New Ideas To Cut Fuel, Lubes Ash Problems In Pm Filters

Diesel Fuel News, March 19, 2001 by Jack Peckham

Detroit -- A host of innovations point the way to reduction or elimination of ash build-up problems inside diesel particulate matter (PM) filters caused by fuel and lube additives, according to leading technology developers at the Society of Automotive Engineers World Congress here.

Examples:

PSA Peugeot Citroen, developer (with Rhodia) of the world's first commercial passenger car with a PM trap/fuel additive system, announced it's investigating a new PM trap (from Degussa's dmc2 division) that contains both platinum and a proprietary oxygen storage catalyst (see. SAE 2001-01-0907).

Potentially, this system could eliminate the injection of cerium (or other metal) additive altogether, thus helping to avoid ash buildup that requires inconvenient (and costly) occasional trap cleaning, catalyst developer Marcus Pfeifer of dmc2 explained here.

While it's well-known that vanadium-containing molten salts provide superior ability to reduce soot combustion temperature in PM traps, vanadium is too unstable at high temperatures inside a PM trap (up to 1,000[degrees]C). However, a combination of platinum and oxygen storage compound, combined with post-main-injection of hydrocarbons, could provide the right conditions for soot oxidation under normal driving conditions, the investigation indicates.

This "vanadia-free catalyst coating" scheme cuts soot ignition temperature by 2060[degrees]C, helps promote [NO.sub.2] (for soot oxidation), and oxidizes CO, hydrocarbons and soluble organic fraction during filter regeneration. As a result it now seems possible to design a complete diesel exhaust clean-up system including a PM trap and a NOx adsorber or SCR system, possibly without a pre-catalyst upstream of the PM filter, although that remains to be proven in further tests.

Lube oil consumption typical of 40,000 kilometer operation had zero impact on catalyst performance, but had the expected backpressure impact from ash-loading the filter, the PSA/dmc2 investigators found.

* Combined with post-injection to support PM filter regeneration, just 10 ppm of iron (or iron/strontium) additive instead of 30 ppm of cerium could extend periodic filter clean-out interval to at least 115,000 kin, and probably a lot more, Associated Octel and Renault found in a new investigation (see SAE 2000-01-1286).

"Logistics of washing out clogged DPF's [diesel particle filters] in a production car are quite involved and represent a potentially costly exercise" because of the delicate ceramic material, Octel researcher Paul Richards explains. "As this process may take a minimum of several days, a replacement DPF unit will need to be fitted to the vehicle when cleaning becomes necessary."

The Octel/Renault researchers tested their theories on a modem, direct-injected and turbocharged 1.9 liter Renault F9Q engine with common rail, oxidation catalyst, and silicon carbide (SiC) filter (by Ibiden). Test fuel was an ordinary European diesel (about 250 ppm sulfur).

* Compares Cerium, Iron

The investigators contrasted results from a 30 ppm cerium additive to a 10 ppm iron additive and a 10 ppm combination iron/strontium additive. (The latter combination has received some but not all required regulatory approvals in Europe).

Tests showed that "metal treat rate could be reduced from 30 ppm for a cerium additive to 10 ppm for an iron based additive while maintaining the regeneration control strategy developed for the engine," they found. "Desired carbon combustion rates were maintained at the lower metal treat rate."

* Clean Diesel Technologies (CDT)

found excellent results from mixing a low-dose (8 ppm) platinum/cerium fuel additive in tests on a 1998 Series 60 Detroit Diesel heavy-duty engine at Southwest Research Institute (SwRI). CDT's technology met U.S. EPA's 2007 PM limits of 0.01 grams/brake horsepower hour even on a 50 ppm sulfur, 22% aromatics California Air Resources Board (CARE) diesel fuel, considerably higher in sulfur than the 15 ppm sulfur limit CARE and U.S. EPA are imposing in 2006. On a 450 ppm sulfur fuel -- today's EPA highway diesel -- the CDT fuel/additive combo achieved under 0.02 g/bhphr, not quite EPA's highway 2007 limit, but nevertheless a huge 80% reduction from engine-out levels, SwRI's Robert Fanick reported.

Combining the fuel additive/trap system with a 40 engine timing retard reduced NOx by about 20% with no increase in fuel consumption as measured on 8-mode steady-state cycle. The additive improves fuel combustion and thus compensates for economy demerits expected with timing retard (see SAE 2001-01-0904).

The low-dose platinum-cerium combo has an advantage over "cerium-only systems [that] tend to require 30-60 ppm of metal and still have balance points [continuous soot oxidation temperature] of 370-420 [degrees]C," Fanick said. "These high levels of metal can lead to ash build-up in the filter" and thus require periodic ash clean-out.

"Because of the potential for these significant reductions, the city of Houston is currently planning to test CDT's 'cleaner-burning diesel fuel' in eight of its vehicles as part of the city's effort to find cost-effective fuel emission reduction strategies," CDT announced.