Voith Turbo credits LMS Imagine.Lab with critical transmission design improvements
As gas prices continue to be turbulent and pressing environmental concerns are everywhere, paradigmatic behavioral shifts are taking place. The energy crisis is not only turning consumersí preferences to smaller and more fuel-efficient vehicles, but it is also engendering a necessary awareness of when and how frequently people use their cars.
More and more people are finding their way to public transportation as a cheaper and more environmentally friendly alternative for getting around. According to a recent US PBS (Public Broadcasting Service) report, mass transit ridership has increased significantly in nearly all US cities: in Boston, subway ridership has grown 9 percent; in Seattle, 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 in particular, this economic situation puts significant constraints on bus and transportation services, paradoxically at a time when demand is on the rise. Furthermore, increasingly stricter emission standards, like the new Euro 5 standard, raise the performance bar another notch. Buses today are more fuel-efficient and less polluting yet still superior in 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 answer to many of these questions is the automatic transmission.
Part of one of the largest family-owned companies in Europe, Voith Turbo, a leading specialist for power transmission based in Heidenheim, Germany, 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 for ensured fuel economy, passenger comfort, operating efficiency and reliability. Worldwide, Voith Turbo has delivered over 150,000 DIWA automatic transmissions to bus customers in Asia, Europe and the US.
The difference behind the DIWA
Voith Turbo’s DIWA transmission is characterized by its unique differential torque converter principle: the moving-off 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 where more conventional gearboxes must be shifted 2 to 3 times. Not only does this reduce the amount of gearshifts by 50 percent, it also means 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 5 or 6 speed gearbox, its prolonged drive range allows shift-free driving which leads to higher driving comfort and less maintenance. For example, a typical oil change interval would be every 180,000 kilometers. This makes DIWA the epitome of economic driveline bus operation, according to a company spokesperson.
Intelligent transmissions
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.
LMS Imagine.Lab steps in
Like most automatic transmissions, the DIWA transmission is hydraulically operated and this is where LMS steps 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. They wanted to analyze their systematic processes and understand the test results. Voith Turbo 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 LMS Imagine.Lab Transmission Comfort solution with three different LMS Imagine.Lab AMESim libraries: Hydraulic (HYD), Hydraulic Component Design (HCD), and Hydraulic Resistance (HR). The LMS Imagine.Lab Transmission Comfort solution helps Voith Turbo visualize hydraulics systems and its related interactions.
"To recourse to 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," said Bernhard Höfig of Voith Turbo’s mechatronics and simulation department.
The LMS Imagine.Lab AMESim software package and libraries gave Voith Turbo a systematic way to examine and develop the hydraulic valves in an automatic transmission. An innovative way of working, the integrated LMS Imagine.Lab AMESim platform and dynamic system analysis helped Voith Turbo gain a better understanding of the overall hydraulic behavior. Thanks to the simulation studies and validate physical 1D models for complex hydraulic systems, it became possible to develop a new design for the hydraulic valves for an automatic transmission that were able to meet the tough functional requirements for more efficient and environmentally sustainable city buses.
What differentiated the LMS Imagine.Lab AMESim software from previous solutions was the scope and quality of the AMESimí libraries, the various model representations and the fully-functional validated 1D models for design and testing purposes. The overall testing time was considerably reduced and fewer prototype iterations were necessary.
"For us, LMS Imagine. Lab AMESim is a powerful and flexible simulation environment for a systematic and detailed analysis of dynamic system behavior. This helps us to continuously improve our design and development process," commented Bernhard Höfig of Voith Turbo’s Mecatronics and Simulation department. "LMS Imagine.Lab AMESim rapidly proved to be the ideal solution for the design of valves in our automatic gearboxes," added Bernhard Höfig. "It definitely helped to streamline the design and development of our transmission systems, making them readily available for the transportation market."
