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.

DPFs, DOCs and regeneration DPFs, DOCs and regeneration

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.

Future NOx encounters

When meeting the EPA's NOx regulations for 2010-model heavy-duty, on-highway diesels, engine manufacturers may opt to comply with varying technologies. For example, according to Construction Equipment's truck editor, Tom Berg, Cummins will use more sophisticated in-cylinder techniques to meet 2010 NOx standards in its larger on- and off-highway diesels. The company's medium diesels, however, likely will be made compliant with a NOx aftertreatment system. (See CE's November 2007 issue.)

For those engine manufacturers opting to meet the EPA's 2010 NOx regulations with an aftertreatment system, then vehicles using these engines may be equipped with a NOx adsorber or a selective catalytic reduction (SCR) system — most likely the latter.

The NOx adsorber uses a catalyst to initially trap and hold harmful compounds of nitrogen. Some have likened the catalyst to a "molecular sponge." But like a sponge, it can only hold so much before it must be emptied — or regenerated. Regeneration is initiated with periodic injections of diesel fuel, which reacts over the catalysts involved to first liberate the NOx, then to convert it to benign nitrogen gas (N₂). But this process is not simple, and poses technical challenges.

Although the NOx adsorber is extremely efficient at trapping NOx, it is finicky about its regeneration environment, requiring an oxygen-deficient exhaust stream that is difficult to achieve, thus posing a technical barrier to the NOx adsorber's practicality. In addition, the NOx adsorber is susceptible to fuel-sulfur poisoning (even with ultra-low sulfur fuels), and the temperature at which it regenerates is higher than that required by the DPF, a situation that might make the two systems incompatible.

So, at the moment, says Donaldson's Schmidt, SCR seems to be the winning technology.

The SCR process, like the NOx adsorber, uses a catalyst to trap NOx. But instead of diesel fuel as the "reductant," SCR employs ammonia (NH₃), which reacts with captured NOx in the presence of a catalyst to form N₂. The favored source of ammonia at present is an aqueous solution of urea, which decomposes, when periodically injected into the exhaust stream, to form ammonia and carbon dioxide. In addition, a DOC may be used after the SCR system to neutralize excess ammonia that might slip past when exhaust temperature is too low for regeneration or when too much urea is injected.

In Europe, SCR is accepted technology. But in the United States, the EPA has expressed concerns — not about the effectiveness of the technology itself — but about the possible lack of infrastructure to distribute urea and about the possibility of operating the engine without urea. Recently, the EPA issued guidelines that address the design of SCR systems to mitigate the latter concern.

Retrofitting for cleaner air

More and more governmental bodies in "non-attainment areas" (geographical regions that fall short of the EPA's standards for clean air) are including the reduction of pollutants from older diesel engines in their clean-up strategies. Some jurisdictions, in fact, might require that machines used on public works projects meet specific emissions standards.

In some instances, incentives, such as tax credits or grants, may be offered for cleaning up an older diesel's exhaust; and qualifying measures might include re-powering with emissions-compliant engines, rebuilding engines to include emissions controls, using ultra-low-sulfur fuel, and installing exhaust aftertreatment systems. Our scope here is to look briefly at retrofit aftertreatment systems.

When considering any type of aftertreatment for retrofit, first make sure that the system is on either the EPA's Verified Technologies List (http://www.epa.gov/otaq/retrofit/index.htm), then click on "Verified Technologies List," or on a similar list from the California Air Resources Board (CARB) (http://www.arb.ca.gov/diesel/verdev/vt/vt.htm), then click on "Currently Verified Technologies."

Remember, too, that before installing a retrofit device, a qualified supplier might have to instrument and monitor your machine's operation for several days in order to competently recommend (or not recommend) a particular system. For example, does the machine's duty cycle allow exhaust temperatures to remain high enough for sufficient periods to passively regenerate a catalyzed DPF?

According to Schmidt, three basic retrofit technologies ("good, better and best," he says) typically are available for particulate control on older diesels: the DOC, the partial filter, and the DPF.

When the diesel oxidation catalyst is used alone (not in conjunction with a DPF) as a means for reducing particulates, it uses a different catalyst than when its function is to clean the DPF. The catalyst used in the stand-alone DOC basically strips off the "soluble organic fraction" (SOF) portion of PM. The SOF consists essentially of unburned portions of diesel fuel and lubricating oil that condense on the sponge-like carbon particles.

Compared to the DPF, the stand-alone DOC is perhaps 20 to 30 percent effective at reducing total PM, but does little to reduce the volume of solid carbon particles. Reducing the SOF is a plus, however, and the stand-alone DOC does not require ultra-low-sulfur fuel. The price for a retrofit DOC might be in the neighborhood of $2,000 to $4,000, maybe more if a dual system is required or if installation is difficult.

Partial filters are 40 to 70 percent effective at capturing soot and may use filtering material such as metallic fleece that is laminated between layers of corrugated steel. The filter traps a portion of the carbon particles present in the exhaust stream, but usually does not trap ash. Partial filters are passively regenerated, either by using a catalyst on the filter or by employing a diesel oxidation catalyst, thus requiring ultra-low-sulfur fuel to protect the catalyst from sulfur poisoning. The partial filter might cost between $5,000 and $6,000.

(According to some aftertreatment specialists, particulate filters that use passive catalyzed regeneration systems are limited to 1994 and newer non-EGR [exhaust-gas-recirculation] engines. Pre-1994 engines might overwhelm a passively regenerated soot-filtering system, and [to add a twist of irony here] cooled-EGR engines may not produce enough NOx for a passive system to convert NO to sufficient quantities of NO₂ for combusting the soot.)

Older diesels also can be fitted with a full DPF, usually at a cost (depending on engine horsepower) between $7,000 and $10,000. But costs can vary dramatically. Some large engines, for instance, may require dual systems to handle high exhaust flows, driving the cost to perhaps $20,000 or more.

Retrofit DPFs typically require higher exhaust temperatures to passively regenerate, compared with temperatures required for partial filters. If temperatures are insufficient, then an active system is required, such as an integral electrical heater plugged in overnight. In theory, non-catalyzed, actively regenerated particulate filters could be retrofitted to any diesel.

"Low temperature" DPFs, which employ more catalyst to produce more NO₂, can passively regenerate at temperatures below those required for a standard DPF. But the possibility of this process resulting in excess NO₂ at the stack has the EPA and CARB concerned. (The sidebar, "DPF Passive Regeneration and NO₂ Concerns," addresses this issue.)

For older trucks and machines that might require both particulate and NOx control, the "lean NOx catalyst" combined with a catalyzed DPF may be a retrofit possibility. Cleaire Advanced Emission Controls, for example, manufactures a retrofit system (the Longview) that combines these two technologies.

According to Tom Swenson, director of sales and verification for Cleaire, the lean NOx catalyst uses continual injection of a small volume of diesel fuel into the exhaust stream to reduce NOx over a proprietary platinum catalyst, producing benign nitrogen gas and oxygen. Unlike the NOx adsorber, the lean-NOx catalyst works well in an oxygen-rich environment, but it is not nearly as effective at reducing NOx as the adsorber, and thus is not considered for use on new engines.

With ultra-low-sulfur fuel, the Longview has a stated NOx-reduction capability of at least 25 percent, but, says Swenson, that number might be closer to 35 to 40 percent in off-road applications, where exhaust temperatures generally are higher. Minimum particulate reduction is rated at 85 percent. The proprietary catalyst that coats the Longview's particulate filter promotes oxidation of collected soot and also converts CO and hydrocarbons into benign gases and water.