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Development of an Axle Efficiency Methodology for Evaluating Fuel Economy Potential of Synthetic Lubricants

August 01, 2013
Vasu Bala
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ABSTRACT

Legislature for commercial heavy-duty trucks to meet lower emissions standards since 2004 has increased activity in designing efficient transmission and rear axle drivelines for improved fuel economy.  This paper discusses an axle efficiency test methodology developed for simulating line-haul duty cycles in rear axle gear boxes.  Axle efficiency data will be presented for an axle lubricant optimized for line-haul applications.  Comparisons against other axle lubricants will be made to highlight desired rheological properties recommended for maximizing fuel economy for line-haul applications.

1. INTRODUCTION

The drive for improved fuel economy in rear axle drive cars and trucks has renewed importance today as the global demand for gasoline or diesel has increased significantly.  The recent climatic catastrophes near refineries coupled with increasing fuel demand have automotive manufacturers and lubricant suppliers collectively looking at improving torque efficiency and temperature reduction in active automotive drivelines.  Today’s rear axle driven vehicles have improved fuel consumption over their predecessors because of continued improvements to the design and operation of these drivelines. 

The drive for improved fuel economy is passionately pursued around the world.  In the US, mandated Corporate Average Fuel Economy (CAFÉ) limits placed on rear axle vehicles such as sports utility vehicles (SUVs), passenger cars challenge the automotive manufacturers in meeting them.  These automotive manufacturers are incorporating improved aerodynamics, efficient drivelines, reduced weight, and hybrid engines in new models. 

There have been several past studies on evaluating gear lubricants for their fuel economy benefits.  Studies with low viscosity axle lubricants in improving fuel economy claimed their potential of up to 6.3% (1, 2).   Also, axle efficiency tests at 40 to 70 mph showed the potential of up to 1.8% improvement in torque efficiency using multigrade gear lubricants (3).

A number of studies (4, 5) reported improved fuel economy and axle efficiency when using friction modifiers.  Others have claimed up to 4% improvements in fuel economy when using low viscosity SAE 75W-90 gear lubricants (6-9).  Since the year 2000, there has been a steady rise in the numbers of significant studies conducted to evaluate the axle efficiency of single or multigrade gear lubricants.  Examples of significant studies are listed at the end of this paper (10-17).

All these drivers in the automotive industry for improved fuel economy have had the affect on additive and lubricant suppliers.  Additive and lubricant suppliers continue to meet and challenge themselves in offering multigrade mineral and synthetic gear lubricants.  As improving fuel economy is one paramount challenge facing the automotive and trucking industry, this paper highlights the setup of an axle efficiency test to evaluate several multigrade gear lubricants for their effect on spin loss and torque efficiencies on city, highway and towing duty cycles.

2. AXLE EFFICIENCY TESTING

Figure 1 illustrates the axle efficiency test rig and its major components.  An electric motor rated at 200 horsepower (hp) with a variable frequency/speed drive between 30 to 3600 rpm is used to provide ranges in torque and drive speed.  This is required for simulating spin loss at different speeds and the various driving test stages.  Two Himmelstein non-contact type torque meters were installed on the driveshaft after the motor and one of the half-shaft before the dynamometer.  A total of six multigrade gear lubricants were tested against the spin loss and discreet driving duty cycles.  The conclusions of these tests are summarized in the next section.

Figure 1. Schematic of Axle Efficiency Test Stand

3. CONCLUSIONS

This study arrived at the following conclusions:

  • Several mineral  and  synthetic  axle  lubricants with the viscosity grades meeting SAE 80W90, SAE 75W90 and SAE 75W140 were evaluated
  • The dominant  properties  that  affect  spin  loss were low viscosity  and  moderate  VI  using  SP additive systems
  • Spin hp losses increased with speed and decreased with operating temperature
  • Synthetic PAO base oils and thickeners ranked higher than PIB or brighstock thickeners  for  minimizing  spin losses
  • Axle  lubricants  with  low  viscosity  and   moderate  VI were desired for City and Highway Duty Cycles
  • Axle  lubricant  with  efficient  additive  system  for low boundary friction is desired for Towing Duty Cycle

4. REFERENCES

[1]  Bartz, W. J., “Fuel economy improvement by engine and gear oils”, 5th CEC International Symposium on the Performance  Evaluation  of  Automotive Fuels and Lubricants, May 1997, Goteborg, Sweden.

[2]  Bartz,  W.  J.,  “Some  considerations  regarding fuel economy  improvements  by engine  and  gear oils”, ASLE/ASME    Conference,   New   Orleans, LA , October, 1981.

[3]   Chamberlin, W. B. and Sheahan, T. J., “Automotive fuel   savings   through  selected   lubricants”,  SAE Paper No. 750377.

[4]   O’Connor,   B. M.,   Graham, R.  and   Glover,   I., “European experience with fuel efficient gear oils”, SAE Paper No. 790746.

[5]  Naman, T. M., “Automotive fuel economy – potential improvement through selected engine and gear lubricants”, SAE Paper No. 800438.

[6]  Stambaugh, R. L., Galluccio, R. A., and Koller, R. D., “Multigrade gear lubricants in truck fleet testing – analysis for fuel economy effects”, SAE Paper No. 818178.

[7]   Moat, N. W., “Canadian experience with multigrade gear oils”, SAE Paper No. 811204.

8]  Watts, R. F., and Willette, G. L., “Newtonian multigrade gear lubricants: Formulation and performance testing”, SAE Paper 821180.

[9]   Adams, J. H., Frost, K. A., Hartmann, L. M. and Painter, L. J., “The effect of gear lubrication on fuel economy as measured in a line haul truck fleet”, SAE Paper No. 810179.

[10]  Bala, V., Brandt, G., and Walters, D. K., “Fuel economy of multigrade gear lubricants”, 12th International Colloquium Tribology, Esslingen, Germany, 2000.

[11]  Bala, V., Rollin, A. J., and Brandt, G., ”Rheological properties affecting the fuel economy of multigrade automotive gear lubricants”, SAE Paper No. 2000-01-2051.

[12]  Wincierz, C., Schweder, R., Kreutzer, I., and Neveu, C., ”Influence of VI improvers on the operating temperature of multigrade gear oils“, SAE Paper No. 2000-01-2029.

[13]  Akucewich, E. S., Vinci, J. N., Qureshi, F. S., and Cain, R. W., “Developing next generation axle fluids: Part I – Test methodology to measure durability and temperature reduction properties of axle gear oils”, SAE Paper No. 2002-01-1691.

[14]   Vinci, J. N., Akucewich, E. S., Edward, S., Cain, R. W., and Qureshi, F. S., “Developing next generation axle fluids: Part II – Systematic formulating approach”, SAE Paper No. 2002-01-1692.

[15]   Akucewich, E. S.,  O’Connor,  B.  M., Vinci, J. N., and Schenkenberger, C., Developing next generation axle fluids: Part III – Laboratory CAFÉ simulation  test  as  key  fluid  development  tool”, SAE Paper No. 2003-01-3235.

[16]   Vinci,  J.  N., Grisso,  B.  A., Schenkenberger,  C., Qureshi, F. S., Gahagan, M. P., and Hasegawa, H., ”Systematic  formulation  of  efficient and durable axle  lubricants  for  light  trucks  and sport utility vehicles”, SAE Paper No. 2004-01-3030.

[17]  Bala, V., McCombs, T., and Brandt, G., “Effect of rheological properties on the transient and steady state axle efficiencies of multigrade gear lubricants”, 13th International Colloquium Tribology, Esslingen, Germany, 2002.

[18]  Bala, V., “Development of an Axle Efficiency Test  Methodology   for   Evaluating    the    Rheological Effects  of  Axle  Lubricants   for  Improving  Fuel Economy   in    Heavy   Duty   Trucks”,   14th   Int’ Colloquium Tribology, Germany, January 2004.

[19] Bala, V., “Development of an Axle Efficiency Test Methodology   for   Evaluating   the    Rheological Effects   of   Axle   Lubricants   on Spin Loss, City, Highway  and   Towing   Duty   Cycles”,15th  Int’ Colloquium  Tribology,  Germany,  January  2006.

[20] Bala, V., “Developing Methodology for Quantifying Fuel  Efficient   Axle   Lubricants   for  Commercial Trucks”, 18th Int. Colloquium Tribology, Germany, January 2012.

Vasu Bala works at BASF Corporation. His contact information can be found in STLE's membership database.

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