Engineers have been meticulously refining the diesel engine's combustion process during the past decade or so, and the result has been a spectacular reduction in the volume of pollutants exiting the exhaust stack. The design of the combustion chamber itself has been overhauled; electronic, high-pressure fuel-injection systems have evolved; variable-geometry turbochargers and charge-air coolers precisely regulate and condition intake air; and exhaust-gas recirculation retards pollutant formation at the point of combustion.
The two diesel-exhaust pollutants that engineers have had chiefly in their sights, as you may well know by now, are particulate matter (PM), which is soot resulting from incomplete combustion, and oxides of nitrogen (NOx), primarily NO (nitrogen oxide) and NO2 (nitrogen dioxide), which have a poisoning effect on the air around us.
For the most part, when bringing 2007-model, heavy-duty, on-highway diesels into compliance with the ever-tightening regulations of the Environmental Protection Agency (EPA), engineers had to look beyond the combustion process in the cylinders and to the diesel's exhaust stream — that rush of hot gases from the exhaust valves — for ways to further reduce PM at the stack. The technique of cleaning the exhaust stream (versus controlling pollutants within the cylinders) is termed "after-treatment." Thus, to diminish PM, most 2007-model, heavy-duty, on-highway diesels are operating with a diesel particulate filter (DPF) in the exhaust stream.
Looking ahead, as NOx regulations become even more stringent, many engines in this category, by year 2010, will likely also be fitted with a NOx aftertreatment system. And it's possible, of course, that as off-highway diesels become subject to the EPA's Tier-4 Interim and Tier-4 Final regulations, they, too, may be equipped with similar PM and NOx aftertreatment systems.
The typical diesel particulate filter is a ceramic-like cylinder — perhaps 12 inches in diameter and up to 15 inches long — encased in a metal sleeve. The cylinder has row upon row of small square channels running between its two faces. Because the channels are plugged at alternate ends, exhaust gases must pass through the channel walls (where soot is deposited) and into adjacent channels to find outlet at the other face. The DPF usually is fitted with clamp-on inlet and outlet sections, which give the assembly the appearance (and typically also the function) of a large muffler.
Quite often, another piece of hardware, a diesel oxidation catalyst (DOC), is clamped into the DPF assembly between the inlet section and the filter section. The DOC is a flow-through, honeycombed, stainless-steel or ceramic structure coated with a catalyst to promote chemical reactions. When used in conjunction with a DPF, the DOC's main job is to keep the filter clean by burning away accumulated soot — a process called "regeneration."
DPF regeneration can be either "passive" or "active." Passive regeneration, which is used primarily with DPFs in retrofit situations, occurs continuously and automatically — assuming that the exhaust stream meets certain requirements. In the passive process, the DOC oxidizes a portion of the NO in the exhaust gases to NO2. The NO2, an extremely reactive gas, burns away the soot and leaves primarily NO and CO2 (carbon dioxide).
The effectiveness of passive regeneration depends on exhaust temperatures being around 500 F for a significant portion of the engine's operating time, and also on the ratio of NOx-to-PM being in a suitable range. On the latter point, if the engine is efficient at limiting its production of NOx, then its exhaust stream may not support passive regeneration effectively.
In some instances, as with Johnson Matthey's Catalyzed Continuously Regenerating Trap (CCRT), the particulate filter itself has a catalyst that promotes further production of NO2, thus supplementing the action of the DOC and potentially allowing regeneration at lower temperatures. The Donaldson Emissions Group has passively regenerated DPF units that use no DOC, only a catalyst on the filter.
Active regeneration, on the other hand, uses the DOC primarily to raise exhaust temperature. When, at the proper time, diesel fuel is injected into the exhaust stream ahead of the DOC, the catalyst becomes a "flameless heater," says Fred Schmidt, director for Donaldson's Emissions Group, and boosts exhaust temperature to around 1,300 F. At that temperature, oxygen in the hot gases combusts the soot, leaving primarily CO2 and water. Some passive soot burning occurs in an active system, says Schmidt, but that's not the primary purpose.
Most heavy-duty trucks rolling out of the factory today are equipped with an active regeneration system for the DPF. Even though the active system requires electronic intelligence to control the fuel-injection process and to decide when conditions are right for regeneration, it is the more reliable of the two methods, and its efficiency is not influenced by the NOx/PM ratio. Truck manufacturers have built safeguards into the process to ensure that the vehicle and its surroundings are protected when regeneration occurs, a process that typically cleans soot from the DPF in 15 minutes or so.
Just to keep the record straight, not all active-regeneration systems employ a diesel oxidation catalyst. Notable among these non-DOC systems is that used by Caterpillar. It's our understanding that the Caterpillar system uses a separate diesel-fired burner to elevate exhaust temperatures for regeneration.
In addition to soot, however, the DPF also collects "ash," which is primarily the residue of additives in the engine's lubricating oil. Because ash does not burn away during regeneration, the DPF must be periodically cleaned of this substance. Unclamping the DPF assembly's sections allows relatively easy removal of the filter for cleaning.
A number of companies have developed proprietary equipment for cleaning the DPF of accumulated ash. Systems from SPX, Donaldson and Cleaire, for example, use patented techniques involving compressed air, vacuuming and ash collection. Cleaning ash from the DPF may be required at intervals ranging from 150,000 to 300,000 miles, and the process likely will take about the same time as an oil change.