We appear to be learning from our mistakes in aftertreatment system maintenance, but we still have a way to go before we’re at peace with the things.
Strictly speaking, diesel particulate filters, diesel oxidation catalysts, selective catalytic reduction systems and their associated hardware have proven pretty reliable over time. It’s not so much the basic hardware that stymies maintenance personnel, fleet managers and drivers, but the difficulty in predicting when maintenance is required or when upstream problems will cause downstream failures.
“The biggest problem I see is there’s just not enough attention paid to the preventive side of aftertreatment maintenance,” says transportation fleet maintenance consultant Bruce Stockton. “Fleets tend to wait until a light comes on or the system throws a code. By the time the light comes on, the problem could be such that the truck could be down for two or three days at a remote location. If fleets stayed ahead of the problems, maintenance on the system could be done, along with the regular PM work, in a day.”
Stockton says if some preventive measures were put in place at some prescribed time or mileage, much grief and downtime could be prevented.
“A lot of fleets seem to think that because the aftertreatment warranty goes out to 500,000 miles or more, they’ll hope that nothing goes wrong in the interim,” he says. “That’s like saying I bought a 500,000-mile warranty on my engine, so I won’t bother changing the oil.”
Based on Stockton’s experience working with many different fleets, he estimates the safe DPF cleaning interval for an over-the-road truck running 400 to 500 miles a day is 350,000 to 400,000 miles. He has tracked data on these intervals and says they can be applied fairly universally to trucks with fuel mileage in the 6-mpg range and moderate idle time.
“Newer trucks enable you to see soot loading and other factors that impact the cleaning intervals,” he says. “Use that data to monitor the systems, and when levels start to rise, have the DPF cleaned or switch it out with an exchange.”
The beauty in using an exchange is the downtime can be cut by a half to a third. It typically takes a couple of days for a proper cleaning procedure, and then you have the disassembly and reassembly and recalibration time. Dropping in an exchange takes a couple of hours. You’re probably thinking an exchange DPF costs a lot of money compared to a cleaning, but when you factor in the downtime, the cost benefit begins to make sense.
You only need a few exchange DPFs to get started, and then the originals are sent out to be cleaned and are then cycled back to other trucks at their PM intervals.
Staying ahead of your DPF maintenance can improve the prospects for successful cleaning and reuse of the ceramic filters. As ash and soot accumulate in the filter, active regens will eliminate most of it in the early stages of life, but not all of it. As the filter ages, more material is left behind after a regen, which eventually requires more frequent regens. At some point it will need to be physically cleaned. The extent of the cleaning process depends on the condition of the filter.
“A stage 1 cleaning, which is a pneumatic cleaning, will get the majority of the ash and soot out,” says Jeremy Anderson, national sales manager, cleaning equipment supplier, FSX Equipment. “Stage 1 cleaning will return about 80% of the filters back to spec and ready for reinstallation.”
If there’s still a significant amount of material in the filter, it may require a stage 2 cleaning, which involves baking the filter to oxidize the soot and loosen up the ash plugging the channels before it’s pneumatically cleaned, Anderson explains. “This combination of thermal and pneumatic cleaning will return about 90% of the filters to near original spec. The remaining 10% of the filters, such as those left on the truck too long or those with oil or coolant contamination, will need even more intervention.”
Fleets that push the cleaning interval run the risk of contaminating their DPF filters to this point, and this is where it gets expensive and time-consuming.
DPF cleaning 101
Larger fleets that have their own cleaning equipment can turn a stage 1 cleaning around in a day, and maybe two days for a stage 2. If you have to send the filters out to have them cleaned, you’re probably looking at double that time, and that can keep affected trucks on the sidelines.
If you get to the point where your filters require a stage 3 cleaning, which includes a wet wash as well as the thermal and pneumatic cleaning, you are basically giving the filter up for reconditioning. Stage 3 is a process used by OEMs and really large fleets to turn badly contaminated filters back into service. From an end user’s perspective, when you buy a reconditioned filter, it has usually gone through something like a stage 3 cleaning.
“There are some question marks about buying reconditioned filters,” cautions Anderson. “They are much less expensive than new filters, but they come without a history. If they have gone through a stage 3 cleaning, they will have been exposed to some higher level of contamination and there may be some internal damage to the filter.”
So when it comes to cleaning, if you’re proactive and stay ahead of the need for a stage 2 or stage 3 process, you’ll be time and money ahead. You’ll know the filters you are using, as opposed to running yours out to the bitter end and then trading it for a reconditioned product of unknown origin.
However, there is something else to think about. A stage 1 or stage 2 cleaning will get most of the material out of the filter, but according to Daimler Trucks North America, not all.
“Ash left behind from air cleaning results in hot spots within the substrate, which can lead to cracks which allow unfiltered exhaust to pass through the DPF,” says Dale Allemang, DTNA’s director of field service. “Air cleaning will clean the center portion of the filter, but it’s inclined to leave material around the circumference of the filter. For that reason, we recommend a liquid wash as well.”
Determining service intervals
A combination of better technology and better monitoring has allowed engine makers to extend DPF cleaning intervals. A predetermined number of miles or hours combined with fuel consumption are reliable predictors of how hard the engine is working, and how likely the DPF is to need cleaning.
“Since 2007 we have learned a lot, and with the addition of SCR in 2010 we have been able to tune the engine system differently, which is allowing us to reduce ash accumulation,” says Russ Poling, Cummins on-highway marketing communications manager. “We are now producing more torque at lower engine rpm, which reduces the amount of fuel we burn, and that is producing higher exhaust temperatures and steadier flow. All of which lead to more effective regenerations.”
More effective regens mean longer intervals between ash removal clean-outs. For example, Cummins now bases its recommended cleaning intervals on fuel economy. The published intervals for an X15 in on-highway service reads: 5.5 mpg: 250,000-400,000 miles, 5.5-6.5 mpg: 400,000-600,000, and 6.5+ mpg: 600,000-800,000. (That’s a lifetime average by the way, not just one particularly good day’s running.)
There’s still quite a spread in there, but it is possible to be a great deal more precise if you have engine data showing soot load and restriction in the DPF. Service tools such as Noregon’s JPro diagnostic software can reveal the condition of the DPF relative to its need to be cleaned.
“We can give you the codes and tell you what’s going on, and then we’ll give you recommended actions,” says Shane Gilliam, Noregon vice president of sales. “We can lead you down that path, so you don’t have to be extremely well-versed in truck repair. We can help you with that.”
Of course, that still leaves open the door for contamination of the DPF by oil or coolant due to internal leaks or failures. Gilliam says that won’t show up in the codes, per se, but if there is a high restriction reading and a low coolant or oil code, you could put two and two together.
“There are other ways of determining internal problems that may affect the DPF,” says Gilliam. “JPro can provide the bread crumbs; it’s up to the tech to follow the trail.”
Why oils matter
Engine lubricants are typically the single largest source of ash in the DPF. Specifically, it is certain additives in the oil, such as metal-based detergents, anti-wear additives and anti-oxidants, developed before the introduction of DPFs. The new API oil classifications that came into being in late 2016 contain less of this material, which means that if you’re using a CK-4 or an FA-4 oil, there will be less of that type of ash winding up in your DPF.
“These newer oils contain less ash than older oils, so there’s less chance of DPF fouling,” says Jim Nachtman, Navistar heavy-duty product marketing director. “The new cleaning intervals we publish for our A26 engine are based on these new oils. You can continue to use older oils, like a CJ-4 or CI-4, but you’ll have to dial back the DPF cleaning interval.”
Even “healthy” engines consume small amounts of lube oil, and most of that ends up going into the exhaust, explains Shawn Whitacre, senior staff engineer - engine oil technology with Chevron. “Because lube oils contain metallic components (from additives and from wear metals), some of these incombustible materials accumulate in DPFs and other exhaust components.”
When choosing oil, be sure you’re using the right product for your engine. The CK-4 oils are for current and older engines, and are backward compatible to older oil categories. The FA-4 fuel-efficiency oils are designed for newer engines (2016 and beyond) and are not backward compatible because of lower high-temperature/high-shear viscosity.
Oil has other ways of making it into your DPF as well, such as leaks and internal oil consumption. Any liquid, such as fuel, oil or coolant, that makes it into a cylinder and eventually into the exhaust stream will foul the DPF and lead to premature plugging, and perhaps permanent damage to the diesel oxidation catalyst, the DPF or worse, the SCR.
“Contamination of the SCR can interfere with NOx conversion,” says John Moore, Volvo Trucks product marketing manager for powertrains. “Once they have been damaged by coolant or oil contamination, they have to be replaced. They cannot be cleaned or repaired.”
Since the SCR is downstream of the DPF, the likelihood of contamination is slim, but if a severe problem goes unchecked for a long time, the risk is higher.
Most of this story has been devoted to DPF maintenance because that’s where most of the expensive and time-consuming problems occur. The fact is, the SCR side of the aftertreatment system requires relatively little maintenance.
Nachtman says the only scheduled maintenance for Navistar’s SCR system (Cummins Single-Module aftertreatment) is changing the diesel exhaust fluid tank neck filter, which is in place to keep grit and debris out of the tank. “It’s a scheduled item at 30,000 miles,” he says. “That’s all. If there’s a problem with the DEF line return pump, you replace it. There’s really no other maintenance except to clean the tank if it becomes contaminated with something like diesel fuel.”
Volvo and Mack have added a DEF-quality sensor to the tank to alert the driver to a potential contamination problem. “That came with the on-board diagnostics updates in 2016,” Moore says. “Previously the system would not detect bad DEF. Instead, it would guess at the source of the problem, sometimes targeting a sensor. You’d get some incorrect fault codes and possibly an engine derate because it wasn’t seeing the proper conversion rate. Now, you’ll see a code for bad DEF.”
Mack and Volvo have a scheduled DEF filter and pump filter replacement at 150,000 miles. Paccar has a service interval for the DEF filter and dosing unit at 300,000 miles.
Still, DEF tanks can be contaminated, and that can cause problems with the DEF quality and ultimately NOx conversion. If only water has been added to the DEF tank, drain the DEF tank, flush with distilled water and refill with new and/or known good DEF, advises Paccar. With other contaminants, such as diesel fuel, hydraulic fluid, coolant or windshield washer fluid, the tank/pump/filter could require replacement dependent upon the severity of contamination.
The good news for fleets is that aftertreatment system maintenance is getting easier and less complicated, but only for those with newer trucks. Original equipment manufacturers have improved the efficiency of these devices and in some cases the packaging. Those changes mean some fleets will see fewer regen events on their DPFs and their cleaning intervals have in many cases been extended. Maintenance of the SCR and DEF systems remains minimal.
Fleets with older trucks, however, will likely see their DPF regen intervals shorten as ash clogging builds up to solid blockages. Many with 2014-ish trucks will be coming up to the DPF cleaning/replacement intervals. Rather than get caught on-road in an expensive predicament, take a proactive step and schedule a cleaning or replacement before the DPF plugs up for good and sticks you with a $1,000 bill and three days downtime.