This selective catalytic reduction device converts NOx to harmless nitrogen gas and water vapor — to achieve near-zero emissions. Photo: Cummins Inc.
A cleaning machine loosens and blows white ash out of the filter's ceramic cells. Photo: FSX Inc.
By Paul Abelson
The promise of 2010 pollution-control standards was that proven technologies will eliminate potential problems. Therefore, this year's engines would be problem-free.
Not necessarily. Except for incorporating European selective catalytic reduction (SCR) that adds up to $9,000 to vehicle price, the 2010 models aren't much different from their predecessors. It's the 2007 diesel particulate filters (DPFs) with which 2010 engines are equipped that are raising maintenance requirements for all medium- and heavy-duty vehicles.
For those not familiar with the technologies that have virtually eliminated particulate matter and nitrogen oxides (NOx) from diesel exhaust, here's a brief review:Historically, diesel had contained 0.5% sulfur, or about 5,000 parts-per-million (ppm), by weight. In 1993 EPA limited sulfur to 500 ppm, a 90% reduction.In 1998, engines were releasing 4 grams/horsepower-hour (g/hp-hr) of NOx and about 0.10 g/hp-hr of soot (or particulate matter). The first step in controlling emissions was to reduce the amount of acid-forming sulfur in the fuel. The acid attacks emissions technologies and contributes to acid rain.In 2002, faced with NOx limits of 2.4 g/hp-hr, manufacturers launched exhaust gas recirculation (EGR).In 2006, sulfur was limited to 15 ppm.In 2007 particulate matter was restricted to 0.01 g/hp-hr and manufacturers introduced diesel particulate filters. NOx emissions were reduced to 1.2 g/hp-hr, requiring increased EGR.
In response to this year's decreased NOx allowance of just 0.1 g/hp-hr, manufacturers offered two solutions.
International's MaxxForce engine uses Advanced EGR and EPA credits for an in-cylinder solution.
All other manufacturers added SCR, a chemical exhaust after-treatment using urea injection to neutralize NOx. The technology has been used for years in Europe, so a great deal of development and field experience went into making the engines virtually foolproof.
And the engines themselves are. But fleet operations have been plagued with difficulties related to the filters. Exhaust with soot from combustion and ash from motor oil additives accumulate in the filter's cells, while the gaseous exhaust permeates the walls to adjacent cells that carry exhaust gases to the tailpipe.
When the cells are so loaded with soot that exhaust flow is restricted, a regeneration process burns the soot to convert it to carbon dioxide. Diesel fuel is injected into the exhaust, which then passes over a diesel oxidation catalyst. The resulting combustion raises exhaust temperature above 1,100° F, oxidizing the soot. A relatively small amount of inert ash remains. Over time, ash builds up and must be removed, or else the controller will derate the engine and the engine will lose 25% of its power.
To clean the filter, the ceramic section must first be removed. Through heat, vibration, and carefully applied compressed air, the ash is physically removed. Then the filter is reassembled.
Engines that run continually at peak temperatures automatically regenerate, sometimes several times daily. Given the stop-and-go duties of most public works vehicles, though, engine temperatures are rarely high enough to trigger regeneration. To avoid power loss and eventual shut down, the operator must launch regeneration manually, a process that takes up to an hour and is potentially dangerous.
Exhaust temperatures approach 1,200° F, so park far from anything flammable. Don't park vehicles with vertical stacks under trees, wires, or building overhangs. Secure all booms and other equipment.
Once you learn how often vehicles require regeneration, you can schedule in advance in the yard.
Pickup and SUV filters are usually welded in place and are good for the vehicle's expected life. Cleaning filters from larger vehicles can be done in-house or contracted out to the vehicle's dealer. Prices for cleaning equipment and service contracts vary, but the break-even point for 250 medium- and heavy-duty vehicles with engines built after Jan. 1, 2007, is generally less than one year.
On (so far) rare occasions, ash buildup coupled with improper handling has caused filters to crack — remember, hollow ceramic is fragile and should be handled accordingly, especially when removed from its protective housing. Cracking from thermal shock occurs when coolant or oil enters the exhaust.
— Paul Abelson (firstname.lastname@example.org) is a former director of the Technology and Maintenance Council of the American Trucking Associations, a board member of Truck Writers of North America, and active in the Society of Automotive Engineers.