Today's Trucking
products Engines/Drivetrains Other

THE DOWNLOW ON DPFs

Posted: October 30, 2014 by Rolf Lockwood

ArvinMeritor’s Thermal Regenerator is a particulate filter ideal for vocational trucks.

Escape is not possible. Every medium- and heavy-duty 2007 engine needs a diesel particulate filter, or DPF, to reach mandated emissions levels next year. They add cost, they add some weight, and they add maintenance demands.

And in some cases, drivers may become involved as well. As an end user, there isn’t much of a difference in the ’07 engines compared with the 2002/04 offerings – but is a DPF mounted somewhere on the truck. Count on it.

The need for this filter arises because 2007 Environmental Protection Agency (EPA) rules call for the reduction of particulate matter (PM or soot) in the exhaust of diesel-powered vehicles by 90 percent or more from current levels, and that’s where the DPF comes into play. Its job is to trap the soot, and to burn off the residue as it accumulates.
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, and the DPF doubles as a muffler. DPFs are somewhat larger than standard mufflers, so frame space could be an issue in some cases. In addition, the DPF will have to be mounted as close to the turbocharger as possible in order to achieve the high internal temperatures necessary to facilitate efficient regeneration of the DPF.

While the DPF – in most cases – will look much like a conventional muffler, though slightly larger, they’re considerably different beasts. DPFs are equipped with a temperature sensor and two internal pressure sensors – one upstream of the filter substrate, the other downstream – to determine the necessity of ridding the filter of soot. While they’re promised to be as physically robust as any muffler, the filter substrate is sensitive to contamination by high levels of sulfur found in pre-’07 diesel fuel blends, and the high ash content of contemporary engine oils.

Proper functioning of any DPF will depend on new ultra low-sulfur fuel (15 parts per million sulfur) and on a new lube oil – called PC-10 for now – designed specifically to control ash build up in the DPF.

With the exception of Cat’s ’07 engines rated at 550 hp and above, trucks will require one DPF. The big Cats will need two.Active Regeneration
In low ambient temperatures or low-load/low-speed conditions where exhaust temperature just doesn’t get high enough, some sort of thermal device will be needed to provide serious heat up in the 1000°F range. This is active regeneration.
In an approach typical of all engine brands, 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 1300°F, resulting in very high temperatures across the filter. Cat does the same thing by igniting a small quantity of diesel in an auxiliary heater mounted directly behind the turbocharger. 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.

And don’t sweat those high internal temperatures. We’re assured that the external temps will be no higher than you’d find on a conventional muffler.

On a test drive of Detroit Diesel’s 2007 Series 60 engine in Portland earlier this year, my co-conspirator, Jim Park, witnessed an active regeneration event by way of a dash-mounted laptop computer. The screen showed numeric values representing exhaust temperature, inlet pressure, outlet pressure, and a diminishing value representing the oxidizing soot. For the sake of the test, the event was manually stopped, and the exhaust temperature returned to normal. Upon resuming the event, the temperature rose, and the soot began to disappear.

Park stressed that during the course of the active regeneration event, he noticed no difference in the driveability of the engine, observing that if he hadn’t watched it on the computer screen, he never would have known it was happening.

As described by Mack Trucks powertrain manager Dave McKenna in a presentation to the Technology and Maintenance Council (TMC) in Tampa, Fla. this past February, there is 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 would have to be stationary and the engine at high idle. 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 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 backpressure sensors would see it and order a heavy derating of the engine. The driver would have no choice but to pull off the road.

Cat’s on-highway DPF is expected to need cleaning every 300,000 miles.

More “What Ifs?”
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 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 worse yet, it also seems likely they’ll demand some training for drivers as well.

Share

  • This field is for validation purposes and should be left unchanged.
Related Articles
TodaysTrucking
TruckNews