New catalytic converter stores heat to cut emissions

Automotive Industries, Jan, 1998 by Norman Martin

It's an unlikely place to start a revolution. Under stark blue Colorado skies, a team of scientists is reinventing automotive pollution control for the 21st Century. Their idea is the highest-tech Thermos bottle you've ever seen.

Using a concept influenced by battery design, the group at U.S. Department of Energy's National Renewable Energy Laboratory (NREL) in Golden, Colo., believes they can cut more than 80% of vehicle tailpipe emissions by revamping the traditional catalytic converter so that it stays hotter, longer -- as long as 24-hours.

The complex, three-phase design conserves heat, stores heat, and when it gets too hot, releases heat. This is all done passively at a cost competitive to that of current converters, claim the inventors. "It has the potential to revolutionize the catalytic converter that we see today," says Matt Keyser, a mechanical design engineer at the NREL. Benteler Automotive Corp. in Grand Rapids, Mich., holds the patent rights to the converter and says the new design should be ready for production in 2001.

Catalytic converters are very effective once hot. But until they "light off," they don't work; untreated exhaust gas basically shoots straight out the tailpipe. In those first two to three minutes of warm-up, 60% to 80% of tailpipe emissions occur. As the EPA tightens its emission regulations, one of the few places automotive engineers can get emission improvement is in those first two or three minutes. And the only thing to do is get the converter hot, fast. To do that automakers are moving or have moved converters as close to the exhaust ports as possible. Other methods under consideration are, electrical heating, flame heaters and some type of exhaust gas combustion system.

These take the approach of heating the catalyst as fast as possible. NREL's approach is to keep the converter hot from the last drive. According to the EPA, 90% of the vehicles in the U.S. will be restarted within 18 hours of the time they were last shut off.

The new converter, which was developed by NREL's David Benson and Thomas Potter, operates in three steps:

Step one conserves the heat using vacuum insulation, much like that used in a normal Thermos bottle. But this bottle is a little more sophisticated. Inside the core of the converter is step two, a phase change material used to store extra heat.

In this case, "phase change" means a material that melts -- in other words from solid to liquid. The phase change material is a proprietary light-metal alloy. Benteler Automotive would not disclose the exact nature of the material, other than to say it is a benign, recyclable, non-toxic, low-cost commodity grade material.

Step three is a way to cool off the converter by using what's known as a hydride in the vacuum portion of the converter. A hydride is a material that is able to store hydrogen gas between its atoms. When the few grams of hydride in the converter get hot, a small amount of hydrogen gas is expelled. The result: the vacuum space is no longer a vacuum. It helps that hydrogen gas is a good thermal conductor.

"Typically, we would look at holding the temperature of the core converter at or above 200-degrees C for 18 to 24 hours," says John Biel, manager of advanced technology development, exhaust components, at Benteler Automotive. "It will all depend on the final design."

Biel says do in one of its own tests in a V-6-engine, emission reduction -- compared with a normal catalytic converter -- after 24 hours was 95% for hydrocarbons, 75% for nitrous oxide, and 93% for carbon monoxide. The company is now prototyping and providing samples for OEM evaluation.

The original concept for the insulation device came from a contract with the U.S. Advanced Battery Consortium to develop a vacuum insulation around a sodium sulfur battery. Later, NREL scientists brought the idea to Benteler Automotive as part of a cooperative research and development agreement (CRADA).

"The function of a national laboratory is to come up with technologies, develop a product and pass it on to industry," NREL's Keyser says. "That's what we have done here."

The question now is how quickly can the variable-conductance insulation converter, as it's called, bring those clear Colorado skies to the rest of the world.