Transmission specialist reinvents the dual clutch
Dual-clutch transmissions provide an excellent example of how long it takes to bring some automotive innovations to the mass market. Although the dual-clutch transmission for shifting gears without any interruption in tractive force was first patented in 1940, it was not until the 1980s that it achieved practical success in the Porsche 962 Group C racing car and subsequently in the Audi S1 rally version.
But at that time, the technology was too expensive to allow series production. Around the year 2000, Volkswagen revisited the Porsche dual-clutch transmission and made it suitable for series production. The experts at Getrag Ford Transmissions are now contributing their knowledge in the field of transmission technology to the further development of this transmission.
Getrag Ford Transmissions was founded in 2001 as a joint venture between Ford and the transmission specialist Getrag. The company develops and manufactures manual and automated manual transmissions as well as dual-clutch transmissions for all Ford brands as well as for other OEMs.
The joint venture will soon launch a new generation of dual-clutch transmissions that allow automatic gear shifting without any loss of power flow. In these transmissions, the oil does not just act as a lubricant but also provides the hydraulic control. Using computer simulation, engineers were able to ensure proper oil circulation under all operating conditions.
Oil quantity demands balance
The biggest problem in the development of a dual-clutch transmission is how to place two clutches together with all the associated hydraulic and electronic components in a space no larger than that required for a conventional transmission case. "To transfer maximum torque using a clutch that is only half the normal size, we use 'wet' clutch systems that operate in an oil bath for high performance engines," explains the company's head of CAD Product Development, Rüdiger Muth. "This means that oil circulation through the complex internal mechanism must be guaranteed under all operating conditions."
Design engineers want to know as early as possible how much oil they must put into the transmission to reach a given oil level, and how this level changes when the vehicle accelerates, brakes hard, or corners at speed.
The issue of oil quantity is by no means a trivial one, as Ronald Engel, technical specialist for pump and lubricant technology in dual-clutch transmissions at Getrag Ford Transmissions explains, "The financial people and the experts responsible for transmission efficiency want to have as little oil as possible in the transmission, whereas the people responsible for durability want as much as possible. And, if possible, we want to guarantee that the customer can drive 150,000 miles without having the transmission system serviced. All this must be calculated up front in order to find the right balance."
In the past, engineers used a graduated beaker to fill prototypes with a given quantity of oil. There was a certain amount of spillage, but even more expensive and time-consuming were examinations of the oil level under driving conditions—which not only required a tilting test rig but also the fabrication of a transparent housing (at a cost of about 100,000 Euros!).
To help avoid these costs, and to obtain this information earlier in the development cycle, engineers tried using mathematical fill level simulation. Ford's engine development department had such a tool but it turned out to be unsuitable for transmission development since a gearbox, unlike an engine sump, consists of hundreds of individual parts. The various assemblies had to be painstakingly simplified and the parts fused in order to calculate the enclosed volume. That operation alone took nearly four weeks.
Reliable data early in the process
Since no commercial software could be found to determine oil levels digitally, Getrag Ford Transmissions partner, Siemens PLM Software, offered to develop a solution.
The result, based on the NX® digital product development system, allows the calculation of volumes enclosed by multiple individual parts. Now commercially available as the SpaceFinder function in NX, it performs a type of mathematical filling simulation that is suitable not only for digital oil level measurements but, in principle, for all applications where engineers must calculate complex design spaces surrounded by a variety of individual components. Other examples include the calculation of trunk space or an analysis of design spaces involving models from different CAD systems—in other words analyzing spaces that are not really mathematically closed, which are frequently found in real-life situations.
SpaceFinder readily calculates an oil fill level. The fill simulation provides the user with a model of the "frozen" oil, which is then used as a basis for further analyses ."For example, I can specify an oil level that I need for optimal transmission operation and the software determines the corresponding volume," says Engel. "This optimization takes into account a minimum level for lubrication and a maximum amount that is dictated by price and efficiency considerations," he notes.
