Equal to 12,665 miles a gallon The Shell Eco-Marathon, a race that takes place annually in different cities, is a showcase for fuel efficiency. Teams from around the world attempt to complete a distance of about 15 miles with a minimum average speed of 18 mph using the least possible amount of fuel. In 2005, for the first time in many years, the world record was broken by a large margin by a car called PAC-car II, a hydrogen-powered vehicle that achieved the equivalent of 12,665 miles a gallon! The PAC-car II weighed about 66 pounds and ran solely on a hydrogen fuel cell that powered two electric motors.

PAC-car II was designed by students at the Swiss Federal Institute of Technology in Zurich. Most of them came from the departments of mechanical and process engineering. Industry partner, Tribecraft AG, contributed core skills in initial conceptualization, design and engineering. Other industry partners contributed to the project as well. All development of the low-profile, aerodynamic PAC-car II was done using NX™ digital product development software in conjunction with Teamcenter® digital lifecycle management software, both part of the product lifecycle management (PLM) portfolio from Siemens PLM Software.

Answering conceptual questions When the development of the PCA-car II began, the world record stood at 9,455 miles a gallon. Several fuel cell-powered cars had competed in previous Eco-Marathons and this technology seemed to hold the most promise. A fuel cell system converts hydrogen into electrical energy. The core element is the stack of fuel cells – layers of graphite plates, reaction membranes and seals – pressed together under high pressure. A clamping technology, developed some years ago by Tribecraft and later patented, offered the combination of lightweight construction with low manufacturing costs.

Business challenges

Develop a highly fuel-efficient vehicle for competition in the annual Shell Eco-Marathon

Keys to success

Parallel development process using NX master model

All processes based on CAD models

NX system-level product control structure, referred to as WAVE technology, maintained assembly dependencies even through design changes

Extensive freeform modeling and surface analysis capabilitie

Results

The most efficient vehicle in the world (getting the equivalent of 12,665 mpg)

A major contribution to the development of environmentally friendly vehicles

Swiss car breaks world record for energy efficiencyA hydrogen fuel cell-powered vehicle, developed with NX, smashed the previous record

TRIBECRAFT AG

Automotive and transportation

PLM Software

www.siemens.com/plm

NXTEAMCENTER

NX rendering of PAC-car II.

After deciding on fuel cell technology, the development team still had many other questions to answer. These included the number of wheels (three or four), type of steering (from the front or the rear) and engine placement (whether to apply power at the front or rear). Aerodynamic questions such as the integration of the wheels into the chassis versus placing them outside, ground clearance and the shaping of the outer surfaces had to be weighed against driving considerations such as the speed of cornering and the risk of tipping over. The conceptual phase was deliberately extended so that these questions and many others could be satisfactorily answered. The result was an optimized vehicle with two rigid, inclined front wheels and one steered, braked and driven rear wheel. This configuration met the demands of both lightweight construction and of aerodynamics.

Parallel development processNext the project proceeded into the design phase, with NX providing a consistent and uniform platform to all the project partners. NX was used to design all individual parts, to model the bodywork with freeform surfaces and to implement the industrial design of the vehicle.

This work was done in parallel using a master model approach. According to Jörg Evertz, an engineer at Tribecraft, this approach was the key to parallel, consistently parameterized work. “The master model concept made it possible to represent important vehicle parameters consistently, and to transfer them to individual parts by means of the ‘NX WAVE’ tool,” he explains. “This ensured that individual parts fit together later, without the need for continuous collision checks.

“CAD was the central working tool for the process,” Evertz adds. “All processes were based on NX models. Limiting factors were found more in the fields of assembly production and test than in the creation of the geometry.”

Taking full advantage of a versatile CAD solution The design phase involved extensive use of the modeling, assembly, freeform feature and drafting capabilities of NX. Modeling was done with full parameterization. Mutual dependencies for all assemblies were defined and individual parts were based on the underlying geometries created for them. NX supported this top-down modeling approach through its WAVE technology, which assigns underlying geometries to individual parts, and permits special analytical tools to be used to display the dependencies.

Shaping the body (simultaneously the inner and outer skin) was one of the critical modeling challenges. The inner side represented the limit of the available space for the driver, the wheels and all other components. The outer side provided the aerodynamically minimum frontal area with a low coefficient of friction. Tribecraft brought many years of experience in the fields of top-down modeling and creation of freeform surfaces and trained students in these techniques. NX data in STEP format was used to create models for wind-tunnel testing and computer simulation.

Aerodynamics was a central aspect of the project. It was optimized by creating a small frontal area and a shape that offered little wind resistance. Continuity of surface curvature was important in

NXTEAMCENTER

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Jörg Evertz Engineer Tribecraft AG

The core of the PAC-car II: the fuel cell stack.

Several curvature analysis methods.

achieving this and NX provided many tools for generating surfaces with continuous curvature. Even when changes were made, NX provided full control of surface curvature. The extensive analysis tools in NX were used to check the results of the modeling process. It was possible, for instance, to display the curvatures and gradients of the composite surfaces.

Looking good – part of the planEven though much of the vehicle design was constrained by physical and engi-neering parameters, the team did not neglect the overall appearance. Their aim for the visual design of the PAC-car II was to express technological intelligence with an appealing image. “As a contrast to the petrol camp, the quiet, clever, cool hydrogen drive was chosen the theme, and not just in the form of applied graphics,”explains Evertz. “Appearance was an integral element of the product.”

To obtain the perfect shape for the vehicle, curve analysis tools in NX were used to ensure that every initial curve on which the surfaces were based was correct from the very beginning. A great challenge lay in the creation of a faultless form without the surface boundaries usually found in automobile design.

A success for the planetThe PAC-car II did more than succeed in its mission of fuel efficiency and success in the Shell Eco-Marathon. The technology that was used is contributing to the development of lighter and more environmentally friendly vehicles.

The solutions provided the broad range of functionality the PAC-car II needed to design the different elements of the vehicle with the high degree of accuracy the project required. The team also credits the technology for providing something else that was critical to its success: time, which let them answer the difficult conceptual questions. “Taking the NX master model approach and using NX?WAVE technology made it possible, after a long initial conceptualization phase, to work in parallel to design all the components of the car in a short time,” Evertz concludes.

Solutions/Services

NX www.siemens.com/nx

Teamcenter www.siemens.com/teamcenter

Client’s primary business

Tribecraft AG makes use of systematic methods as well as extensive design and engineering knowledge to create innovative new products, working from the beginning of development through to series manufacture. www.tribecraft.ch

Client location

Zurich Switzerland

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Jörg Evertz Engineer Tribecraft AG

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Automotive and transportation

NXTEAMCENTER

Comparison of new and old clamping device.