In the wake of federal clean-air mandates in 2007 and 2010, maintenance technicians are working furiously to keep the aftertreatment systems of their fleets’ trucks operating.
Diesel particulate filters, or DPFs, and selective catalytic reduction, or SCR, have become standard in North American heavy-duty trucks over the past decade. They have enabled the nation’s Class 8s to significantly slash emissions of particulate matter, or soot, and nitrogen oxide compounds, but maintenance professionals interviewed for this story were unanimous in describing significant difficulties in keeping them running.
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There has been lots of trial and error for matching maintenance protocols with the type of work to be done. Maintenance executives also have learned to hunt for upstream problems that throw contamination downstream to DPFs. They also said they have to grapple with diesel exhaust fluid that crystallizes around SCR dosing valves.
“In 2005, we spent 55 hours a week on emissions exhaust in our shops, and in 2015, it was 662 hours a week,” said Lee Long, director of fleet services at Southeastern Freight Lines, a less-than-truckload carrier based in Lexington, South Carolina.
Long said SEFL’s fleet size did grow by 5% to 6% over the decade. However, the complexity of the systems is the main reason for the increase to the equivalent of about 11 technicians doing nothing but exhaust work each week.
“When we first started, we used the standards based on what OEMs recommended, but the DPFs were clogging fairly quickly,” said Long, the 2012-2013 chairman of the Technology & Maintenance Council, a division of American Trucking Associations.
“The frequency of these problems is fairly high,” said Tim Moore, a vice president of roadside operations for FleetNet America, a nationwide third-party provider of maintenance services. “It was a real problem for me on the fleet side as a maintenance director. You could see it every day, a truck running down until it derates. There have been a lot of self-inflicted wounds on DPFs.”
Moore said he knows of drivers who have disabled the DPF regeneration function and failed to turn it back on, and that leads to filter clogging and then a derating of the truck.
“These problems are definitely prevalent. … The problem is developing components that can live in this environment,” said Michael Buck, president of MCB Fleet Management Consulting on St. Simons Island, Georgia. “If you do everything right so the engine runs optimally, the aftertreatment doesn’t have to work its butt off. But that hardly ever happens,” Buck said, adding that hitting the optimal point is “a pretty tall order.” Buck left a 25-year maintenance career with UPS Inc. in 2001 and has been an independent consultant since 2005.
DPFs and SCR are considered aftertreatment systems because they process exhaust, which is the result of internal combustion. DPFs are the slightly older technology and appear to generate problems more frequently. They were known to be sensitive before they were put into use. The maximum sulfur limit in diesel fuel had to be sliced to 15 parts per million in the fall of 2006, from 500 ppm prior to that, to keep the sulfur from destroying the filters.
DPFs are designed to collect soot, or unburned fuel, as exhaust passes through. DPFs get cleaned through a regeneration process, or a regen, in which heat from the engine bakes the soot. This is called passive regeneration, and it is ideal because the filter gets cleaned through normal operation.
The alternative is a forced regen, in which diesel fuel is splashed on the DPF, creating a fire to bake the soot.
While the most logical place to look for DPF issues is the filter itself — pull it out and blow it out, technicians said in recommending cleaning — maintenance providers and even drivers also need to be trained to look for upstream fluid and air leaks that manifest themselves as DPFs clogging early.
“Emissions, to some degree, are still new to us,” said Darry Stuart, president of DWS Fleet Management Services in Wrentham, Massachusetts. Another former TMC chairman, he said maintenance staffs still are learning how to deal with aftertreatment.
“If there’s a leak, it has to be replaced. … Any one little thing with a tendency to fail will create an emissions issue,” Stuart said. A leaking exhaust manifold gasket, for instance, robs the DPF of heat needed for a passive regen.
In terms of fluid leaks, exhaust gas recirculation (EGR) coolant and turbocharger oil were named most frequently as problems. They have been found to enter the engine’s cylinders and get burned in combustion. At that point, they are part of the exhaust and they enter the DPF and clog it.
All maintenance executives who were interviewed said the problems are not unique to a specific truck or engine maker but spread across original equipment manufacturers. The problem is that rumbling over hundreds of thousands of highway miles, and even more than 1 million miles, exacts a price.
“Over time, with road vibration and wear and tear, what starts as a tight machine becomes less so. These are very fragile systems, and the older a truck is, the more leaky it gets,” said Drew Taylor, director of global sales and part owner of FSX Equipment in Granite Falls, Washington, a manufacturer of DPF cleaning machines.
Taylor said the upstream contaminants can clump into an “ash island,” which he compared with “a cancerous tumor on a DPF.” The longer they remain, the harder they get until they become like “concrete,” he said.
“A DPF is a sophisticated garbage can, and like any garbage can, you have to dump it,” Taylor said. Actually, the filters are a collection of many little garbage cans, as there are about 6,000 cells in a DPF and all of them need to be cleaned.
Volvo Group’s two North American heavy-duty truck-making units, Mack Trucks and Volvo Trucks, are introducing a new, combined aftertreatment system, ClearTech One for Mack and Exhaust AfterTreatment System (EATS) for Volvo. For use on trucks made in 2017, they combine the DPF and SCR in one unit.
“Two key factors drove the development of Mack ClearTech One: improved emissions performance and up to 11 inches of additional frame rail space,” said Scott Barraclough, a Mack technology product manager.
The new system has a copper-zeolite catalyst, designed for better low-temperature conversion of nitrogen oxides. Combining two components in one location also allows the company to save weight, up to 17 pounds, and provides easier access to the DPF, Barraclough said.
Volvo Director of Product Marketing Wade Long described EATS in a similar fashion. Both men said their respective companies have extended their DPF service intervals to 400,000 miles from 250,000.
The maintenance executives said one size does not fit all for DPF cleaning. Stuart said an over-the-road carrier whose trucks “run across Kansas” on a daily basis will be able to enjoy an extended cleaning interval — if there is no upstream contamination.
However, grocery store distribution, LTL pickup and delivery, refuse haulers, school buses, medium-duty trucks and vocational vehicles were cited as trucks that will need more frequent cleaning and attention.
An important condition to monitor, everyone said, is pressure differential. DPF systems have pressure sensors on either side of the filter. If the entrance side has high pressure, but the exit side is low, that’s a sure sign of filter clogging.
Beyond cleaning and inspection, there was consensus on training drivers, especially about the topping off of fluids. Drivers have to communicate with their maintenance colleagues about check-engine lights and other warnings. Furthermore, if drivers are carrying 5 gallon jugs of coolant or diesel exhaust fluid and topping off tanks that empty quickly, that is a classic warning of leaks upstream that probably are flowing toward aftertreatment systems.
“You have to rely on the drivers to communicate,” said Southeastern’s Long, who added that his 400-person maintenance staff also relies on reading fault codes reported through vehicle telematics.
On the SCR side, the big problem is crystallization around dosing valves. The valves feed other hardware that mists the exhaust with DEF, a solution of urea in water.
If the valves do not shut off precisely, DEF will accumulate and crystallize into a hard, white, chalky material, similar to what pool players apply to their hands to minimize friction. If left untreated, the valves can become completely encased and they are no longer able to dose the exhaust. If that happens, they must be replaced.
Long said, though, he has had success soaking the chalk-like deposits in warm water and then scraping them away from the decomposition tubes that are fed by the dosing valves. He said problems often become noticeable at 180,000-mile intervals.
Another recommendation for DEF was buying a good product. Test the fluid purchased with a refractometer, find a reliable vendor and stick with that line.
Carriers should make sure their DEF inventory does not top six months, and they should not expose it to sunlight while being stored or let it freeze.
Stuart of DWS Fleet Management said equipment is improving over time and “becoming more manageable.” FSX Equipment’s Taylor agreed, saying products from major OEMs have improved, but he warned about bargain-basement, low-quality DPFs that can damage the truck.
Long said Southeastern has brought the problem under control, “but you have to stay on top of this or it will eat your lunch.”