Wednesday, March 30, 2011
High-Tech Fuel Tanks
We wouldn’t be surprised if the average driver thinks of a vehicle’s fuel tank as roughly the technological equivalent of a bucket. In today’s world of ever-stricter emissions regulations—which also have ramifications for the fuel tank—that couldn’t be further from the truth.
Prior to 1971, when the EPA mandated sealed fuel systems, vapors emanating from the tank made up a significant portion of a car’s overall hydrocarbon emissions. The corking of the tank was—and still is today—accomplished by temporarily storing excess vapor in a charcoal canister. From there, it is periodically purged to the engine, where it is burned along with fuel from the injectors.
But even in closed-off systems, there’s a tiny amount of fuel that escapes through the tank and its associated plumbing, and this—called “permeation”—is regulated. The EPA’s current Tier II Bin 5 passenger-car standard allows for a maximum leakage of 95 milligrams per day. To achieve CARB’s squeaky-clean Partial Zero-Emission Vehicle (PZEV) rating, a tank must emit just 20 milligrams a day, a 79-percent reduction.
But even in closed-off systems, there’s a tiny amount of fuel that escapes through the tank and its associated plumbing, and this—called “permeation”—is regulated. The EPA’s current Tier II Bin 5 passenger-car standard allows for a maximum leakage of 95 milligrams per day. To achieve CARB’s squeaky-clean Partial Zero-Emission Vehicle (PZEV) rating, a tank must emit just 20 milligrams a day, a 79-percent reduction.
For this reason, starting in the early 2000s, most PZEV-capable vehicles have employed steel rather than plastic fuel tanks for their superior permeation resistance. But TI Automotive, which supplies fluid systems and delivery hardware (everything from fuel tanks and lines to HVAC components and brake lines) to nearly every automaker around the globe, came up with the first weight-saving plastic tank to earn PZEV status.
The key to the reduced emissions is that TI’s tank—launched on the 2005 Ford GT—relocates all the fuel-delivery components, which were the source of the most significant leaks, from the exterior of the tank to the interior. In addition, the plastic, which is blow-molded around the other components in a process that TI calls “ship in a bottle,” is a six-layer design developed to greatly reduce leakage.
TI says that roughly 98 percent of today’s European vehicles have plastic tanks versus about 75 percent in the U.S. Compared with an equivalent steel tank, TI claims its plastic ship-in-a-bottle tank is about 20 percent, or five pounds, lighter. And the use of plastic tanks should continue to increase in the face of rising fuel-economy standards where every last ounce of ?weight savings counts.
But the problems are far from over. Right now, the most common complaint that TI hears regards unwanted fuel-slosh noise, made noticeable because hybrids are running farther and farther in electric-only mode, in which there’s no obscuring engine sound. To solve this, TI adds carefully designed, splash-reducing interior walls.
Another hybrid problem is that when the engine is not running, the vapor stored in the charcoal canister can’t be purged. So to keep vapor from filling the canister, the pressure inside the tank is increased. Although this technology is not yet in production, TI has working prototypes that add reinforcing structure to the tank described above (main image) as well as one with two six-layer walls (inset) with lattice work in between that can tolerate as much as 7 psi of internal pressure. And the weight savings for one of these pressurized tanks can be as high as 50 percent compared with a pressurized steel tank.
But despite all the engineering activity, the new-tech tanks operate just as seamlessly as before. So most people will probably just keep pumping gas mindlessly without a second thought. BY DAVE VANDERWERP
Source: www.caranddriver.com
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