
                 UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
                NATIONAL VEHICLE AND FUEL EMISSIONS LABORATORY
                             2000 TRAVERWOOD DRIVE
                           ANN ARBOR, MI  48105-2498


									     OFFICE OF
									AIR AND RADIATION
July 2016

MEMORANDUM

SUBJECT:	Effectiveness of Technology to Increase Axle Efficiency 

FROM: 	James Sanchez, Mechanical Engineer, OTAQ/ASD

TO: 	Greenhouse Gas Emissions Standards and Fuel Efficiency Standards for Medium- and Heavy-Duty Engines and Vehicles - Phase 2 - Docket EPA-HQ-OAR-2014-0827


      This memorandum describes axle technologies that were reviewed to determine their effectiveness with respect to their ability to improve the fuel efficiency of heavy duty vehicles.  
Improvements in axle efficiency are generally achieved by reducing mechanical and spin losses.  Mechanical losses can be reduced by reducing the friction between the gears that are in contact with each other.  This friction is reduced mainly by improving the surface finish of the gears or sometimes by reducing the sliding distance of the gear faces where they come in contact with each other.  Spin losses are a function of speed and not torque.  One of the main ways to reduce the spin losses of the axle is by using a lower viscosity lubricant.  Spin losses can also be reduced by lowering the volume of lubricant in the sump.  This reduces the surface area of the gears that are churning through the lubricant.  Other technologies that were evaluated include changes in axle ratio and high efficiency carrier and wheel bearings. 
The benefits of these technologies depend significantly on duty cycle as well as lubricant temperature, but reduction in fuel consumption from these technologies combined, covers a range of 2 to 3 percent.  The 0.5 to 1 percent effectiveness of high efficiency lubricants is based on CBI data from Lubrizol and a SwRI report on advanced lubricants.  GEM simulations with default axle losses show that the efficiency of the axle changes by 0.5 to 1 percent when the axle ratio is reduced from 3.5:1 to 2.5:1, depending on vehicle and duty cycle.  The effectiveness of lubricant level, gear surface finish, and high efficiency carrier and wheel bearings is based on CBI data that was provided to the EPA by Meritor.   
      The effectiveness of these technologies was analyzed for both 6x2 and 6x4 axle configurations.  The effectiveness for the same technologies is nearly double for 6x4 axles versus the 6x2 axles since the 6x4 axle have roughly twice the losses of a 6x2 axle.   
      
      
