Manganese additive promotes soot oxidation

Diesel Fuel News, Nov 25, 2002 by Jack Peckham

San Diego -- The manganese-based "MMT" fuel-borne catalyst not only acts as a sulfur/phosphorous scavenger that could reduce poisoning of a nitrogen oxides (NOx) trap, but also lowers the temperature at which soot oxidizes in a diesel particulate filter (DPF), according to a new study.

With about 23 ppm of MMT added to diesel fuel, the DPF soot oxidation rate improves, as oxidation starts taking place at about 150[degrees]C lower than with a non-additized fuel, according to Ethyl Corp. test results presented to Diesel Engine Emissions Reduction (DEER) workshop here.

A higher treat rate (over 40 ppm) pushed the down the temperature for soot oxidation even further.

Ethyl first reported the sulfur-scavenging benefit of MMT for diesel NOx traps last year (see Diesel Fuel News 9/3/01, p3). Now, its latest tests show about a 4-5% improvement in NOx trap efficiency with an MMT-additized fuel, versus a 30-ppm sulfur baseline diesel fuel, at exhaust temperatures between 300-400[degrees]C.

The additive also sharply reduces soot-induced backpressure buildup in DPFs, compared to a baseline (non-additized) fuel, when filter inlet temperature was between 275-300[degrees]C. That's the temperature range normally cited for maintaining a "balance point" where soot oxidation roughly equals soot accumulation in a DPF.

On transient cycle tests, the MMT additive prevented backpressure from exceeding original equipment manufacturer (OEM) backpressure limits, compared to a non-additized baseline fuel.

At an estimated cost of less than 1 c/gallon, the additive has potential to pay for itself through fuel-economy benefits of lower backpressure, reduced costs of DPF/NOx trap systems, and improved catalyst system durability/efficiency, Ethyl's senior engineering specialist David Human explaine here.

The additive potentially could allow engine/aftertreatment systems developers to reduce warranty and initial equipment costs, and reduce the fuel-economy penalty from temperature-boosting schemes used to regenerate DPFs. It also might allow reduced precious-metal catalyst loadings or active regeneration hardware, Ethyl suggests.

Future research will include treat-rate optimization, various after-treatment combinations, field demonstrations and an analysis of possible OEM or retrofit applications. (Ethyl also reported its MMT/DPF results in a paper (SAE 2002-01-2728) to Society of Automotive Engineers Powertrain & Fluids conference here last month).

Automakers continue to criticize MMT however. A recent Alliance of Automobile Manufacturers study cites several problems in gasoline car catalysts that were exposed to gasoline-MMT blends, under certain test cycles. U.S. EPA is also a harsh critic of MMT and is demanding exhaustive health effects studies for the additive. Ethyl contends it has and continues to address these concerns with test programs and continuing studies.