E&MU: SCR Exhaust Systems Could Reduce Soot Output

This story appears in the November/December 2009 issue of Equipment & Maintenance Update, a supplement to the Nov. 23 print edition of Transport Topics. Click here to subscribe today.

Today’s new diesel-power trucks are, by law, equipped with particulate filters, and in many models the DPF will soon will be joined by another aftertreatment device — selective catalytic reduction, or SCR, in a canister.

SCR will share the role of nitrogen oxides reduction with exhaust gas recirculation in the engine systems of all but one of the truck makers serving North America.

Operators have to know how the diesel particulate filter works and what they must do, if anything, to keep the truck running efficiently.

Similarly, the workings of SCR will demand operator awareness. As with the particulate filters, getting comfortable with SCR will require training for drivers and technicians for issues such as the significance of a new dashboard warning lamp.

The key issue for SCR will be when and how to refill the containers of urea fluid.

SCR systems work by injecting a urea-and-water mixture — which truck and engine makers are calling diesel exhaust fluid — into the exhaust stream just before it enters the SCR canister.

Generally, the SCR unit will follow the DPF in what could now be called the exhaust “chain.”

SCR systems need the fluid to clean up the last margin of NOx in the exhaust stream. Dashboards of 2010-compliant trucks will get an additional warning lamp or gauge to alert drivers when the fluid is running low.

To deter drivers from operating the trucks without the fluid, the engine will be configured to reduce power so the truck can operate only in a slow, “limp home” mode and prevent the truck from starting once it has stopped if the fluid tank is not refilled.

For medium-duty truck users, this should be a minor issue.

 “Since most Class 7 and smaller trucks come home each night, mechanics can look at [urea] levels the same [as] they do with any fluids,” said Alex LaBrie, principal of Air Blue Fluids, Brea, Calif., a supplier of SCR fluid.

The urea mixture is fed into the exhaust stream, following the particulate filter, at rates controlled by onboard sensors and computers. The dosing rate will be based on engine size and activity, but the consensus of several SCR engine representatives is that the rate of urea use will be about 1½% to 2% of diesel fuel use.

The refill frequency for the onboard fluid containers will vary, depending on container size, the truck and other factors. For example, a Freightliner Custom Chassis walk-in van with a 10-gallon container getting 10 miles per gallon, running about 15,000 miles a year with a dosing rate of about 2% will require about three container fill-ups a year, said Michael Stark, senior technical sales manager.

Todd Bloom, vice president of marketing for Isuzu Commercial Truck of America, said the Isuzu N-Series truck would typically use about one gallon a week, and one tankful of urea will last six to 10 tanks of fuel.

Maintenance of SCR systems should be minimal, according to suppliers. The catalyst — the largest component of the Cummins SCR package — will

not require periodic cleaning, Cummins spokeswoman Christy Nycz said.

Also, most SCR systems will have a filter for the fluid, either in or near the container or near the SCR canister. Nycz said the maintenance interval for its filter will be about 200,000 miles.

Isuzu’s SCR filter will have a replacement cycle of between one and two years of use.

A complete SCR system, including the catalytic canister and the fluid delivery, will add weight to the vehicle. To start with, the fluid weighs 12 pounds per gallon.

The SCR system in the Dodge 3500/4500/5500 chassis cabs, for example, will weigh about 250 pounds, a company spokesman said. Freightliner’s SCR system in the Business Class M2 model, with a six-gallon tank, will weigh 200 pounds, said Stephen Morelli, a marketing manager.

One thing for certain is that 2010-compliant trucks will cost more, regardless of which technology — SCR or enhanced EGR — is used.

Earlier this year, Volvo Trucks North America said SCR would add $9,600 to the cost of its Class 8 trucks. The typical cost premiums for medium-duty trucks likely will be less, in the range of $4,000 to $6,000, said Glenn Ellis, vice president of marketing and dealer operations for Hino Motor Sales.

Recently, Daimler Trucks North America said an SCR surcharge of $9,000 would be added to 2010 vehicles equipped with the Cummins ISX15 engine.