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!