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."