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Porsche's new supercar the Carrera GT is a study in lightweight material use. The chassis and engine subframe are fashioned from carbon fiber reinforced plastic, and there is extensive use of magnesium, ceramics, aluminum and a new lightweight steel grade – H400.
If you remove Detroit from the ranking, in the world of distributed automotive centers, Stuttgart is preeminent. Home to DaimlerChrysler, Porsche and mega-supplier Bosch, Stuttgart is a place where many automotive suppliers, ranging from software developers to plastic injection molders, feel they must have a presence in order to stay on the cutting edge. And unlike many areas where the importance of the automotive industry has been eclipsed by other high-tech industries, the Stuttgart region has actually seen the percentage of its total manufacturing output represented by automotive manufacturing rise from 28.7% in 1980 to 41.1% in 2000. The majority of that growth has been in small and medium-sized companies; to keep them flowing into the area the Stuttgart Region Economic Development Corporation has helped create a network of 17 “Competence Centers,” each of which focus on a specific area of technology like telematics or fuel cells. These centers allow smaller companies to pool resources with local universities to carry out R&D projects that would otherwise be out of their reach. But many of the more well-established players have the deep pockets needed to stay at the forefront of technological development. Here’s a look at some of the latest developments to come from companies based in Stuttgart.
DAimlErChryslEr: Fuel cell forefront. In 1994 the company then known as Daimler-Benz unveiled its first fuel cell vehicle. It was a full-size van with a massive fuel cell unit that took up the entire rear cargo area, effectively turning the van into a heavy two-seater with no storage space. Not exactly a marketable commodity. Since then, DaimlerChrysler’s research center at Nabern near Stuttgart has refined its fuel cell system to the point that it now fits into an A-Class without any intrusion into the passenger compartment. It can be assembled into vehicles on the same line as its internal combustion-powered counterparts. Dr. Andreas Truckenbrodt, director, Fuel Cell and Alternative Powertrain Vehicles, says that the tremendous advances in reducing the size and increasing the range of fuel cell vehicles have put commercialization within reach. He outlines DaimlerChrysler’s four-stage fuel cell strategy:
Arriving at this strategy required abandoning some other alternatives. For example, there’s using hydrogen as a fuel for modified internal combustion engines (an approach often touted by BMW). About that, Truckenbrodt says, “We have spent a lot of money on hydrogen combustion engines and we have given up.” He explains that the lower power density of hydrogen compared to gasoline or diesel fuel together with the greater inefficiency of an internal combustion engine (due to the excessive amount of energy that must be dissipated as waste heat) essentially drives a stake in the heart of this alternative. Truckenbrodt also reveals that DaimlerChrysler has essentially given up on on-board fuel reformers that crack hydrogen from fuels like methanol: “Reformation is a non-starter from the efficiency point of view. And the reformer itself adds too much weight and complexity. Also, you usually need to provide a separate water supply which opens up problems with freezing.” Though he embraces hydrogen, Truckenbrodt rejects using it in liquid form which has to be stored hundreds of degrees below zero, “You can’t stop it from getting warmer and it will evaporate from the tank within two weeks,” he explains. Compressed hydrogen is what DaimlerChrysler engineers think is the best form, and though range is a problem, the standard for hydrogen compression in the industry will soon double from 5,000 psi to 10,000 psi, which Truckenbrodt says will increase range by 80%.
DaimlerChrysler’s research teams are currently focusing on ways to improve the power density of their fuel cells by experimenting with the use of bi-polar metallic plates that would more efficiently lead the hydrogen over the membranes, and membrane materials that can function at higher temperatures. With an eye toward reducing cost and complexity the teams are also determining if the sophisticated electronic sensors that measure the voltage in each fuel cell are really necessary. And though every vehicle development program targets weight reduction, it is particularly important for the A-Class since the fuel cell version is currently 300 kg heavier than a conventional model.
BEhr: global cooling. Behr GmbH and Co. is the number-one supplier of air conditioning systems for the European passenger car market—a fact that is all the more impressive when you know that in the last decade automotive air conditioning installation rates in Europe have soared from about 20% to 80%. But Europeans’ new found love for A/C has raised environmental concerns about increases in global warming gasses. So the European Union is discussing a ban of the refrigerant R134a that would begin phasing in in 2008. That doesn’t give suppliers much time to come up with alternatives, but Behr already has a working system that uses simple carbon dioxide (CO2) as its refrigerant. The benefits of using CO2 are clear: its global warming potential is 1,300 times less than that of R134a, and if A/C units are charged using the CO2 that is the waste byproduct of many industrial operations, the impact on the environment is nil. CO2 also cools more efficiently. Behr estimates that its CO2 system consumes 14 to 25% less fuel for the same cooling output as current units, reducing both operating costs and emissions. So, what’s the catch? Pressure. CO2 systems must operate at much higher pressures (120 bar vs. 30 bar for R134a) to be effective, which means that inexpensive rubber seals must be replaced with costlier metal (though not necessarily stainless steel) that can stand up to the increased pressure. Still, the efficiency savings alone should outweigh the increased part costs once the units are in mass production. “We think CO2 will be the future of automotive air conditioning,” says Dr. Thomas Heckenberger, director of Behr’s Group Technology Center.
PorschE: material benefits. The grounds of the Porsche Development Center in Weissach are a vision of a world in which everyone owns a Porsche but are forbidden garages. 911s, Boxsters and Cayennes are shoehorned into every nook and cranny of asphalt along the road, leading to a Porsche-per-square-foot ratio that must top even Southern California. But the one Porsche product not found in this cheek-by-jowl arrangement is the yet-to-be-released Carrera GT supercar. Since the car will go for about $400,000 a copy when it debuts this fall, avoiding door dings is a high priority.
In addition to being by far the most expensive vehicle in Porsche’s model line, the Carrera GT is by far its most technically advanced. It is essentially a street-legal F-1 machine, so it’s no coincidence that its development criteria would be familiar to any F-1 designer: minimal weight, maximal stiffness, lowest center of gravity, and a midship engine. To help meet the first two criteria Porsche designed the most carbon fiber-intensive chassis in the world. The passenger box portion is fashioned from carbon fiber reinforced plastic (CFRP) and integrates the windshield frame and roll bar for maximum rigidity. (The Carrera GT is an open top car but it has better torsional stiffness than the closed 911.) The CFRP is made up of an upper and lower layer of carbon fiber that sandwiches a honeycomb layer of aluminum or a resin-impregnated material called Nomex, which is widely used in aircraft and spacecraft. The rear subframe which houses the engine and is bolted onto the passenger box also uses CFRP, but it features a heat-resistant honeycomb and a special resin in the matrix that can withstand the heat generated by the engine without deforming. (In a gentle jab at archrival Ferrari, Porsche engineers point out that the subframe of their competitor’s supercar the Enzo is merely constructed of welded aluminum.)
In all, 1,000 different pieces of CFRP are used on the chassis and each one is arranged by hand to provide maximum strength. For example, where bending loads are high—like in the door sills—uni-directional weaves are used to enhance bending strength. After the CFRP pieces are laid up, the chassis is rolled into an autoclave where it is subjected to 180°C and 6 bar of pressure for four hours in order to harden the structure. Each chassis takes five days to produce, so Porsche will need to have 10 identical sets of tooling on hand to reach the planned production of two Carrera GTs a day. All of this handwork is not cheap, but according to Michael Holscher, the supercar’s general project manager, “We use the most expensive way of making carbon fiber because it is the most efficient.”
Porsche didn’t stint on using other lightweight materials in the Carrera GT, either. The wheels (19-in. in the front and 20-in. in the rear) are made of forged magnesium which offers what the company describes as “optimum durability” while being 25% lighter than aluminum. Even interior parts like the center console panel and the gearshift lever frame are made from magnesium. In fact, Porsche had to use a special magnesium alloy that is heated to 400°C before stamping in order to get the garnish right.
When Porsche’s engineers were forced to use something as quotidian as steel they managed to make that exotic, as well. The front and rear side members, suspension push rods and rear-axle wishbone are all made of a newly developed grade of stainless steel, H400. H400 was chosen because it has high formability and a lower gauge of the steel can be used to absorb more impact energy in a crash while reducing overall weight.
Porsche also went to extraordinary lengths to ensure that the Carrera GT would have the lowest center of gravity of any production car in the world. The first step in doing that was to make sure that it had the lowest crankshaft height, which is key to determining center of gravity. The car is powered by a 68° V-10 engine instead of one of Porsche’s signature horizontally opposed designs. The reason, as Holscher explains, is that though the crankshaft height on a horizontally opposed engine is lower than that of a “V,” the exhaust components hang below the bottom of the block, inhibiting the engine from being mounted as low as possible. On the Carrera GT’s V-10 the exhaust components sit above the bottom of the block, allowing the crankshaft height to be lowered a few crucial (at least to Porsche) millimeters.
An even bigger factor in achieving the lowest center of gravity is an all-new compact, lightweight clutch that was developed specifically for the Carrera GT. Porsche engineers needed something small and light but knew they could not use the kind of carbon fiber clutch common in race cars since customers would have to replace them more often than their oil. So they developed a clutch that uses carbon fiber reinforced ceramic discs mated with titanium backing plates that achieves maximum power density at a low rotational mass, while giving a service life ten times that of racing clutches.