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. Exhaust aftertreatment devices will replace mufflers, ultra-low-sulfur diesel fuel will be mandated and the engines will require a new type of engine oil, classified as CJ-4. There are unavoidable consequences, such as a higher purchase price and a small weight gain. Yet, as manufacturers begin to roll out their solutions, it appears performance will remain comparable to current industry standards.
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. "On a heavy-duty truck, it is about $7,000 to $10,000."
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."
Even the mounting of the radiator was scrutinized. "On a heavy-duty vehicle, we actually mounted the radiator to the engine," says Morelli. "That allowed us to go with a tight fan-tip tolerance so that we got a more efficient cooling system."
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."
Aftertreatment replaces 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.
Fuel and oil sensitivity
The '07 engines will require the use of CJ-4 oils. "Keep in mind that the majority of the ash collected in the diesel particulate filter is a by-product of engine oil, not through the regeneration of soot because that oxidizes pretty cleanly," says Dutko.
The new oils have lower ash content, and tend to be more expensive. But on a positive note, they are backwards compatible.
Fuel is also a concern since some customers, such as construction contractors, may have access to fuels other than ultra-low-sulfur. "You have to use ultra-low-sulfur diesel, which is 15 ppm or less," says Dutko.
The systems are not designed to handle higher sulfur contents. "The sulfur will coat the precious metals [used in the catalyst]," says Morelli. "Over time, continuing to use diesel with high levels of sulfur will compromise the system. It could lead to having to replace the diesel oxidation catalyst prematurely, which would be a relatively expensive proposition."
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 6 in. to the side and 12 in. 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."
What about fuel economy?
Fuel efficiency is still a question, and the response depends on who you talk to. In the case of Mack, a net fuel economy gain is expected because a new generation MP Series engines will replace the company's ASSET engines, which were a derivative of its predecessors. "We are going from an overhead valve engine to an overhead cam engine," says McKenna. "We are going from a high-swirl combustion system to a no-swirl combustion system. We are going from high injection pressures to ultra-high injection pressures. This is essentially a ‘clean sheet of paper' engine."
Freightliner has been pleased with its initial fuel economy results. "The fuel economy was surprisingly close to where the '04 engines are running," says Dutko.
But the fuels currently being used are not production volume fuels, and the ultra-low-sulfur fuel will have slightly less energy per unit of volume. "The initial information we are getting back from test fleets is the engines are able to maintain the performance levels they had on the pre-'07 engines," says Dozier. "Where we are likely to see the impact is on the fuel itself. Ultra-low-sulfur fuel has less energy density. With the engine, all things being equal, you take energy density out of the fuel and you are going to see a slight decrease in fuel economy. It now ranges 1% to 3%. I think those are safe numbers that you will hear from a lot of people."
Matsil agrees, adding, "Ultra-low-sulfur fuel has less BTUs per gallon than 500-ppm sulfur fuel. I think you will lose a percent or half a percent of fuel economy. But in terms of power, we have adjusted for it. You will get the same performance. We just understand the fuel base we are using."
Spec it right
The '07 trucks are going to be more sensitive about the routing of the exhaust system. "We keep advising our dealers and customers that in '07, it is vital that the spec be submitted to the factory properly because revising or modifying the exhaust system after the fact is not something they are going to want to do," says Morelli. "It is not going to be cheap. It is best to get it right the first time."
The relationship between the aftertreatment device and hot exhaust gases coming from the turbocharger are critical. "[Everything] from the front of the aftertreatment device is fixed by the OE so the certification process works correctly," says Dutko. "That is a big switch."
Dozier explains, "From the outlet of the turbocharger, there are defined windows of where [the aftertreatment device] has to be placed to maintain a specified range of heat loading. It depends on the engine and the horsepower of the engine."