Heavy haulers that transport loads above 120,000 pounds gross combination weight (GCW) must carefully specify their trucks for both durability and power. The challenge is to achieve the right balance between the truck's job requirement and expected annual mileage to help produce the lowest operating cost per mile.
This year, that challenge is increased with 2010 federal engine emissions standards in the United States and Canada. When it's time to purchase new trucks, these standards may necessitate some changes to the heavy hauler's current equipment configurations.
"The extent of these changes depends upon each heavy hauler's choice between two available engine technologies, which may also affect truck performance and operating costs over its lifetime," according to Samantha Parlier, vocational marketing manager for Kenworth Truck Company.
2010 ENGINES -- SCR OR EGR?
Heavy haulers can choose an engine aftertreatment approach that utilizes selective catalytic reduction (SCR) technology in combination with exhaust gas recirculation (EGR), or an in-cylinder approach through increased EGR.
Both technologies use EGR to circulate a portion of an engine's exhaust gas back to the engine cylinders and a diesel particulate filter (DPF) to remove particulate matter from the exhaust. A critical difference is the amount of exhaust gas that is recirculated back to the engine; the enhanced EGR approach uses a significantly higher level of recirculated exhaust gases. SCR also mixes a reactant -- most commonly a solution of urea and de-ionized water known as diesel exhaust fluid (DEF) -- with the nitrogen oxides (NOx) in exhaust gases. The exhaust then passes through a decomposition reactor, where the DEF reacts with the NOx to convert it into nitrogen and water.
Increased EGR reduces NOx by boosting the amount of exhaust gases in the engine cylinder, then slowing and cooling the combustion process and burning off pollutants. The increased heat created with the enhanced EGR approach requires greater engine cooling capacity. Increased EGR also requires more fuel to be injected into the DPF for active regenerations.
"SCR doesn't rely on engine heat to treat emissions, so SCR-based engines offer the advantage of higher fuel economy," Parlier said. "Since SCR doesn't narrow the engine's maximum speed range for optimum efficiency, or its 'sweet spot,' to attain emission reductions, fleets also can still maintain fuel economy at lower or higher engine speeds."
Not all SCR technology engines are the same, however. "An aftertreatment catalyst using copper zeolite is much more efficient than one with iron zeolite at reducing NOx at normal engine operating temperatures," Parlier said. "Engines using copper zeolite may enjoy up to an additional 2 percent fuel economy improvement over engines using iron zeolite." PACCAR engines and Cummins engines both use copper zeolite.
Heavy haulers rarely run trucks with engines under 15 liters. Most choose engines rated at 475 hp and 1,750 lb-ft of torque and higher. Engine cooling is always a serious consideration.
"These trucks can spend extended periods pulling a heavy load up hills at slow speeds with little air circulation, so the radiator package is critical," Parlier said. "Kenworth's 1,780 square-inch radiator -- our largest -- and wide hood for the Kenworth T800 help handle the most demanding heavy haul applications."
Parlier recommends dual, 15-inch cowl-mounted air cleaners to keep that big engine breathing easy. "These give the engine cooler more air than an under-hood air cleaner, and the 15-inch duals have four times less restriction than the typical single air cleaner," she said.