Efficient Solutions for Automatic Transmissions
In the automotive industry, the global trend to favor more efficient systems continues with the goal to further reduce CO₂ emissions. As a result, internal combustion engines continue to be optimized through the extensive use of cylinder deactivation and turbocharging. It is not just the weight of the complete vehicle – and thus of the drive train – that must be reduced, but also the mass moment of inertia of the components in order to offer improved driving dynamics and to rapidly reach the optimum-efficiency operating point of turbocharged engines.
In addition, the reduction of CO₂ emissions also requires a lock-up clutch that is capable of precisely controlling slip in all operating conditions. In new-generation transmissions, there has been a tendency to increase the number of gears; resulting in additional reductions of installation space for the torque converter.
This paper aims to show technical solutions for the challenges described above.
The ability to control the lock-up clutch in the converter is essential for the efficiency and vibration isolation of the system. It is important to ensure that the clutch can be opened or closed on demand in all operating conditions and that the targeted slip speed can be maintained precisely. Allowing slip in the torque converter clutch is an effective method to reduce drivetrain vibration, however, efficiency is lost in the form of heat as a function of increased slip speed.
Figure 1 Comparison of a two-pass and a three-pass converter
In simple terms, the torque converter clutch capacity is characterized by the following:
• The active surface area of the piston, the effective radius of the friction material and the number of friction surfaces
• The friction coefficient between the friction material and steel
• The pressure applied to the piston.
While the geometry and the friction coefficient of a given design are fixed, the pressure that controls the clutch can be varied by the hydraulic control system of the transmission. In an ideal system, the torque capacity of the clutch and the required slip would be controlled exclusively by varying the apply pressure.
Torque converter clutch interference factors
In reality, there is no ideal system because there are other factors that affect the transmitted torque of the clutch. The interference factors below represent the areas in which two-pass and three-pass systems have shortcomings, making it difficult to accurately control clutch slip.
1. Pressure drop across friction material: In two-pass converters, the friction material not only serves to transmit torque, but it is also a sealing component on the outside diameter of the piston. In order to cool the clutch, a groove pattern is often pressed into the friction material. When ATF flows through the grooves from the high pressure side of the piston to the low pressure side, it experiences a pressure drop. The magnitude of this pressure drop is dependent on the groove geometry, consistency of the friction faces, temperature, and slip speed.
2. Absolute system speed: After the ATF has flowed through the friction material grooves in a two-passage converter, it must be transported radially from the outside diameter of the converter to the inside towards the transmission input shaft. Since the entire system is spinning, the fluid particles are subjected to Coriolis forces on their way to the inside, leading to the formation of a spiral flow in front of the transmission input shaft. This results in back pressure that reduces the effective pressure on the piston.
3. System pressure variation: Fluctuations in converter charging pressure affect the high-pressure side of the piston in a two-pass system and the low-pressure side of the piston in a three-pass system.
4. Differential speed (slip): During open or slipping conditions, two and three-pass systems have components such as the damper, turbine or cover on either side of the piston which are rotating at different speeds. These components dominate the mean rotational speed of the ATF on either side of the piston, which results in a different centrifugal force, creating a relative pressure across the piston.
Potential solution: Converter with four hydraulic passages
Figure 2 Design of the four-pass torque converter
Like the three-pass system, two of the passages are used for the flow through the converter, and the third passage serves to control the clutch. The unique feature of the four-pass torque converter is the additional fourth passage, which feeds a pressure compensation chamber. This results in identical fluid speed conditions on both sides of the piston. The dynamic centrifugal force of the ATF is identical on both sides of the piston because the outside diameters of the activation and compensation chamber seals are the same. This means that the piston pressure is now independent of slip speed, and furthermore, the pressure chambers of the clutch are shielded from system pressure variations, i.e. from charge pressure fluctuations.
Comparison of systems
Figure 3 Comparison of the slip speeds of two-pass, three-pass and four-pass systems
The comparison shows that in this specific four-pass application the lock-up clutch can be engaged even in first gear. Besides fuel consumption savings, this also means that the lock-up clutch can be used as a launch device in line with the torus of the converter. This allows a smaller and more lightweight design of the torus. In higher gears, the four-pass converter can be operated at a very low slip speed due to its precise controllability. As a result, the damper can be designed on a smaller scale, allowing a more space-saving design of the converter as a whole.
Figure 4 Design of a conventional converter (left) and an iTC (right)
Figure 5 Integrated torque converters (iTC, top) for front-wheel (FWD, left) and rear-wheel drives (RWD, right)
A comparison shows that many of the conventional applications have a dual-surface clutch while the iTC uses a single-surface clutch. This reduces complexity, but to ensure sufficient clutch capacity to accommodate engines of similar or higher torque, the clutch in the iTC is designed with a high taper, resulting in higher torque capacity due to the increased angle. Furthermore, the higher taper impeller surface improves stress distribution, allowing the use of thinner material, thus reducing mass. Additional savings are achieved by using Schaeffler’s stamping technology that replaces heavy sintered hubs and other components.
Coast engagement diaphragm spring
In the pursuit of efficiency, the precise controllability of the torque converter clutch during coast is a fundamental part of the modern powertrain. This can present a challenge when utilizing a 2-pass clutch, such as the one in the iTC; but this is addressed through some of the unique characteristics of the design. Since the clutch function is combined with the turbine, the axial forces of the hydrodynamic circuit have an effect on the controllability of the clutch. In drive mode, the iTC principle offers an advantage because the axial forces in the converter ensure that the clutch is biased towards engagement.
During coast, however, the opposite is true. The clutch is biased open by the hydrodynamic axial forces. In order to override this condition, ATF flow can be used to generate increased pressure, but this may cause the clutch to rapidly close causing an undesirable torque bump. An alternative to the control strategy of keeping the clutch closed at the transition to coast is to support the clutch mechanically in coast in order to achieve a seamless engagement.
Figure 6 Design and function of the coast engagement diaphragm spring
Laser etching surfaces
With the introduction of the iTC, new processes were developed to accommodate the differences in design while still meeting all customer requirements. Integrating the clutch function into the turbine requires the friction material to be bonded to the turbine instead of to a separate plate. As an industry standard, blasting with aluminum oxide (Al2O3) has been used to prepare and roughen the steel surface for the bonding process. Due to the geometry of the turbine, with its numerous blades, there is a risk that aluminum oxide residue will remain on the component even after washing and eventually enter the transmission. This could damage the transmission valve body and would not be acceptable for meeting the increasingly stringent contamination requirements. Schaeffler has found the answer in the laser etching process. In this process, individual particles are vaporized on the steel surface by a laser, resulting in a rough and clean surface.
Figure 7 Preparation of adhesive surface by laser etching
Potential of the centrifugal pendulum-type absorber
The advancement of absorber technology is critical given the ever increasing requirements for NVH isolation and efficiency. In this pursuit, consideration must be given to optimizing both the packaging space needed and the effectiveness of the absorber, specifically to provide excellent NVH at low engine speeds with a fully locked clutch.
Figure 8 Isolation comparison of torsion dampers with and without a CPA
In the future, even lower lock-up speeds down to 800 rpm will be required to further increase the efficiency of the drive train.
Torsion damper with double centrifugal pendulum absorber
The torsion damper (TD) and the double torsion damper (DTD) are the most frequently used damping systems with a CPA. While the DTD has been established as a CPA damping concept for applications with rear-wheel drive, the TD with a CPA is particularly suitable for front-wheel drive vehicles. This is because the axial design space required is small and the sensitivity is in the medium speed range, which is lower than that for rear-wheel drives, allows the elimination of a spring damper behind the CPA.
Figure 9 Isolation comparison of torsion dampers with CPA and double CPA
If the CPA were instead arranged with the turbine at the damper output, the natural frequency of the transmission input shaft mode would shift to the medium speed range. Compared to the DTD with a CPA, this series torsion damper (RTD) with a CPA offers much improved isolation below 1,000 rpm. However, this is at the expense of the resonance point at medium speeds between 1,000 and 2,000 rpm.
A potential solution could be the use of a DTD with a double CPA. The innovation of the double CPA is the combination of the strengths of previous damper concepts. Both CPAs are adjusted to the same order. Above 1,000 rpm, the CPA offers excellent isolation on the intermediate flange, and below that, the second CPA at the damper output offers the best possible isolation for lock-up speeds down to 800 rpm. A double CPA utilizes smaller masses than a torque converter with a single CPA, but it achieves significantly better isolation. This clearly demonstrates the importance of arranging the CPA. The concept is also suitable for applications with cylinder deactivation. Here, one of the CPAs is designed for full engine operation and the other for cylinder deactivation.
When designing the DTD with a double CPA, the primary goal was to make the overall system as compact as possible. In order to reduce additional axial space requirements for the other CPA, the “inline CPA” was developed, as shown in Figure 10.
Figure 10 Design of the inline CPA using a DTD with a double CPA as an example
In this design, the masses and rollers are in the same plane as the flange. The mass is centered axially by a thin cover plate on both sides of the flange, which helps increase the weight of the pendulum masses. With a comparable effective mass and inertia, the axial design space requirements of the second CPA are reduced by around 2 mm. In addition, the first CPA on the intermediate flange can be designed smaller due to the use of the second CPA. This allows the additional design space requirements of the damper system to be minimized.
Cylinder deactivation in four-cylinder engines
Figure 11 Isolation potential of torsion dampers with CPA and double CPA upon cylinder deactivation of two cylinders of a four-cylinder engine
Combining a CPA on the intermediate flange and a two-stage damper, where the CPA and the first damper stage are designed for two-cylinder operation, significantly increases the isolation effect and allows driving with a fully closed lock-up clutch down to 1,200 rpm. Isolation in four-cylinder operation remains nearly constant.
A good solution for further improving this isolation is the addition of a CPA for full engine operation, i.e. a concept with a double CPA. The second damper arranged behind the CPA is replaced by a CPA so as not to increase design space requirements. In addition, the remaining upstream damper is enlarged to ensure good pre-isolation. The resulting TD+dCPA damper concept offers very good isolation in four-cylinder and two-cylinder operation for nearly the entire speed range.
Effects of gravity on the CPA
In applications with a main excitation order of one, a solution must be found for not only drivetrain isolation, but also for the influence of gravity. These gravitational effects are typically trivial. This is not the case for a first order CPA.
Figure 12 Effect of gravity on the pendulum motion in a 1st order CPA
Figure 13 Damper concepts TD and DTD with double CPA
With the CPA, excellent isolation is achieved across the entire driving range due to its ability to cancel specific order content regardless of engine speed. However, for applications with excitation orders below one, CPA design solutions become increasingly challenging due to CPA envelope requirements (track length and lack of curvature).
At these extreme low orders, it is also evident that the ability to adapt the speed only rarely provides a significant advantage. A fixed frequency absorber can be an alternative to the CPA. Designed as a Tuned Mass Absorber, the Turbine Tilger (Absorber) uses the turbine inertia as an absorber mass during the closed lock-up clutch condition. Since the turbine is already an integral part of every torque converter, its mass/inertia can be used as an absorber for “free” (no additional absorber mass/inertia to the TC). Of course, additional absorber mass/inertia can be added to improve the absorber’s effectiveness. A reasonable embodiment of the Turbine Tilger concept consisting of the spring rate, mass distribution and friction can achieve an improvement in isolation in the lower speed range compared to a conventional serial damper, as shown in Figure 14.
Figure 14 Isolation comparison of torsion dampers with and without turbine tilger
The first damper concepts with Turbine Tilgers were developed as far back as 2007, sharing the same timeframe as the CPA. Initially, the CPA took center stage with its speed adaption advantage. Recently, cylinder deactivation applications (e.g. three-cylinder engines with static cylinder deactivation) have increased demand for sub-first order isolation solutions. Turbine Tilger optimization quickly followed.
Initial Turbine Tilger concepts had the turbine connected to the series torsion damper through an additional third torsion damper (the absorber spring). Optimization led to the integration of the absorber spring element into the main torsion damper envelope. The newly freed-up design space can then be utilized for increasing the absorber mass/inertia, if required. The stamped/formed part used for this can be easily adapted to the given design space. Schaeffler has been able to use its early experience with Turbine Tilgers for the rapid development and introduction of a volume production solution since the start of 2018.
The previous chapters discussed technologies for torque converters and related damping systems. These concepts can also be used for other components, assemblies and systems in the drivetrain. Examples of these technologies include the following:
• Forming technology: Expensive, heavy sintered or cast iron parts can be replaced by stamped parts in a lightweight design or integrated into other components.
• Joining technology: Numerous innovations have been achieved over the past few years, particularly in riveting technology, included in combination with forming technology.
Figure 15 Concept for clutches with an optimized design space
Figure 16 Concept of a clutch with self-amplifying actuation
This paper has presented various approaches by Schaeffler for optimizing space, mass, control and efficiency of damper and torque converter systems for automatic transmissions. For improved controllability of the lock-up clutch in all driving conditions, a four-pass torque converter is an excellent solution. The integrated torque converter (iTC) offers a simplified setup and significantly optimized design space utilization by integrating the piston into the turbine. Additional improvements with regard to design and process technology have been described for the iTC.
Vibration isolation also plays an important role in the converter. The potential of the centrifugal pendulum-type absorber (CPA) is far from being fully utilized. A new approach, shown here, is the use of a double torsion damper with a double CPA in the torque converter. Further, the paper describes a damper concept for four-cylinder engines with cylinder deactivation and the use of the turbine mass as an absorber mass to be able to offer an alternative to a CPA in certain applications.
Schaeffler’s expertise in torque converters and related damping systems is also applied to electrified drivetrains. Schaeffler is always ready to support transmission manufacturers and developers in future challenges.
Schaeffler offers technologies that improve the efficiency of the transmission through reduced losses while also allowing the use of new engine technology. In terms of the requirements for the torque converter lock-up clutch and damper, this means that vibration isolation becomes significantly more challenging . One potential solution could be light-weight components, and another could be the intelligent integration of existing masses into the system to isolate the vibrations.
The potential of the centrifugal pendulum-type absorber (CPA) for isolation in the drive train has been clearly recognized by the automotive industry. After Schaeffler introduced the CPA for the dual mass flywheel (DMF) in a dry environment in 2006 and started volume production in 2008 , volume production for the CPA for the torque converter began in 2010. The CPA was designed to function in a wet environment and to provide excellent vibration isolation with a fully locked torque converter clutch.
• Friction systems: Schaeffler has been actively involved in the development of friction material for many years. A special friction material has been developed for torque converters that is now used in almost all Schaeffler converters for the lock-up clutch. This expertise in wet friction material  can be easily applied to other clutch functions in the automatic transmission because the requirements and environmental conditions are similar.
Drivetrain electrification will include the automatic transmission. At Schaeffler, the technology transfer from conventional converters to hybrid solutions results in a P2 hybrid module with an integrated converter . Instead of a traditional sandwich design, which places the hybrid module between the engine and the converter, the rotor of the electric motor is riveted to the converter cover. This eliminates an intermediate wall, which saves on axial design space. In addition, the wet K0 separating clutch is designed as a stamped component and uses the same friction material utilized in converter clutches. Volume production of a P2 hybrid module, similar to the one described here, will begin in 2019.
An advanced concept of a self-amplifying clutch is shown in Figure 16. Here, a leaf spring generates additional contact pressure. This makes it possible to reduce the clutch actuating pressure as well as hydraulic losses .
 Freitag, J.; Häßler, M.; Lehmann, S; Raber, C.; Schneider, M.; Wittmann, C.: The Clutch Comfort Portfolio – From Supplier’s Product to Equipment Criterion. 10. Schaeffler Kolloquium, Baden-Baden, 2014