Ulsd: Encouraging Sign For Diesel Nox/Pm Traps

Diesel Fuel News, May 28, 2001 by Jack Peckham

Brussels -- The 2005/6 emergence of 10-15 ppm ultra-low-sulfur diesel (ULSD) coincides with new research showing strong promise for a sulfur-sensitive diesel nitrogen oxides (NOx) trap.

Combined with a particulate matter (PM) trap, the NOx trap system potentially could allow compliance with very tough emissions standards for both light- and heavy-duty diesels.

NOx traps benefit from ultra-low-sulfur diesel (ULSD) to maximize NOx conversion efficiency, minimize sulfur fouling/fuel economy penalty, and promote trap/catalyst durability.

Examples of new research findings:

* Toyota presented a technical paper on a combined Nox/PM trap (Diesel Particulate-NOx Reduction, or "DPNR") system (see Diesel Fuel News 8/4/2000, p10) at the Hart Europe Fuels Conference here.

DPNR cleverly combines a NOx/PM trap-catalyst with "pulsed" air-fuel ratio lean/rich switching. This scheme employs a "low temperature combustion" concept, as Toyota-Europe engineer Thiebault Paquet explained here.

The system combines common rail fuel injection system and an electrically controlled exhaust gas recirculation (EGR) system, he said.

The scheme requires multiple fuel injections including post-main injection, allowing "rich low-temperature exhaust with massive EGR." Rich pulses every 60 seconds promote quick PM oxidation.

Exhaust gas temperatures in modern light-duty diesels often aren't high enough by themselves to support soot oxidation. So, Toyota looked for ways to stimulate low-temperature oxidation.

DPNR partly achieves this via pulsed lean/rich switching (boosting the oxidation rate of PM) and "active oxygen" radicals that are conveniently released in the process of NOx storage and reduction. This allows continuous soot oxidation at temperatures as low as 250[degrees]C For even lower-temperature soot oxidation, massive EGR and multiple injections may be added. The frequent-pulse scheme also avoids soot fouling that otherwise would interfere with Nox conversion.

Test results with a turbo direct-injection (TDI) diesel on Japan's 10-15 light-duty mode show that both PM and NOx can be reduced by over 90% with this system, Paquet explained.

As for fuel sulfur sensitivity, "at the moment a thorough investigation is underway, but it's a little too early to show data. However, the mechanism for sulfur poisoning on DPNR is very similar to gasoline [direct injection] NOx traps," he said.

In Toyota bench tests on a gasoline NOx trap that's believed to behave similarly to DPNR's system, NOx reduction efficiency (at 400[degrees]C) declined after several hours from nearly 100% to about 90% due to sulfur poisoning with an 8 ppm sulfur fuel.

Over the same number of hours, NOx efficiency drastically declined with fuels of 30, 90 and 500 ppm sulfur, by contrast.

To restore nearly 100% NOx conversion at 400[degrees]C, the trap poisoned with 8 ppm sulfur fuel took only a few minutes for regeneration. By contrast, traps with higher-sulfur fuels took much longer to regenerate, and still didn't achieve the high NOx conversion levels of a fresh catalyst.

Subsequent tests with 16,000 miles of aging on a gasoline direct-injection NOx trap show "nearly 100% recovery with 8 ppm sulfur fuel, while 30, 90 and 500 ppm fuels are unacceptable," he said.

Now, the emergence of 10 ppm ULSD in Europe from 2005 is a very encouraging sign for DPNR's future.

"This breakthrough technology makes it possible to seduce both NOx and PM well beyond the levels foreseen for 2005," Toyota Europe Vice President James Rosenstein told the Hart conference.

On another front, it's hoped that a similar version of DPNR eventually can be developed for heavy-duty, but that faces even tougher challenges. Heavy-duty catalyst durability target is about 1 million kilometers as opposed to over 250,000 km for light-duty, Paquet explained.

Also fuel consumption penalties (from desulfurization and regeneration of a DPNR) would be a bigger challenge for heavy-duty application.

"At Toyota, we believe the system could also have potential for heavy-duty, although more research is needed," Paquet said. (Not mentioned here was heavy-duty NOx/PM trap research at Toyota's Hino affiliate.)

* VW: New Optimism

Meantime, Volkswagen likewise says it's heartened by the news that Europe seems to be moving toward a 10 ppm sulfur ULSD introduction in 2005.

"If this low-sulfur fuel comes, then we're pretty optimistic" about employing a combination PM/NOx trap on heavier-weight classes of passenger vehicles, VW spokesman Stuart Johnson told us.

That vehicle class continues to broaden, as VW and Porsche now aim to introduce a new sport-utility vehicle (SUV) in the 2003 model year for both Europe and North America.

So, if the ultra-clean diesel fuel becomes widely available, then a diesel version of this SUV could become possible.

* New Desulfurization Schemes

Meantime, AVL List and catalyst developer dmc2 explained in a joint paper at the recent Vienna Motorsymposium that desulfurization of a NOx trap is indeed possible even at light-load, lean-mode operation.