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Voith is an engineering company that employs more than 43,000 people and operates in about 50 countries around the world. It is one of the largest family-owned businesses in Europe and supplies essential markets, including energy, oil and gas, paper, raw materials, and automotive and transport.
As gas prices continue to be turbulent and environmental concerns grow, fundamental behavioral shifts are taking place. The energy crisis is not only turning consumer preferences to smaller and more fuelefficient vehicles, but it is also engendering a keen awareness of when and how frequently people use their cars.
More and more people are choosing public transportation because it is a cheaper and more environmentallyfriendly alternative for getting around. According to a recent Public Broadcasting Service (PBS) report, mass transit ridership has increased significantly in nearly all American cities. In Boston, Massachusetts, subway ridership has grown 9 percent; in Seattle, Washington, the number of commuter rail passengers is up 28 percent; and in San Antonio, Texas, bus ridership has risen by 11 percent. While transit officials welcome the growing number of commuters, they cannot escape the volatile oil prices and elevated cost of mass transportation service.
For governmental authorities, this situation puts significant constraints on bus and transportation services paradoxically at a time when demand is on the rise. Furthermore, the implementation of increasingly strict emission standards, like the new Euro 5 standard, raises the performance bar another notch. Buses are more fuel-efficient and emit less pollution while providing superior passenger comfort. To economize, bus operators must make full use of their resources; this means driving as efficiently as possible and minimizing wear and maintenance.
According to Voith Turbo, the way to answer many of these concerns is to implement an automatic transmission. Part of one of the largest family-owned companies in Europe, Voith Turbo is a leading specialist in power transmissions. Based in Heidenheim, Germany, it has developed a four-speed automatic transmission especially for city buses. Founded in 1867, the company produces state-of-the art hydrodynamic drive, coupling and breaking systems for road, rail, marine and industrial applications.
Voith Turbo’s automatic transmissions for buses – DIWA.3E and DIWA.5 – can be installed in any driveline bus and will enhance fuel economy, passenger comfort, operating efficiency and reliability. Voith Turbo has delivered more than 150,000 DIWA automatic transmissions to bus customers in Asia, Europe and the United States.
Voith Turbo’s DIWA transmission is characterized by its unique differential torque converter principle: the lower-speed range of the DIfferential WAndler (converter) transmission replaces the first two gears of a standard automatic transmission. The differential torque converter principle functions on the basis of power separation: the engine power is split into mechanical and hydrodynamic components, which are later rejoined in the output drive.
The result of this differentiating power process is a continuous acceleration within a speed range in which more conventional gearboxes must be shifted two to three times. Not only does this reduce the amount of gearshifts by 50 percent, but it also results in better fuel consumption in the lower speed ranges thanks to the partly mechanical power transmission as well as a diminished thermal load on the engine cooling circuit using the power-split principle.
Since the four-speed DIWA transmission is comparable to a standard five- or six-speed gearbox, its prolonged drive range allows shift-free driving that leads to higher driving comfort and less maintenance. For example, a typical oil change interval would be performed every 180,000 kilometers.
The DIWA transmission comes with intelligent acceleration-dependent gearshift programs that adapt to topography, vehicle acceleration, axle transmission ratio and load conditions. This optimizes the shift points and minimizes fuel expenditure. The transmission is further equipped with a hydraulic torsional vibration damper for minimal engine speed after shifting as well as standard automatic neutral shifting for interrupting power transmission between the engine and transmission when the bus is stationary.
Moreover, it is possible to select from four different gearshift programs adapted to operating conditions. The transmission state is continuously monitored by patented software to prevent transmission overhauls and, interestingly, comprehensive operational data is recorded for later vehicle design optimization as well as maintenance and repair.
Like most automatic transmissions, the DIWA transmission is hydraulically operated and this is where Siemens Digital Industries Software stepped into the picture. In 2005, Voith Turbo requested a hydraulic valve case study to improve the hydraulic valve behavior in terms of stability and functionality. The company wanted to analyze its systematic processes and understand the test results. It was also interested in introducing a systematic development process for mechatronic products as well as parameter variation studies and other enhanced solutions.
Voith Turbo opted for Simcenter Amesim™ software from Siemens Digital Industries Software with three different libraries: Simcenter Amesim Hydraulics library, Simcenter Amesim Hydraulic Component Design library and Simcenter Amesim Hydraulic Resistance library. Simcenter Amesim helps Voith Turbo engineers visualize hydraulics systems and their related interactions.
“Using 1D multi-domain system simulation is particularly valuable since the dynamic behavior of such systems is hard to predict and every single subsystem needs to be taken into account,” says Bernhard Höfig of Voith Turbo’s mechatronics and simulation department.
Simcenter Amesim libraries gave Voith Turbo a systematic way to examine and develop the hydraulic valves in an automatic transmission. The integrated Simcenter Amesim platform and dynamic system analysis helped Voith Turbo gain a better understanding of the overall hydraulic behavior. Thanks to the simulation studies and validation of physical 1D models for complex hydraulic systems, it became possible to develop a new design for the hydraulic valves for an automatic transmission that was able to meet the tough functional requirements for more efficient and environmentally sustainable city buses.
What differentiated Simcenter Amesim from Voith Turbo’s previous solutions is the scope and quality of the libraries, the various model representations and the fully-functional validated 1D models for design and testing purposes. Moreover, using Simcenter Amesim, the overall testing time was considerably reduced and fewer prototype iterations were necessary.
“Simcenter Amesim provides us with a powerful and flexible simulation environment for a systematic and detailed analysis of dynamic system behavior,” says Höfig. “This helps us to continuously improve our design and development process.”
Höfig notes, “Simcenter Amesim rapidly proved to be the ideal solution for the design of valves in our automatic gearboxes. It definitely helped to streamline the design and development of our transmission systems, making them readily available for the transportation market.”