conventional transmission designs – from automatic planetary gears with individual market preference in the US to the CVT, primarily in Japan, and the double clutch transmission, primarily on the European market, to the classic manual transmission, mainly in Europe and BRIC countries – as well as for new hybrid transmissions and electric axle reduction gears, with these requirements in mind, Figure 2. This includes low-friction rolling bearings, efficient actuators with appropriate power consumption and effective vibration damping with new centrifugal pendulum-type absorber concepts. It is what makes the use of even more efficient engines possible. One requirement for customized transmission systems that are designed precisely for specific requirements is the wide use of simulation technologies and CAE methods that takes aspects such as future load spectra into consideration at an early stage in the design phase.

Figure 2 Transmission system trends and products and system solutions developed by Schaeffler

Another trend in transmission design is the increasing automation. Automated transmissions can contribute to the reduction of CO₂ emissions because they permit new shifting strategies with optimized consumption and more complex driving strategies such as coasting and are able to fully utilize the potential of hybridized powertrain concepts through appropriate recuperation actions. In addition, they take the increasing comfort requirements of many drivers into account.

Based on market assessments by Schaeffler, the percentage of automated transmissions will increase from approximately 60 % to around 70 % worldwide in the next ten years. At the same time, the percentage of hybrid and electric vehicles on the world market will increase to 70 % by 2030. Since the percentage of hybrid vehicles alone will be at 40 %, the percentage of new vehicles sold on the market with an internal combustion engine will still be 70 % by the end of the next decade. Besides the well-known transmission types that involve the insertion of a P2 hybrid module with 48 volt or high-voltage technology in automatic transmissions, CVT and dual clutch transmissions, there are additional transmission designs with growing market shares. The transformation of transmission systems will result in a new diversity, such as the electric variable transmission (EVT) and general dedicated hybrid transmissions (DHT), in which the full functional capability is ensured by the integration of an additional electric motor as a second power source. At the same time, simpler reducing gears for P4 hybrids as well as electric vehicles will gain higher market shares [4].

For all transmission types whose job it is to transmit drive power to the vehicle’s wheels, the reduction of losses occurring during torque conversion remains an essential development goal in order to minimize the use of primary energy and increase the driving range of the given energy accumulator.

II. New concepts for low-friction rolling bearings in all transmission types

Task

In the world of internal combustion engines, it was an important task for developers of transmission bearings to reduce friction in order to decrease fuel consumption. This task is not going to change in the future: It is important to reduce CO₂ emissions and increase the range of electrified drives. Depending on the load conditions, this goal can be achieved by using tapered roller bearings with friction-optimized geometries that have been developed with the help of new CAE methods, the replacement of tapered roller bearings with double-row angular contact ball bearings as well as by locating/non-locating bearing concepts that do not require axial preload. In addition, friction power can also be reduced and high load capacity maintained by completely new bearing concepts such as the angular roller unit (ARU) which is similar to a tapered roller bearing but, unlike the tapered roller bearing, is able to support axial forces in both directions with the help of an innovative arrangement of lips on the inner and outer ring [5].

Angular roller unit (ARU) as a new bearing design

Locating/non-locating bearing supports cannot always be implemented without modifying the design space due to the relatively low load rating of deep groove ball bearings because the required load ratings must be achieved by means of a larger bearing diameter if necessary. This raises the question of which alternative bearing design can be used as a locating bearing instead of a deep groove ball bearing.

Figure 3 shows Schaeffler’s approach with a new locating bearing – the “angular roller unit” (ARU). It has a higher load capacity than a deep groove ball bearing but operates with less friction under axial loads than a cylindrical roller bearing. The ARU can support axial forces in both directions as a self-retaining single bearing. However, it should be mounted in the preferred direction so that the higher axial forces can be transmitted via the raceways, similar to tapered roller bearings.

Figure 3 Comparison of the design principle of a tapered roller bearing and an angular roller unit (ARU) (left) as well as loading conditions for an ARU with preferred direction (right)

The locating/non-locating bearing supports using ARU and cylindrical roller bearings perform similarly well in terms of friction as the solution with deep groove ball bearings. With the ARU as a locating bearing, the required rating life is maintained at the same time. This allows the changeover from adjusted tapered roller bearing supports to locating/non-locating bearing supports without modifications to the design space.

III. Use of CAE tools

Task

Simulation methods are able to make a reliable prediction of varied phenomena such as vibrations in the powertrain, supply valid information on sources of losses and consumption benefits and, if required, to map the complete powertrain system with a very high level of detail. Schaeffler uses CAE tools such as the BEARINX calculation software to optimize transmission bearings. In addition, simulation methods are also used for virtual testing and for the design of other subsystems such as clutches. Here they allow the design of components to be customized for their intended use with the help of load spectra, i.e. neither too big nor too small. If attempts to secure operation close to the design limit by using CAE methods are successful, transmission components can be designed as small as possible. Not only does this reduce costs, weight and design space, it also reduces friction losses, which helps achieve the broader goal of reducing CO₂ emissions [6].

Application example: Thermo-mechanically optimized clutches

In the thermo-mechanical optimization of clutches, the interaction of all relevant factors in the complete system must be analyzed. That is why Schaeffler has combined the previously separate models for thermal behavior, mechanical deformation and friction coefficients in the CLUSYS (Clutch Systems) software. The optimized thermo-mechanical clutch design takes the friction coefficient behavior, component geometries, cushion deflection and system rigidities into account. The software permits rating life calculations for various utilization profiles, the calculation of thermal damage due to misuse and the calculation of clutch capacity for transmitting torques.

The optimized approach is shown using the example of a self-adjusting clutch. The conicity behaviour of the previous clutch is not ideal; however, this could not be taken into account adequately in the classic design due to extremely long calculation times, but it is now fully possible with the new thermo-mechanical model. The optimization results in a significant increase in torque capacity while also reducing wear by preventing excessive local stress. The improved torque capacity can be utilized to reduce contact pressure and thus pedal force and to increase actuation comfort. The excess gained in rating life requirements can be used elsewhere.

Figure 4 shows potential variants. This allows the inside diameter of the clutch disk and the pressure plate to be increased, making lower contact pressure and reduced pedal forces possible. At the same time, space is freed up inside the clutch for installing a centrifugal pendulum-type absorber on the clutch disk with a greater damping effect, for instance. Another variant consists of reducing the outside diameter of the entire system, thus decreasing the required space for the clutch system as such. A combination of small changes to the diameters of the friction partners creates the exact space needed to optimize the components with regard to rigidity, thus further reducing pedal forces.

Figure 4 Optimization variants reached in comparison with a basic clutch

IV. Isolation of torsional vibrations

Task

Internal combustion engine-based measures to reduce fuel consumption such as downsizing, cylinder activation and down-speeding – i.e. driving with a long ratio just above the idle speed – make high requirements on the isolation of torsional vibrations of the crankshaft. In order to prevent undesirable NVH phenomena such as gear rattle, body boom and other noises and vibrations, Schaeffler has developed specific solutions such as the dual mass flywheel for manual transmissions and double clutch transmissions as well as damping systems for torque converters in automatic transmissions and CVTs. Here centrifugal pendulum-type absorbers (CPA) are increasingly being used which are suitable for applications in systems with a dual mass flywheel or hydrodynamic torque converter as well as directly on the clutch disk [7].

Further development of the centrifugal pendulum-type absorber

The great market success of the CPA can be attributed, among other things, to the fact that the physical principle automatically results in a balance between excitation and pendulum vibration with the right frequency or excitation order. In this process, the vibration amplitude keeps increasing until the exciting mass no longer vibrates. This means that the CPA can compensate different phase positions that can occur at higher speeds or when coasting. The natural frequency of the damper changes over the engine speed in relation to the centrifugal force which itself is increasing quadratically with the speed – just like the firing frequency of the internal combustion engine that is the main excitation force, so that, given the relevant adjustment, the main excitation order of the engine is minimized.

However, undesirable, noticeable inherent noises may occur in a CPA. This is because gravity is dominant over centrifugal force starting at a certain point when the engine is turned off at decreasing speeds. This causes the pendulums to lose contact with the rollers. As a consequence, the rollers and pendulums may hit the flange or each other.

Figure 5 shows two solutions by Schaeffler. With the couple pendulum (left), which is already in volume production, the pendulum masses support each other through springs in a circumferential direction. Here the spring preload is selected in such a way that the pendulum remains in the guide track even if the engine is at a complete standstill. The effect of the almost constant spring forces overlapping with the speed-based centrifugal forces is largely compensated by correcting the order of the track. This type of spring arrangement is particularly helpful for first order pendulums, such as those needed for cylinder deactivation from four to two active cylinders. This is because gravity also generates a first order excitation in a rotating pendulum, which is another interference factor in addition to the excitation from cylinder deactivation.

Figure 5 Couple pendulum and iso-radial pendulum (top) with a diagram of vibration amplitudes in relation to the speed (bottom)

The approach using the iso-radial pendulum (right) is entirely different, permitting even lower vibration amplitudes to be achieved (bottom). In this approach, the individual pendulums are connected in one point by a ring not located in the torque flow, which means that the pendulum masses are now synchronized. One of the usual two spherical rollers is eliminated, causing the pendulum to carry out a swiveling motion rather than a purely radial motion. This design eliminates the first order excitation from gravity on the individual pendulum masses. Noise is controlled by means of stop elements to counteract contact loss at low speeds during stops.

In applications with dedicated hybrid transmissions (DHT) without a start-up clutch, it may be necessary to protect the entire powertrain from the occurrence of impermissible high peak torques. Special DHT torsional dampers have been developed for this purpose that include an additional slip clutch as a torque limiter, Figure 6.

Figure 6 DTH damper with integrated slip clutch as a torque limiter

V. Real power-on-demand actuators

Task

Automotive development focuses on energy efficiency. All energy consumers must be taken into account in order to utilize the full potential. These include actuators that actuate the components in the automated powertrain. If real power-on-demand actuators are used, the energy supplied to the electric motor must be converted appropriately and as directly as possible into adequate forces and pressures with accurately fitting travel and volumes. Another aspect is maintaining positions. Theoretically, there is no active energy involved, but in reality, a lot of energy is spent on maintaining a condition [8].

One important approach consists of designing actuators in a way that several consumers can be supplied and modulated. P2 hybrid structure concepts require a third clutch for double clutch transmissions so that another clutch actuator including electronic components must be added. In this case, hydraulic solutions are a better choice than electro-mechanical solutions because they are easier to expand by adding valves and scaling the pump-accumulator module. However, to turn precisely these hybridized double clutch transmissions into a milestone of efficiency requires appropriate actuators for such P2 double clutch transmissions with three clutches. A new approach here is the electrical pump actuator (EPA).

Application of the electrical pump actuator (EPA)

In the EPA, the sequential pressure build-up on the two working ports is achieved with the help of a passive two-pressure valve. Regardless of the EPA’s conveying direction, the two-pressure valve connects the lower pressure with a reservoir. This means that the EPA can be applied to a consumer, such as a clutch, in a forward direction and also modulate it by turning it back and forth. In reverse operation, the clutch pressure can be reduced completely. If the EPA continues to run in reverse operation, pressure builds up in the second working port to shift to a different gear selection. Two independent consumers can be operated sequentially by using an EPA with a two-pressure valve.

Figure 7 shows an EPA application in the double clutch transmission of a hybrid vehicle in P2 arrangement. The transmission can be operated with just two EPA and one hydraulic gear actuator (HGA). The control of the separating clutch (K0) can be mapped with another valve and another pressure sensor, similar to a conventional hydraulic control unit.

Figure 7 Actuator system for double clutch transmissions with two EPA and one HGA

On closer inspection, the electrical system architecture varies considerably. All that is left of the active electric motors of the gear actuator is two simple switching valves that can be controlled with simple valve output stages for the operation of electric motors. This makes it possible to question the entire transmission control unit and to replace it with the two EPA. As a consequence, expensive components can be eliminated, however, the intelligence of the transmission control unit must be mapped by the EPA. This requires a system and software structure that is oriented towards the independent actuation of both subtransmissions. Only the shifting coordinator with the gear selections and the coordination of the overlapping gearshifts must be doubled, distributed or transferred to a superordinate control unit for the powertrain which is usually available in hybrid drives. The elimination of the transmission control unit makes this architecture a very cost-efficient variant.

VI. More efficiency in individual transmission designs

Torque converter for automatic transmissions and CVT

The good controllability of the lock-up clutch in the torque converter of automatic transmissions and CVT is essential for the efficiency and isolation behavior of the system because it allows the torque converter to be locked as early as possible or to be operated with very little and finely controllable slip. The converter lock-up is controlled by applying pressure to the lock-up clutch piston. However, interference factors include fluctuations in the converter charging pressure and differences in the centrifugal oil pressure on both sides of the piston [9].

Figure 8 shows the design of the four-channel torque converter that allows the lock-up clutch to be controlled very precisely and independently of dynamic operating conditions. Two of the channels are used for the flow through the converter. The third channel serves to control the clutch, and the additional fourth channel serves as a pressure compensation chamber. This fourth channel ensures identical fluid conditions on both sides of the piston. The dynamic centrifugal force oil pressure is identical on both sides of the piston because the diameters of the actuation and compensation chamber seals are identical and the oil rotates at engine speed on both sides independently of the slip speed. In addition, the pressure chambers of the clutch are shielded from variations in converter charging pressure.

Figure 8 Design of the four-channel torque converter with four hydraulic connections for compensating centrifugal oil pressure

Double clutch transmission

In transmission development so far, the focus has been on the direct costs of components and systems. Against the background of increasingly strict CO₂ legislation, however, the indirect costs of consumption and power losses that lead to increases in CO₂ emissions due to drag losses and power consumption of actuators and cooling systems must also be reviewed [10].

Figure 9 shows three designs by Schaeffler with varying direct system costs and indirect operating costs. With regard to power loss, the systems with engagement bearings offer an advantage (center and right). The actuation system consists of a clutch slave cylinder and is activated by a hydrostatic clutch actuator (HCA). It uses a leak-free hydrostatic section to transmit the actuation energy to the clutch with little loss. Due to a travel sensor, the HCA does not need any additional return force gradient in relation to the engagement travel. This allows the power losses in the clutch to be minimized.

Figure 9 Wet double clutch with rotary oil feeds and rotating cylinders (left) as well as wet and dry double clutch with engagement bearings (center and right)

A more cost-driven approach is characterized by the use of a mechanically driven hydraulic pump (left), and, if necessary, supported by an additional electric pump to cover volume flow peak demands The transmission of clutch actuation energy is not achieved with the help of bearings but rather with rotary oil feeds, and the actuation of the transmission shift system is achieved hydraulically or electro-mechanically. The system does have its disadvantages because of the continuously running pump, but does well overall in terms of system costs.

Manual transmission

In spite of an increasing variety of transmission concepts, the manual transmission continues to be one of the most important transmission designs with regard to volumes. This means that efficiency gains have a particularly strong effect globally. In order to utilize any additional potential offered by new technologies to reduce fuel consumption and CO₂ emissions, however, it is advantageous to automate the clutch in manual transmissions. Based on measurements by Schaeffler, using a sailing only strategy on an RDE-compliant test track allows fuel and CO₂ savings of between 3 % and 5 %. In addition, the expanded recuperation of brake energy enables improvements of around 5 % with P0 mild hybrids and, when used in combination, of around 8 % [11].

Figure 10 shows three variants of how clutches of manual transmissions can be automated. MTplus (left) is an entry-level system for the partial automation of the clutch that allows driving strategies such as sailing without the driver’s involvement, even for manual transmissions. An additional actuator with a suitably small design is used in parallel with the existing master cylinder on the clutch pedal. Clutch-by-wire systems (CbW) are also perceived as normal manual transmissions with a clutch pedal by the driver. The actual actuation of the clutch, however, is always carried out by an actuator. This allows additional functions such as the start assist, traffic jam assist and slip control. As the name “by wire” suggests, there is neither a mechanical nor a hydraulic connection between the clutch pedal and the slave cylinder of the clutch. This also applies for ECM systems (right) that do without the clutch pedal altogether. Instead, a sensor on the gear selector recognizes the intent to shift gears and controls the opening of the clutch via the actuator. Besides its efficiency potential and new functions, the automated clutch has the added benefit of being able to protect the manual transmission from excessive loads, for instance in cases of misuse.

Figure 10 Variants for clutch automation: MTplus, clutch-by-wire (CbW) and electronic clutch management (ECM)

VII. Summary

With the growing number of drive concepts, the variety of transmission designs and components also continues to increase. This paper has shown numerous examples of the ways that this new variety of individual solutions advances the development of transmissions – including with regard to hybridization in all its complexity and electrical-only mobility. General trends such as automation share the goal of using as little primary energy as possible for driving motor vehicles, thus reducing CO₂ to the lowest amounts technology can achieve as well as increasing the driving range for a given energy accumulator.

Here, all relevant components in the transmission system must be included, from the further optimized transmission bearing to effective damping concepts that make drive strategies for reducing consumption and emissions possible to actuators with minimal power consumption. The implementation of customized concepts requires sophisticated CAE tools that put subsystems and systems in context. The result is efficient solutions for individual transmission designs that only involve minimal losses when transmitting power and converting torques.

Literature

[1] Scheidt, M.: The Combustion Engine: A Drive with a Future! 11. Schaeffler Kolloquium, Baden-Baden, 2018

[2] Faust, H.: Powertrain Systems of the Future. Engine, Transmission and Damper Systems for Downspeeding, Downsizing, and Cylinder Deactivation. 10. Schaeffler Kolloquium, Baden-Baden, 2014

[3] Faust, H.; Schübel, R.: Göckler, M.: Reduction of Energy Losses in Transmission Systems. CTI Transmission Symposium, December 2016, Berlin

[4] Faust, H.; Schübel, R.: Göckler, M.: Reduction of Energy Losses in Transmission Systems. CTI Transmission Symposium, December 2016, Berlin

[5] Petery, G. von; Rumpel, R.: Innovative Bearing Concepts for the Powertrain of the Future. 11. Schaeffler Kolloquium, Baden-Baden, 2018

[6] Heinrich, D.; Kerstiens, J.; Schneider, M.; Wittmann, C.: Innovative CAE – Optimal Layout of Transmission Components. 11. Schaeffler Kolloquium, Baden-Baden, 2018

[7] Kooy, A.; Seebacher, R.: Best-in-Class Dampers for Every Driveline Concept. 11. Schaeffler Kolloquium, Baden-Baden, 2018

[8] Müller, B.; Grethel, M.; Göckler, M.: Innovative Power on Demand Concepts for Transmission Actuation. 11. Schaeffler Kolloquium, Baden-Baden, 2018

[9] Heck, T.; Zaugg, B.; Krause, T.; Vögtle, B.: Efficient Solutions for Automatic Transmissions – Torque Converters and Clutch Packs. 11. Schaeffler Kolloquium, Baden-Baden, 2018

[10] Rathke, G.; Grethel, M.; Baumgartner, A.; Kimmig, K.-L.; Steinmetz, S.: Made-to-Order Double Clutch Systems. 11. Schaeffler Kolloquium, Baden-Baden, 2018

[11] Welter, R.; Kneißler, M.: The Manual Transmission Has a Future: E-Clutch and Hybridization. 11. Schaeffler Kolloquium, Baden-Baden, 2018

[12] Englisch, A.; Pfund, T.: Schaeffler E-Mobility – With Creativity and System Competence in the Field of Endless Opportunities. 11. Schaeffler Kolloquium, Baden-Baden, 2018

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