23 December 2005
The California Air Resources Board (ARB) has verified Donaldson Company diesel multi-stage filter (DMF) device for 1991 through 2002 diesel engines used in on-road applications. The DMF system uses a multi-stage passive “flow-through filter”—incorporated in a muffler unit—to achieve at least 50% reduction in particulate matter emissions, thus qualifying for California Level 2 verification. The DMF system has been verified for use with ultra low sulfur diesel fuel. The device has been verified in two configurations: with and without the Donaldson Spiracle closed crankcase filtration system.
The DMF system uses a two-stage metallic “open” or “flow-through” filter to trap and oxidize diesel particulate matter (PM). Each filter stage consists of alternating layers of a corrugated metal and a porous sintered metal fleece. A catalyst coating is utilized to continuously oxidize the trapped PM, as well as HC and CO emissions.
Certain minimum exhaust gas temperature conditions are necessary for the DMF device to sustain its PM emission reduction activity. According to the conditions of the verification, engines manufactured from 1991-1993 must have a duty cycle with exhaust temperature of 230°C for 40% of time and average exhaust temperature of 215°C or above. Engines manufactured from 1994-2002 must have a duty cycle with exhaust temperature of 210°C for 40% of time and average exhaust temperature of 210°C or above. If exhaust temperatures are too low, the device will not plug (contrary to a “closed” particulate filter), but its PM emission control efficiency will deteriorate.
Few technical details have been released, but it appears the DMF device features a unique catalyst system design. According to Donaldson, an impressive 70-77% PM emission reduction was demonstrated in the verification testing (FTP), while meeting the NO2 emission requirements of the California verification program. Other commercial systems utilizing “flow-through filters”—such as the PM-Kat device on MAN Euro IV engines—are believed to have a PM emission reduction efficiency on the order of 50%.