Mode Test cycle. Figure 7 shows a set of Volvo D12D FE test results for lubricants with different SAE Viscosity Grades. The presented data suggest that the high temperature viscosity determines the outcome of the test to a greater degree than the low temperature viscosity and SAE 10W-30 and 5W-30 engine lubricants provide more fuel economy improvement potential than SAE 5W-40, 10W-40 or 15W-40 lubricants for over-the-road Heavy-Duty diesel engines. They have also reported fuel economy improvement with increasing lubricant temperature; however this leads to increased oxidation, and can increase engine wear if a low viscosity grade lubricant is used in combination with non-optimized additive technology. Therefore, it makes a good case for usage of lower SAE grades only with High performance HDDEOs of performance level such as API CJ-4 or the forthcoming US PC-11.
Figure 4
Figure 7 Figure 5
Emerging requirements of Fuel Economy With the emerging focus on greenhouse gas (GHG) reduction there is growing interest in fuel economy (FE) improvement. Low viscosity lubricants can play a crucial role in improving fuel economy. The viscosity of a lubricant is actually a measure of its internal molecular friction. Looking at the Stribeck curve, in the hydrodynamic lubrication phase, the greater the viscosity of the lubricant, higher the energy loss. However, wear prevention is the foremost concern as fuel economy or efficiency can be lost if the film thickness is lost due to inadequate viscosity, leading also to mechanical failure.
Impact of Fuel Economy challenge on VII choice Both Selby5
and Van Dam et al4 have reported Viscous Fuel
Efficiency Index (V-FEI) gains by reducing HTHS viscosity @ 150C and stabilized Fuel Economy (FE) improvement in the Volvo D12D engine by reducing HTHS viscosity @ 150C after 90 cycle shear in the Kurt Orbahn Shear cycle, respectively. Figure 8 shows Fuel Economy Improvement (FEI) with respect to Kinematic Viscosity and the HTHS Viscosity of the lubricants after they have been run in the Bosch Shear Test for 90 Cycles.
Figure 8 Figure 6 Van Dam et al4 have reported the following on the basis of
work carried out in a Volvo D12D Fuel Economy engine test, where specific fuel consumption was measured in the ESC 13
The Kinematic Viscosity in this case is the viscosity after permanent shear loss (PVL), whereas the HTHS measurement accounts for both the permanent shear loss in the Bosch Shear Test as well as the temporary shear loss in the HTHS measurement and has been designated as the Overall Viscosity Loss (OVL) by Selby6
. Thus, HTHS viscosity requirements have a
critical role for both assurance of good wear protection through the formation of an adequate film thickness between engine
Continued on page 10
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LUBE MAGAZINE NO.124 DECEMBER 2014
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