The Aftertreatment Equation

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.

Alternatively, Caterpillar offers the Caterpillar Regeneration System (CRS). "The combuster provides heat directly into the exhaust and into the diesel particulate filter," says Mike Dozier, Kenworth chief engineer. "The CRS is mounted just after the turbocharger in the traditional turbo pipe location. It works with the same control system methodology where it is identifying, based on temperature, when the diesel particulate filter needs to be regenerated."

Since Kenworth uses both Caterpillar and Cummins engines, it has witnessed both the dosing and burner approaches to aftertreatment devices. "At this point, from a standpoint of application and performance, I haven't seen anything that indicates a significant difference one way or another," says Dozier. "We are seeing excellent results from both."

Operators will not likely notice the automated regenerations during operation. "Other than indicator lights telling them it is happening, it will be largely transparent," says Dozier.

Initially, a black cloud hanging over the use of catalyzed particulate filters was the realization that you would need to clean out the ash. "Ash will accumulate in the low part of the particulate filter," says Matsil. "At some point, you are going to have to remove that ash. That is a service item."

The EPA set minimum limits of 110,000 to more than 150,000 miles between cleanings, depending on the size of the truck. But manufacturers report being able to greatly exceed these limits. In vocational applications, it could be years before you need to clean ash out of the particulate filter.

"There is maintenance, but they are not maintenance intensive," says McKenna.

HOT PIPES

An area of concern with regeneration is the heat coming out of the exhaust. "During an active regeneration, the exhaust plume gets up around 1,000° F," says Morelli.

"One area we are concerned about is this concept of having real hot exhaust gases coming out of the tailpipe during regeneration with a horizontal exhaust system," says Matsil. "During our development programs, we have actually melted and puddled the asphalt underneath the vehicle during regeneration."

To help prevent these extremely hot emissions during regeneration, exhaust aftercooling devices are being developed.

"Kenworth has developed an exhaust temperature reduction device," says Dozier. "The manufacturers are also working together to develop driver interface systems to help further control the regeneration process."

General Motors is also developing an exhaust cooler. "We are going to design and validate an exhaust cooler device, which we will mount downstream or after the diesel particulate filter, that will cool the exhaust gases," says Matsil. "It is a patented device we have developed that will allow us to mix and blend air. Some manufacturers are actually looking at pumping in air with an auxiliary compressor."

Freightliner, as well as other manufacturers, are working on similar solutions. "We have a diffuser that we have looked at for our horizontal exhaust," says Morelli. "In our testing, we have found that when you mix the exhaust plume with ambient air with any kind of swirl, it cools off six inches to the side and 12 inches to the back very quickly. We are pursuing that solution, and it has been very effective so far getting that hot exhaust down to a much lower level."

FEEL THE HEAT

With only a few months to go before these systems appear on production trucks, time is of the essence. Manufacturers will need to solve their heat issues before 2007 trucks can take their places in vocational fleets.

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