$4.00 per gallon gasoline and impending CAFE regulations are causing vehicle manufacturers to look beyond the usual powertrain and body structures for weight-reduction opportunities. Every system and subsystem is under scrutiny for potential weight saves. Even brake systems.
Italian brake system manufacturer Brembo (www.brembo.com) is touting its carbon ceramic brake disc technology (CCM) as a significant breakthrough for vehicle manufacturers looking for weight reduction. The system has been used on several Ferrari and Maserati vehicles for the past few years; its first U.S. OEM application is on the 2009 Corvette ZR1. The carbon ceramic discs are a mixture of carbon fiber and phenol-based resin; Brembo says its discs are 40-45% lighter than traditional cast iron disc systems.
The manufacturing process for the discs, explains Emanuele Bruletti, engineering manager for CCM at Brembo, covers seven different steps, beginning with mixing the materials to a proper composition, after which they are molded into a near-net shape using a specially developed hot molding process that heats the materials up to 1700oC. The discs are then transformed into carbon by pyrolytic decomposition under a 1000oC nitrogen gas flow. This converts the two materials into a carbon fiber and porous carbon matrix. Next, the porous carbon material is infiltrated with liquid silicone and temperatures above 1430oC before the disc is put under vacuum at 1700oC. That converts the porous carbon almost totally into silicone carbide that forms a barrier to protect the carbon fibers from oxidation. Finally, diamond grinding is used to machine the rotors down to within 5 microns. This process, incidentally, is nearly identical to those used to manufacture aircraft brake systems. Because the process is so complex, it leads to the key drawback of Brembo's brakes: cost. CCM discs are at least four times more expensive than their cast iron counterparts.
But this is not to say that there aren't some advantages that could offset the cost of CCM: longevity and part stability. "Under normal driving conditions, carbon ceramic rotors don't wear and therefore there is no need to replace the rotor for the entire life of the vehicle. On top of this, the dimensional stability of the rotor doesn't change, which means it maintains its shape and does not deform. In essence you are always driving around on new brakes," Bruletti says.
The use of carbon fiber also helps with heat and energy management, since the material can withstand enormous heat without degradation. This, however, poses another problem for manufacturers: the need to develop corresponding components-ball joints and bushings-that can handle the same amount of heat as the rotors themselves. "We have to now look at properly cooling the entire corner, not just the brake system," Bruletti says.
While Brembo is under no illusion that CCM technology will be adopted for more high-volume programs in the near term, the company expects automakers to look at these technologies within the next 10 years as the cost of the materials and higher production volumes reduce piece-cost levels. "We're continuously working toward optimization of the processes and materials because we know that weight and cost reduction are the biggest challenges for automakers," Bruletti says.
A System Approach
While the use of exotic materials remains a challenge, TRW Automotive (www.trw.com) is hopeful it has a solution that can help automakers reduce weight and improve brake system performance through a simpler, yet more comprehensive approach. The supplier has developed a six piston electronic stability control system-traditional systems have two pistons-that can help boost brake system pressure on vehicles with direct injection (DI) or diesel engines.
Dan Milot, chief engineer for TRW's North American advanced control systems group, points out that as automakers migrate toward increased adoption of DI and diesel engines, engineers will have to overcome the inherent reduction in vacuum pressure generated through the intake manifold used to supply boost to the brake system. "There are some conditions where these engines cannot produce the normal levels of vacuum for brake boost, particularly in low-temperature and high-altitude scenarios," Milot says. Some automakers currently offering DI and diesel engines rely on an additional mechanical vacuum pump or some other supplemental pump system to provide the added vacuum to boost fluid fill. This not only adds weight to the overall brake system, but also places a drag on the engine that reduces output and fuel economy. TRW claims the next-generation six-piston ESC will do away with the added pump, thus reducing cost and overall system weight while improving engine operation.
An additional advantage of using this system is that most automakers currently have an ESC architecture already in place. Therefore, the cost to upgrade to the six-piston system will be minimal and require little or no change in the current vehicle architecture. "If you have an infrastructure in place for an ESC, this can slide right in that space. The six-piston system does add cost, but not as much as an entire vacuum pump and an additional ECU," Milot says. The six piston system can also better accommodate advanced braking technologies, including active emergency braking system. "This is a stepping stone to adding brake-system based technologies that will improve vehicle safety," Milot adds.
Automakers remain concerned, however, over how the system will impact brake pedal response and feel to the driver: "The biggest immediate challenge we have is pedal feel transparency," Milot admits. However, he is confident TRW will have the system installed in production vehicles by 2012.