The Aftertreatment Equation

Heat issues with '07 aftertreatment devices.


It's not a well-kept secret that the diesel engine emissions standards that go into effect for the '07 model year require more complex solutions. Many in the industry were initially concerned about how performance might be affected. "The anxiety levels were pretty high. But the more we got into it and started testing our trucks and getting our systems optimized, the less customers are going to see," says Larry Dutko, EPA '07 project manager, Freightliner.

The largest impact is likely to be the initial purchase price. Manufacturers have been pretty consistent about the estimated price increases. "On a medium-duty truck, it is about $4,000 to $6,000," says Steve Morelli, sales and marketing manager for EPA '07, Freightliner.

KNOCKING OUT NOx

Truck manufacturers had to substantially cut two tailpipe emissions—NOx and particulate matter. Most tackled the NOx reduction challenge by increasing the amount of exhaust gas recirculation (EGR). A larger percentage of the exhaust gas is routed back into the combustion cylinder. But this also raises the heat rejection.

Freightliner addressed this issue by increasing the size of its radiators. "We widened our radiators a little, but we didn't change any hood lines," says Dutko. "On the engine, there is a slightly larger EGR cooler. With the radiator, we had to increase the water pump flow." The results look promising. "Our '07 engines are actually running slightly cooler than they were in '04."

In addition, closed crankcase ventilation systems will replace open breathers. All of this hardware takes space.

"Looking under the hood, to say the least, it is going to get crowded," says Steve Matsil, vehicle chief engineer—medium-duty commercial trucks and full-size vans, General Motors. "The additional cooling capacity for the EGR cooler is going to take up more real estate, probably on the side or top of the engine. The closed crankcase system, with its hoses and tubes and maybe a filter element, is going to add more space."

NO MORE MUFFLERS

Despite all the changes centering on NOx reduction, the biggest challenge for '07 was dealing with particulate matter (PM). All of the manufacturers found it necessary to go with an aftertreatment device that includes a diesel particulate filter.

"The muffler as we know and love it today is gone," says David McKenna, powertrain products marketing manager, Mack Trucks Inc. "In its place is a diesel particulate filter."

The majority of aftertreatment devices are going to use a catalyzed diesel particulate filter. The particulate filter, typically made of a ceramic substrate, captures the soot generated during operation.

Catalyzed diesel particulate filters combine passive and active regeneration. "On a typical short-haul truck, like a transit mixer or a dump truck, you will generate enough heat in a duty cycle that you will passively burn off a large amount of soot that will build up in the diesel particulate filter. However, it will not get all of it," explains McKenna. "So every once in a while, we will have to go into an active regeneration mode, which basically means spraying a small amount of fuel into the exhaust stream that is not combusted. That wets down the catalyst, raising the catalyst temperature. As the exhaust flows through the catalyst, it gets heated up to the point where it will then bake off the soot."

The need for active regeneration is based on exhaust pressure. Pressure sensors, located upstream and downstream of the diesel particulate filter in the exhaust pipe, measure the pressure differential. When the filter starts to restrict the exhaust gas and hits a predetermined pressure differential, atomized fuel is released into the exhaust stream. This is called the "dosing approach."

A catalyst in the aftertreatment device reacts with the atomized fuel and creates a large amount of heat, which burns off the soot and creates a small amount of ash.

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