- Hydraulic torque converter
- Multi Function Torque Converter (MFTC)
- Lock-up clutches
- Damper for lock-up clutches
- Super Slim Stator
Hydraulic torque converter
In principle, the converter hydraulically transfers engine torque onto the transmission intake shaft. The pump and therefore the entire converter housing has a torque-proof connection to engine and turbine via hub overlap with the transmission intake shaft. The entire converter is filled with gear oil. The pump and turbine accommodates blades, which at differential speed cause a circular oil flow between pump and turbine. The oil is extracted from the internal diameter of the pump and forced outwards by the centrifugal force. The oil is then cast from the pump into the turbine, where it is deflected by the turbine blades. This generates a torque in the turbine and/or the transmission intake shaft.
At start-up or high differential speeds between pump and turbine, the oil flow is deflected into the turbine and forces the guidewheel backwards. However, installed inside the guidewheel is a freewheel that causes the guidewheel to be disabled by the stator shaft when reversing. This generates a guidewheel torque, which increases over engine torque by up to factor 3 due to the torque equilibrium in the converter. Converter efficiency is therefore particularly high in start-up situations.
It must be considered that the hydrodynamics of the converter can only transfer torque when there is a speed differential between pump and turbine. When, during driving, the speed between pump and turbine has equalised, a bridge clutch hydraulically activated by the gearbox engages. This eliminates slip and power loss during converter operation, thereby reducing fuel consumption. LuK guarantees early bridging and thus economic consumption through the use of innovative dampers.
On the engine side, the converter is mounted to a flexible flywheel. The pump collar usually also acts as a drive for the oil supply pump on the automatic transmission. To achieve optimal performance, LuK consistently employs Computational Fluid Dynamics (CFD) simulation tools for optimal flow control and thus reduced consumption in the vehicle.
Multi Function Torque Converter
The torque converter has proven itself to be the most robust vehicle launch device for over 100 years. The secret to its success is that the torque it transmits is always proportional to engine speed and decreases as the vehicle reaches cruising speed. This reliable performance has come with a price – the converter can produce only one torque for each operating condition.
That means that even at idle speed the converter is consuming torque and therefore fuel. Furthermore, the launch event can only be optimized for a certain acceleration profile. Even worse, if the engine produces a high torque at a low speed, for example, in the case of turbo diesels, the torque converter prevents the engine from accelerating due to the high torque it must transmit. An additional challenge is isolation of engine vibration. Every torque converter includes a damper for this purpose, but as engine torques rise, isolation becomes a more difficult challenge. Large and complex dampers must be designed to meet this challenge, increasing cost and inertia.
All of these issues are resolved by the Multi Function Torque Converter. This product offers an elegant solution to the challenges of the modern drive train by the addition of one simple which allows redistribution of existing inertia and provision for converter disconnect.
Idle Disconnect realized seamlessly
Disconnecting the fluid circuit of the torque converter reduces the torque demand on the engine at idle from 30 or 40Nm to nearly zero. This fact alone accounts for an improvement in fuel economy of approximately 2%. The Multi Function Torque Converter provides this disconnect function within the converter itself with the addition a simple clutch between the converter cover and the impeller. The ability to disconnect and reconnect the converter at will, also allows control over the launch event.
The engine speed can be controlled based on throttle position. Higher throttle position would mean the transmission controller allows a higher engine speed before completely re-engaging the converter. For lower throttle positions, a lower engine speed can be selected. For the first time in the history of the torque converter, the calibrator has the ability to calibrate the launch event.
The impeller inertia is fairly small and therefore very easy to accelerate when reconnecting the converter, resulting in a very smooth event compared to shifting a transmission into gear. The inertias involved can be seen by comparing Condition 1 and 2 in the cross sections at the right. A measurement of reconnection compared to a shift from neutral to drive can be seen below. This reveals why many transmission disconnect systems are rough and difficult to calibrate – the inertia of the turbine, damper and transmission parts is much greater and therefore more difficult to engage smoothly.
Performance Launch enabled!
Low inertia is very important to rapid engine acceleration. The figure for condition 1 reveals that a very small percentage of the converters inertia is actually connected to the engine when in disconnect mode. The engine is therefore free to accelerate rapidly to the desired speed. A high engine speed means a higher turbo boost. When the converter is reconnected, the torque to the wheels is higher throughout the launch event. The turbo diesel launch simulation shown elow illustrates the achievable advantage.
Isolation without complexity
Good isolation of vibration requires a low torsion spring rate and a large inertia to move the natural frequency below the driving range. An elegant solution to this problem has been in production in manual transmission vehicles for 25 years – the Dual Mass Flywheel. The principle is to split the existing inertia in half and move the existing damper between these two halves. A similar arrangement is offered by the Multi Function Torque Converter, as seen in Condition 3 of the cross sections. Here the red inertia is coupled to the transmission and the blue is coupled to the engine with the normal damper between them. The resulting NVH measurement is shown below. This excellent isolation allows operation in lock-up mode over all of the driving range, improving fuel economy significantly.
An effective system provides multiple benefits
The final measurement shows the fuel economy achieved with the Multi Function Torque Converter and the recalibration of the system which it enables. An increase of 3.5% in miles per gallon while increasing driving comfort and enabling new possibilities for turbo-diesel launch. The good, old torque converter is ready for the demands of the 21st century!
Lock-up clutches
Lock-up clutches prevent slip under normal driving conditions, as a converter can in principle only transfer torque with slip, which would result in higher consumption. The lock-up clutch is actuated and controlled by the gearbox. In many applications, lower continuous slip is set selectively under certain load conditions, in order to prevent gearbox noise. Modern gearbox control units are able to selectively set a constant slip of approx. 5 rpm, which has practically no influence on consumption. Also, in certain driving situations, the dynamics of the vehicle can be improved in the short-time by selectively increasing engine speed.
The friction occurring during these operations (a few watts through to several kilowatts) can overheat the automatic transmission fluid (ATF) at certain points, therefore significantly reducing the life of the gearbox. An effective system for cooling friction linings is therefore required. Oil has to be selectively routed through or over the friction linings. Standard materials are high-performance paper linings or carbon fibre linings. Diaphragm cooling units are also used, whereby the oil is passed through a hole in the piston near to the friction surfaces. In this case, however, the efficiencies of lining flow cooling are not achieved.
Luk therefore always goes for flow-cooled systems. At LuK, non-fluted carbon linings that allow cooling oil to pass through the carbon fibres and/or fluted paper linings with impregnated continuous flutes etc. are now the accepted components. Flow-cooled conical clutches offering special benefits were also tried and tested (LuK TorCon System). A host of LuK patents in this area highlights the complexity and importance of efficient high-performance cooling. For optimal application, specially developed simulation programs are used for flute design.
Damper for lock-up clutches
Torsion dampers minimise torsion excitation within the powertrain and therefore enhance comfort. This can also be achieved by slip on the lock-up clutch, although this inevitably increases fuel consumption. Torsion dampers have practically no influence on fuel consumption. There are various arrangements and shapes within converters of torsion dampers. It is often necessary to optimise these to the vehicle application in question. This where LuK can draw on its long-established experience of standard production and use its calculation tools to optimise damper arrangement (turbine dampers, conventional dampers, dual dampers etc.), spring shapes (straight or elbow springs), spring rates and the necessary damper frictions.
Conventional dampers
The damper within the powertrain is arranged before the turbine. These dampers are ideally suited to 4-cylinder front-cross applications as an internal damper with straight springs or as an outer damper with elbow springs. However, because on automatic transmission gearboxes the transmission intake shaft is realised with soft torsion and therefore acts as a spring itself, in certain applications, the converter turbine can vibrate under its own resonance in the drivable range, thus causing higher noise levels.Turbine dampers
The damper is arranged in the force flow behind the turbine, so that damper springs and transmission intake shaft are connected in series, creating a very soft spring. The turbine is arranged as a mass on the engine side. This eliminates the need for the turbine shape common to the conventional damper. What’s more, this damper is always arranged in the force flow, enabling hydrodynamic vibrations in open converter operation to be damped. The turbine damper is designed primarily for 6 and 8 cylinder engines with standard and 4-wheel drive.Dual torsion damper
Series-connected damper arrangements can also offer benefits for special applications.Super Slim Stator
A breakthrough in torque converter technology which uses stamped parts to redefines the space in the converter. Axial space savings of 60% or 18mm is achieved in the most critical dimension of the converter, along the hubs and shafts. In addition, the torque capacity or MP2000 characteristic is improved by10%. This pace can be used for high-capacity dampers, multiple plate clutches or for simplification of other components. These advantages are possible through a new blade design and a high torque-capacity one-way clutch concept developed by LuK.The various elements of the torque converter: fluid circuit, lock-up clutch and damper, have undergone tremendous optimization efforts over the last 100 years. Today, a torque converter with a length of 80mm can be used in an application where 125mm was once required. However, this effort has not had much influence on the stator and one-way clutch. It’s axial length is about the same and it now represents a roadblock to further axial length reduction. To break open this roadblock, LuK has developed two new paradigms.
First, the blade length has been reduced half. Since, the blades play the critical role of turning the fluid in the fluid circuit, thereby creating torque multiplication, their function must be maintained. This is accomplished by doubling the number of blades. The blades now carry only half the torque per blade and so their thickness can be reduced. This results in an increase in flow-area of the stator, especially near the coupling point. This improves torque capacity at the coupling point, improving fuel economy. These blades can be easily made from sheet metal as two stampings. Each stamping carries half of the blades and they are assembled around the one-way clutch.
LuK’s second paradigm shift is in the one-way clutch concept. Since the blade plates present adequate sheet metal, a ratchet one-way clutch can be created from only a few additional stamped components. The high torque density of this type of clutch allows a significant length reduction, complementing the new blade design.
Together, these new paradigms create the Super Slim Stator assembly. In the first application, an axial length savings of 60% or 18mm was achieved. With an overall converter length of only 80mm, this is a significant achievement. This space can be used for a variety of purposes depending on the application. If cost is the key driver, a more efficient piston plate can be created and the damper connections can be more easily made. This allows a thinner piston plate and saves welding operations. If axial length is the key issue, the new space can be used for its reduction. If space is needed for a high capacity damper or a twin-plate clutch, it is available due to the Super Slim Stator from LuK!
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