SCR Diesel Engine Delivers Good Fuel Economy

By Tom Berg, Truck Editor | September 28, 2010
Mack Granite heavy truck

Green Granite with its 2010 SCR diesel is owned by Mack and was loaned to Haines & Kibblehouse for testing. It has worked well and fuel economy is encouraging.

MP7 405 Maxidyne diesel engine

MP7-405 Maxidyne diesel looks like a 2007, and basically is. It pulled well, except when starting from a dead stop, where a misadjusted clutch might not have been the problem. Exhaust is ultra clean.

Heavy trucks

Dan Alderfer, H&K’s fleet superintendent (left), and driver Jimmy Kissling say that topping off the blue-capped tank with diesel exhaust fluid is a minor task. The filler is too small for a diesel-fuel nozzle, so replenishment mistakes shouldn’t occur.

Mack Granite Class 8 truck

Blue lines and red wires lead to DEF injector and chamber below the exhaust system’s coffee-pot-shaped diesel particulate filter. This is a prototype, and production installations will be cleaner, Mack says.


Truck: 2009 Mack Granite MRU613, conven tional cab with setback steer axle, BBC 116 inches

Engine: Mack Power 7, 10.8 liters (659 cubic inches), 405 hp @ 1,500 to 1,900 rpm, 1,480 lbs.-ft. @ 1,200 rpm, w/PowerLeash engine brake, and EGR, DPF and SCR equipment

Clutch: Eaton 15.5-inch 9-spring coaxial

Transmission: Mack T-310MRL 10-speed w/multi-speed reverse

Front Axle: 20,000-lb. Mack FXL20 Uni-Max

Rear Axles: 46,000-lb. Mack SS462 w/4.17 ratio, on Mack Camelback anti-sway multi-leaf suspension

Pusher axle: 20,000-lb. Watson + Chalen,liftable and self-steering

Tires & wheels, front/rear: Bridgestone 425/65R22.5 M884F on aluminum discs/11R22.5 M726EL rear on steel discs Fuel Tank: 116 gallon left-side Body: Benson 17.5-foot steel dump

One year from now, in January 2010, all new diesels for sale in the United States and Canada will have to meet even tougher exhaust emissions limits than now, and many will do it with selective catalytic reduction. SCR equipment, already used in many diesels in Europe and Japan, sprays a urea solution into the exhaust stream to break down oxides of nitrogen — the NOx that causes smog — into harmless nitrogen and water vapor. SCR also promises better fuel economy.

That’s about what they’re finding at Haines & Kibblehouse Inc., a contractor based in eastern Pennsylvania, which ran a Granite dump truck with a 2010-spec Mack Power engine in everyday service. The Granite is the very first Mack built with an SCR engine, and last April it was loaned to H&K for testing. By mid-September, when I visited the company’s Skippack headquarters, the engine had worked well and saved a goodly amount of fuel, according to Dan Alderfer, the fleet’s superintendent.

He showed me the truck early one workday morning, and Jimmy Kissling, the assigned driver, pointed out its clean-as-new exhaust stack. “It looks like it just came from the factory,” he noted, because there was absolutely no soot inside, although it had been working for more than five months. Pipes on 2007-spec diesels with their special filters also look this way. A particulate filter was fitted to this truck’s MP7 engine because it’s part of the 2010 equipment to be used by Mack and others. Exhaust gas from these engines will be cleaner than the air they inhale in some smoggy cities, industry people have said.

The Granite MRU613 was owned by Mack but resembled 255 others in the H&K fleet, down to its Adirondack Green paint and gold company lettering. Its wheelbase of 219 inches is a about a foot longer to accommodate SCR apparatus, including an injection chamber and urea tank that hang on either side of the frame beneath the cab. Kissling said there’s very little work required to keep the 23-gallon tank filled with urea-water solution, also called diesel exhaust fluid, or DEF.

“I top it off maybe once or twice a week, and it only takes one and a half to four gallons each time,” he said. He did it that often because Mack people, including Dave McKenna, power train sales and marketing manager, were keeping close track of DEF and fuel consumption. The amount of DEF used is expected to be about 2 to 3 percent of fuel, and, of course, both are directly related to miles run. Normally, the DEF tank could therefore be topped off once every two weeks or so.

DEF in effect displaces fuel, so decent economy is a result. “It’s now 3.9 low to 6.3 mpg high, with an average 5.7 miles per gallon,” Alderfer reported by phone a couple of months after my visit. “Other trucks that I’m monitoring, including one with an automatic (transmission) and a newer Granite (with a 2007-spec diesel), are 4.2 to 4.8. So it’s every bit of a half a gallon better” than the fleet overall. “I think we’re headed in the right direction” on fuel use. Fuel savings should pay for the stiffer upfront cost of a 2010 SCR diesel in a few years of operation, Mack and other manufacturers say.

Kissling, a former over-the-road trucker who’s been with H&K for 11 years, took me on a couple of runs, and with Alderfer’s OK, he let me drive as soon as I asked. This was just after we took on a load of dirt at a construction site behind the Plymouth Meeting Mall, a big shopping center north of Philadelphia.

“Now, one thing, this engine is easy to kill when you start out,” he advised. “There’s something with this transmission — I can’t get the hang of it.” It was a 10-speed Maxitorque that shouldn’t have been touchy, but sure enough, a minute later I killed the engine when trying to move away from a stop light on a slight upgrade. “Don’t feel bad, I do it, too,” he said as my face reddened and I restarted in Low instead of 1st gear.

Once underway, I got a feel for the clutch and gear pattern, and then had very little trouble up- or downshifting. But at stops, I chose a low enough gear and carefully modulated the clutch and accelerator pedals to keep theengine spinning. Later McKenna said the clutch was found to be out of adjustment, and both fuel filters were plugged. Either or both problems could explain the engine’s be-havior.

While moving, the engine — rated at 405 horsepower and 1,480 pounds-feet — was plenty gutsy. Its Maxidyne torque curve was wide and I could lug it to 1,100 and still accelerate fine, and we could easily keep up with most traffic.

Like most dump trucks in Pennsylvania, this one had a single pusher axle ahead of the tandem which lets the truck legally gross 73,280 pounds. Unlike most others, this pusher was self-steering, so I didn’t have to remember to raise it in tight turns. The tandem rode on a 46,000-pound Mack Camelback multi-leaf suspension with an anti-sway feature that seemed to do its job, as the truck was rock steady in all situations.

We took the load of dirt about 20 miles over county roads and city streets to a quarry at Saratoga, near Pottstown, where we dumped it at the edge of the large pit. Here I used the tranny’s multi-speed reverse function, using a thumb switch to engage reverse and choosing 2nd gear to move backward at a clip faster than most transmissions will allow. It’s especially useful when backing long distancesto a paver or, for a mixer truck, to a poursite.

At the quarry, we picked up a load of gravel and took it to the same construction site at the mall. As before, we passed through Norristown, where traffic lights gave me some additional practice at shifting and I didn’tkill the engine anymore. We got anotherload of dirt, and by now it was late morning, so Kissling dropped me back at the shop where I met my Mack hosts. We chatted some more with Kissling and Alderfer, then departed.

A few weeks after my visit, the truck had been returned to Mack after running six months and about 30,000 miles. Engineers were tearing down and inspecting the engine to see what effects the SCR gear and operating conditions might be having. The truck was to be returned to H&K for more road and street testing, and we’ll be watching its progress.