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Posted: August 1, 2014 by Rolf Lockwood

Escape is not possible. Every medium- and heavy-duty 2007 engine will need a diesel particulate filter, or DPF, to reach mandated emissions levels next year. They’ll add cost, they’ll add some weight, and they’ll add maintenance demands. What’s more, technicians will need training in how to keep them working properly.

And in some cases, they may need driver involvement as well.
No wonder they were the main topic of conversation and the subject of three separate sessions at the recent annual conference of the Technology & Maintenance Council in Tampa, Fla.

For some engine makers not much else will change on 2007 products compared to 2002/04 offerings — at least not as far as you, the end user, will see. In fact, Dr. Steve Charlton, executive director of heavy-duty engineering at Cummins, and ’07 project leader, says active design work began back in 2000 and has been very intense ever since. Meeting ’07 emission standards has not been as simple as sticking a DPF out back.

Behind-the-scenes engineering work at every engine maker has included much study of combustion in general, and the combustion chamber in particular, but cooled exhaust-gas recirculation (EGR) will continue as the main means of attacking nitrous oxides for everyone except Caterpillar. Even those yellow engines will feature a variation on the EGR theme — Cat keeps its ACERT technology but has added something it calls ‘clean gas induction’ which draws exhaust gas from downstream of the particulate filter and then throws it back into the intake air system. All other EGR systems draw exhaust gas from ahead of the DPF, the difference being — and Cat says it’s an advantage — that the gas taken post-DPF treatment is clean. That intake charge is soot-free, Cat says, so it won’t induce the engine wear that cooled EGR produces. Other engine makers, not surprisingly, deny that recirculated air is dirty enough to cause any engine damage.

Some of Cat’s ’07 engines are unique in another way, in that ratings of 550 hp and above will require two diesel particulate filters. For 500 hp and below, as with all ratings from other engine manufacturers, there will be only one DPF.

So, what are the issues with these new filters? What were TMC conference attendees most concerned about? The most common questions were these:

1. What will happen to a DPF if I run high-sulfur fuel in my engines? 2. What will happen to it if I don’t use the new PC-10 oil? 3. How much shop attention will DPFs demand?

The keys to DPF performance and maintenance, and the answers to these questions, are wrapped up in the ‘regeneration’ process — the filter’s self-cleaning routines — and in the far less frequent but more demanding removal of ash that’s left hanging around by the lubricant.

What’s a DPF?

Cat’s manufacturing its aftertreatment system in-house

The need for this filter arises because 2007 Environmental Protection Agency (EPA) rules call for the reduction of particulate matter (PM) in the exhaust of diesel-powered vehicles by 90 percent or more from current levels. More specifically, PM has to come down to .01 g/hp-hr (grams per horsepower hour) by January 2007.

New standards for NOx — and also for non-methane hydrocarbons — are 0.20 g/hp-hr and 0.14 g/hp-hr, respectively. They’ll be phased in for diesel engines between 2007 and 2010 on a percent-of-sales basis: 25 percent in 2007, 50 percent in 2008, 75 percent in 2009, and 100 percent by 2010.

For 2007, NOx will be dealt with by higher injection pressures and especially by higher-capacity EGR. But engine makers need the DPF to filter the soot out of the exhaust and store it until it can be burned off. Proper functioning of any DPF will depend on new low-sulfur fuel and on a new lube oil — called PC-10 for now — designed specifically to control soot.

A DPF cleans exhaust gas by passing it through a wall-flow filter, and the particulate collects on the inlet face. The filtration substrate for all ’07 diesels is a ceramic material. In every engine case, the DPF combines a muffler, and there are many packaging issues for truck makers. Most are somewhat larger than ordinary mufflers for one thing, and the extreme heat that can be created within the filter can’t be exhausted just anywhere, so placement is critical.

International Truck & Engine engineer Tim Shick told a TMC audience that body builders will have to understand this point, that they can’t relocate a DPF without careful consultation with the truck chassis maker, and maybe not at all.

These combined filters/mufflers will have a removable filter section to facilitate cleaning or replacement. A DPF will not be interchangeable across engine brands, so fleets using different engine platforms could have inventory issues.

Passive Regeneration

Once the soot is trapped and accumulated in the DPF, it’s oxidized — or burned off — in a process called filter ‘regeneration’. That will happen by way of naturally elevated exhaust heat or by deliberately raising the temperature inside the DPF by automatic or manual means so the soot will ‘burn’. The difference is important, the former called ‘passive’ and the latter called ‘active’ regeneration.

“Both processes will be transparent, invisible to the driver,” says Charlton, and all other engine makers say more or less the same thing. There are cases, however, where active regeneration might demand driver involvement. Even a driver decision. And on that point there doesn’t seem to be universal agreement.

But let’s look at passive regeneration first.

The heat produced under normal load will be enough to raise exhaust gas temperature and oxidize the collected particulates in most on-highway cases, and this will be a more-or-less on-going process as the truck rolls down the road. In some trucks a lamp on the dashboard may tell the driver it’s happening, but no action will be required.

Passive oxidation of accumulated soot starts happening with exhaust temperatures somewhere between 250 and 450 degrees F, and then it accelerates sharply at about 500 degrees. Steve Charlton says the “vast majority” of DPF regeneration events will be managed with passive intervention.

Active Regeneration

In low ambient temperatures or low-load/low-speed applications like garbage trucks where exhaust temperature just doesn’t get high enough, some sort of thermal device will be needed to provide serious heat up in the 1,000-degree F range. This is active regeneration.

Detroit has a lot of experience with particulate filters in buses.
This highway DPF needs cleaning after 200,000 miles

In an approach used by all engines except Cat, a small amount of diesel fuel will be injected into the exhaust stream across an oxidation catalyst, creating a thermal reaction that will heat the DPF to somewhere between 900 and 1,300 degrees F, resulting in very high exhaust-outlet temperatures. Cat does the same thing by igniting the fuel/gas/air mixture in a small heat exchanger just ahead of the DPF. The process is managed by the engine-control system and is triggered by temperature and pressure changes in the exhaust system as the filter accumulates soot and plugs up. An active regeneration will take about 20 minutes.

As described by Mack Trucks powertrain manager Dave McKenna in his TMC presentation, there are a pair of options on the ‘active’ side of things, or possibly a combination of the two:

1. Automatic regeneration, requiring no driver action. The truck will have to be moving above a threshold speed (to guarantee a certain minimum exhaust temperature). This will work where the truck’s duty cycle produces high exhaust temperatures on a steady-state basis.

2. Manual regeneration, where the driver is alerted that a DPF purge is required and then must simply flip a switch. The truck can be moving or stationary, said McKenna, though some other panelists in the various engine-oriented TMC sessions suggested the truck might have to be stationary and the engine at high idle in some situations. Obviously, this will have to be pinned down, though it may well be different for each truck/engine combination and will likely be dictated by application as well.

Truck and engine makers don’t appear to have reached a firm conclusion about how much driver involvement in DPF regeneration is either required or desirable. Should there be a switch on the dash that activates or inhibits a regeneration event when the driver’s alerted that the DPF needs help? It may be advisable in situations where the extremely high temperatures generated in the exhaust stack would be dangerous — a truck parked indoors, for example, or under some sort of flammable canopy. In that case, the driver would be called upon to judge just how risky a regeneration would be. Training would clearly be required.

And what happens, in a truck equipped with that switch, if regeneration is called for by various dashboard warnings but the driver simply ignores them? Volvo engineer Ed Saxman anticipated that question in a TMC session on engine electronic sensors. The DPF would gradually plug up, he said, then the engine would be derated, and eventually it would be rendered undriveable.

Dashboard warnings will change from truck to truck, in all likelihood, but Caterpillar engineer Gene Schneider said most would have a lamp that lights up when a regeneration need is first sensed, and then flashes with greater frequency as the need becomes critical.

Cummins engineer Kevin Otto said this would likely happen fairly gradually over a matter of days. A red lamp would finally be illuminated when the DPF is blocked and engine shutdown is imminent. Every dashboard will have at the very least a lamp that indicates a regeneration is occurring.

Saxman also posed some ‘what if?’ questions. Like, what happens if there’s a seal failure in the turbocharger that sends oil straight into the DPF? Answer: the DPF would plug up rapidly but its back-pressure sensors would see it and order a heavy derating of the engine. The driver would have no choice but to pull off the road.

The Questions

Yes, diesel particulate filters will demand some maintenance. They’ll have to be cleaned periodically, above and beyond the regeneration processes that rid them of soot. DPFs will also collect inorganic ash, not as a byproduct of combustion directly, rather as residue from spent engine lube additives. And regeneration at any temperature won’t have any effect on it.

In most cases this will demand removal of the DPF (a half-hour job in the case of Cummins) and the use of a special cleaning machine. Every engine maker is developing one, or buying from a third party, and the cost of such machines will vary. Bill Stahl of Cummins said a ‘dry’ machine, one that uses compressed air to blow ash away, will cost in the region of US$8,000. Detroit Diesel’s Chuck Blake told a rapt TMC audience that the range would be US$8,000-12,000, adding that a ‘wet’ cleaner — one that washes the DPF — would cost more like US$50,000. Few Canadian fleets are likely to buy either one.

What will probably happen is that particulate filters will be exchanged by the fleet rather than being sent out for cleaning, Blake said, at a cost of US$300-500.

The EPA says cleaning intervals must be at least 150,000 miles apart, but actual intervals will vary. Cummins, for example, has published a range of 200,000 to 400,000 miles, depending on application. Cat says 300,000 miles, which means that some truck owners on short trade cycles will never have to deal with DPF removal and cleaning.

Every DPF will have sensors — typically three for temperature and two for pressure — that can fail, and there will be both electrical and fuel-line connections as well.

And what happens if non-ULSD fuel is inadvertently used in a 2007 motor? International’s Tim Shick told his TMC audience that a tank or two full of 500-ppm fuel won’t do irreparable harm, and other presenters generally agreed. But what if the use of high-sulfur fuel is prolonged?

“What you’ll get is a deterioration of the DPF’s ability to do passive regeneration,” said Kevin Otto of Cummins. That will force more active regenerations and thus more fuel use.

Volvo’s Ed Saxman answered the implicit question that followed: “I’ve heard that it’s about half a gallon or two liters of fuel used in a 20-minute regeneration,” he said.

Caterpillar’s Bob Wessels, a member of the audience in that particular session, stood up to note that “The engine will be flat-out non-compliant with high-sulfur fuel.

“The ability of the system to regenerate will get real bad real quickly,” he said forcefully. “I’m not sure how quickly, but not long.”

The same lousy result would be achieved if older CH-4 engine lube were used, instead of the new PC-10 oil, soon to be labelled CJ-4 in all likelihood. It’s still unclear whether CJ-4 oils will be backwards-compatible, so for fleets with a mix of 2007 and pre-2007 engines — which is to say just about everyone — the temptation to standardize on the older lube would be strong. Given that the key purpose of the new lube is to manage soot, clearly a CH-4 oil would send too much of that particulate matter back to the DPF and plug it up prematurely. The older oil also has a higher ash content, meaning more frequent removal and cleaning of the DPF would result if it were used in an ’07 engine.

The bottom line here is that a 2007-model diesel engine will indeed be a more complicated piece of machinery even if not too much has changed at first glance. Its particulate filter will demand routine, if not terribly expensive, attention, and also training for technicians. Worse yet, it also seems likely to require new training for drivers too.

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