Hitting the road
Because EGR engines must run cooler than SCR engines to meet the NOx standard, Shick says, shedding more heat outside the engine does not stress internal parts. “For example, the oil cooler is no larger,” he says.
For 2010, Navistar moved the main charge air cooler to the front of the radiator, while retaining the inter-stage cooler between turbos, to improve charge-air cooling. Two other changes will improve reliability: Twin wastegated turbos will replace a variable geometry design; and exhaust flow will be controlled with an improved EGR valve that uses stepper motors to move it in both directions, rather than a spring for closing.
Combustion improvements, such as adding a proprietary double piston bowl and increasing fuel pressures from 25,000-26,000 psi to 31,000-32,000 psi, enabled Navistar to reach its goal of maintaining maintenance intervals and getting engine efficiency back to where it was in spite of ingesting more exhaust.
Navistar has said it will introduce the MaxxForce 15 by the end of this year. The engine will use the cylinder block and internal parts of the Caterpillar C15 with no modifications.
Dave McKenna, Mack powertrain products marketing manager, says the 2010 MP engines “have a reduced EGR flow rate, just about down to 2002 levels. This means the engine breathes a little better.” Better breathing saves fuel. The engine will also employ “multiple injection events,” he adds, which minimizes the use of DEF and reduces engine noise.
“The high injection pressures that were available previously allow us to produce less soot.” This is augmented, he adds, by the use of SCR aftertreatment so the NOx is reduced after the exhaust leaves both the engine and the DPF, allowing the system to “passively oxidize 99 percent of the soot in the DPF.”
For example, the infrequent use of the active regeneration system led one driver to tell Mack representatives he was happy with the truck because he had never seen a warning light or one indicating active regeneration from the DPF system.
The engine has a Borg-Warner magnetic fan drive, which also helps save fuel. Reduced EGR means less heat to carry away, although Mack has kept the same cooling package used in the previous design. The combination of an oversize radiator, less heat rejection, and a fan drive that modulates rather than coming on full blast as soon as it is needed, requires less power to drive the engine fan. Drivers will also no longer feel it when the fan engages or disengages.
McKenna claims typical fuel savings in the range of 5 percent “in the same drive cycle with the same horsepower,” with one vocational fleet seeing a change from 6.5 mpg to 7 mpg. These fuel savings are further improved for those who opt for the Econoboost system that discourages unnecessary downshifting by increasing the torque only when it’s needed for hill climbing.
Reliability of the new SCR components has been no surprise, McKenna says, “because of the way the basic system had been tried out previously in Europe.”
No changes in recommended maintenance intervals have occurred. McKenna says that with engine heat and pressure stresses down and the probability that lower EGR means less oil soot, Mack will continue to study data to see if a change is merited.
Watkins Shepard Trucking, which hauls commodities to both coasts from Montana, tested Volvo’s SCR prototype engine. The fleet has had no problems with SCR or DEF supply, says one of its drivers, Michael Halpin.