21 July 2010
Navistar published a ‘fluid economy’ comparison—including the total of diesel fuel and SCR-urea consumption—of three 2010 emission-compliant trucks: ProStar®+ with MaxxForce 13 Advanced EGR, Freightliner Cascadia® with DD15 and Kenworth T660 with Cummins ISX. The DD15 and the ISX engines use urea-SCR for NOx control, while the MaxxForce 13 engine uses in-cylinder NOx control with no NOx aftertreatment. The test, commissioned by Navistar, was conducted by the Transportation Research Center (TRC).
Consumption of diesel fuel and, in SCR trucks, urea solution was measured over a minimum of three repetitions of a 444 mile test run. Two trucks were driven in a close distance during the tests (i.e., three runs of the ProStar with the Freightliner and three runs of the ProStar with the Kenworth).
The ProStar truck with MaxxForce engine showed the lowest total liquid consumption: 0.90% lower than the Freightliner with DD15 and 2.50% lower than the Kenworth with Cummins ISX. The variability between particular runs within each test was not published, but may have been significant—the difference in fluid consumption of the ProStar truck between the two tests was 2.4%.
It should be noted that these results have a rather limited value for comparing the efficiency of different 2010 diesel engine technologies. First, according to Navistar, powertrain is the least important road load item contributing to fuel economy, contributing only 18%, while truck aerodynamics and rolling resistance are contributing 50% and 32%, respectively. Second, the three engines were certified to different NOx levels, which undoubtedly has an impact on fuel consumption. It is believed that among the three contestants the DD15 was the only engine certified to the 0.2 g/bhp-hr NOx standard, while the MaxxForce engines were generally certified to an FEL NOx limit of 0.5 g/bhp-hr and the ISX engines to FEL NOx limits of around 0.3 g/bhp-hr.