When you hear people referring to 2010 trucks equipped with SCR NOx aftertreatment systems as 40-ton air cleaners, don’t laugh. That’s very nearly what they’ve become. The quantities of regulated emissions emerging from the tailpipe are so infinitesimally small, that Mack’s powertrain marketing manager, Dave McKenna, claims engineers at the Hagerstown engine facility are having a difficult time accurately measuring them.
By now, we’re getting used to the sight of a 100,000-mile exhaust stack having not a trace of soot on the inside. Reductions of NOx going into 2010 are on the same scale, but they won’t be as obvious—at least to the eye. While loading the truck used on this test drive, I had the opportunity to get up close and personal with the exhaust outlet—like sticking my face right into the thing and drawing in a deep breath.
“The things you’ll do for a story,” the driver quipped as I drew my breath. But kidding aside, what I felt and smelled was nothing different from what you’d expect from pointing a hot hair dryer in to your face and breathing in. The exhaust was warm and moist, and it had utterly no diesel smell. Why should it? What actually comes out the pipe is little more than nitrogen and water vapor.
The 17-gallon DEF tank is mounted under the driver’s door on the Mack’s Granite chassis.
SCR, or selective catalytic reduction as a reminder, is the NOx reduction strategy for five of the six North American engine makers at this point. SCR is an exhaust-after-treatment system that reduces NOx produced during the combustion event. Cooled EGR was the chosen method for 2007, but it is widely felt that further NOx reduction couldn’t be achieved using EGR without significant fuel economy penalties. Treating NOx outside the cylinder allows engineers to scale back ERG rates slightly, which they estimate will yield a three- to five-percent improvement in fuel economy over 2007-model engines.
It seems a little odd to report that there’s nothing to report. The test drive went well, the MP7 engine worked like a champ under the two 72,000-lb loads of black-top I moved that day, and then we went for lunch. In many ways, that’s the story. There’s nothing different to see, feel, hear, or smell—and no operational differences save one. We didn’t need to do a stationary regen all day.
So-long to the Regen
Depending on the duty cycles, DPFs on 2007-model engines may need to be cleaned out once a day or more. Active regen events see diesel fuel sprayed across an oxidation catalyst, which produces temperatures inside the DPF high enough to burn off accumulated soot. Under some operational conditions, exhaust temperatures are sufficient to conduct this cleaning passively —without the addition of diesel fuel. In applications like this end-dump, running empty half the time, exhaust temps weren’t consistently high enough to accomplish that task on ’07-model engines. So, drivers often need to park the truck for 20 minutes or more and run a parked active regen cycle.
With this SCR system, the operator has gone a week and longer without a parked regen.
McKenna says that in a highway application, active regen events might take place roughly every 500 miles or so with the ’07 engines, but with SCR-equipped highway trucks, if an active regen is needed at all, it might be 3,000 to 4,000 miles between events.
So, What’s the Downside?
By using SCR to treat NOx downstream of the engine, we’ll get back some fuel economy, Mack tells us—and the other like-minded engine makers agree. Most are saying something in the three- to five-percent range. The operator of this Granite, working it in an urban environment typical of the application, is seeing three percent right now. The truck has only 78,000 miles on it, and to be honest, the driver isn’t managing the throttle as effectively as he could be—he leapt from a 10-year-old DM-model to this one and hasn’t yet caught on to the new driving techniques required by contemporary engines. The conclusion is fuel economy can only improve.
Back-of-cab clearance has been a concern for body builders. Mack says they’re now down to about eight inches.
The extra weight of the DEF tank and the catalyst (300 to 350 lb, on top of a similar gain on 2007 engines) will be a concern for some. Frame space, likewise, will be an issue in some highly specialized chassis such as twin-steer configurations, and trucks with lots of frame-mounted accessories.
In the photos, you’ll notice the air tanks sit under the driver’s door. Mack tells me they are being moved to inside the frame rails, making room under the door for another fuel tank. Mack’s McKenna says the company is looking at moving the battery box too.
At this stage, 13 months before the 2010 roll-out, the DEF distribution network remains a concern to some. Haines & Kibblehouse maintains a bulk DEF tank in the shop, and puts a few gallons into the truck’s tank weekly. DEF is consumed at a rate of about two to three percent of diesel consumption, so local and regional operations will have no difficulty if there’s bulk storage on site. On-highway trucks are seeing about 200 miles from a gallon of urea, so a tank like the one on this truck will get you clear across the country and about one-third of the way back again.
Pilot Travel Centers announced in early October that it would have inventory in place by late next year—both bulk and package sales. Even in the worst case, a couple of gallons stashed in the jockey box will get you nearly a full day’s driving.
The engine and truck makers have a high degree of confidence in this technology, and it’s proven itself already in Europe. There will be a few adjustments required on the user’s part, but nothing significant compared to the advantages.
My day driving the SCR-equipped Haines & Kibblehouse Granite around suburban Philadelphia revealed no driver-related issues. The driver explained that he was more than impressed with the MP7’s performance compared to the old DM he drove, so the transition from old technology to new could be eased considerably by the promise of fewer hands-on maintenance events (and possible lost time running a parked regen) for the driver to be concerned about, and improved performance.
There are still a few uncertainties among SCR truck buyers, but they should be put to rest long before these engines hit the street. One thing is certain: come 2010, we’re going to have a whole bunch of new acronyms to worry about—if little else.