Effects of Lubricant Properties on RingLiner Friction in Advanced Reciprocating Engine Systems

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Effects of Lubricant Properties on Ring/Liner
Friction in Advanced Reciprocating Engine Systems
Rosalind Takata Dr. Victor Wong Nov. 7,
2006 ICEF2006, Sacramento CA
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Overall project
This work is funded by the Department of Energy
Project Low Friction Technology for Advanced
Natural Gas Engines

• Focus on developing higher efficiency engines by
  reducing frictional losses
• Friction reduction strategies to be applied to
  Waukesha stationary natural gas engine

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Lubricant project objectives

• Use MIT ring-pack friction model to reduce
  ring/liner friction by optimizing lubricant
  viscosity
• Averaged flow-factor, Reynolds-based numerical
  model
• Focus on
• Influence of viscosity on hydrodynamic and
  boundary friction
• Viscosity variation during the engine stroke
• Application to specific Waukesha engine operating
  conditions
• Study of best-case lubricants
• Also considered
• Boundary friction coefficient
• Influence of viscosity on ring/liner wear

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Viscosity effects on ring/liner friction

• Direct effect on hydrodynamic friction
• increase in viscosity leads to increase in oil
  flow resistance, causing hydrodynamic friction to
  increase
• Indirect influence on asperity contact/boundary
  friction
• decrease in viscosity leads to decrease in oil
  film thickness, possibly leading to asperity
  contact

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Viscosity effects on ring/liner friction

• oil film thickness increases with viscosity and
  piston speed
• trade-off between hydrodynamic and boundary
  friction
• high speed and viscosity cause high hydrodynamic
  friction
• low speed and viscosity cause low hydrodynamic
  friction and small film thickness, which may
  increase boundary friction

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Waukesha ring-pack friction

• Most (65) of friction comes from oil control
  ring
• Second largest contributor is top ring near TDC,
  combustion

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Waukesha ring-pack friction

• Most top ring friction is from TDC, combustion
  dry region
• high gas pressure/high ring load
• poor lubrication gt do not expect viscosity to
  have much effect

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Lubrication regimes for OCR

• Hydrodynamic friction dominates near mid-stroke
• Boundary friction high near dead-centers
• Most friction power losses come from mid-stroke,
  because of high piston speed

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Ring/liner friction reduction

• Hydrodynamic friction is high near mid-stroke
• Boundary friction is high near dead-centers
• There may be a benefit in reducing viscosity near
  mid stroke, and increasing viscosity near
  dead-centers
• Study both idealized and more realistic cases to
  explore friction reduction possibilities

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Idealized cases

• Cases where viscosity can be controlled at any
  point in an engine cycle were studied
• Idealized cases give insight into viscosity
  effects in different regimes

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Idealized cases high viscosity near
dead-centers

• High-dc case shows effects of the idealized
  case with high viscosity near dead-center

• Viscosity transitions chosen based on transition
  between high boundary friction near end-strokes
  and high hydrodynamic friction at mid-stroke

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Idealized cases high viscosity near
dead-centers

• There is a small reduction in overall friction
  for some high-DC cases
• When high-viscosity region extends into
  hydrodynamic regime, friction increases -gt
  transition point is important

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Friction reduction is small

• Boundary friction reduction balanced by increase
  in hydrodynamic friction
• Contribution to total friction from end-strokes
  is small
• Viscosity near mid-stroke has main influence on
  frictional losses

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Idealized cases effect of mid-stroke
viscosity

• Viscosity in mid-stroke region has a large effect
  on friction
• Effect of controlling viscosity variation is much
  smaller

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Realistic cases Temperature dependence

• More realistic cases were considered, using
  realistic temperature and shear-rate dependence
• For temperature dependence, Vogel relationship
  used

v lubricant viscosity z thickness term T1
temperature dependence term T2 lower-bound
temperature dependence term T lubricant
temperature
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Realistic cases Temperature dependence

• All temperature dependence cases have viscosity
  distributions that are symmetric about mid-stroke

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Realistic cases Temperature dependence

• Changing temperature dependence has little effect

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Realistic cases Shear rate dependence

• For shear rate dependence, the Cross relationship
  was used

,
v lubricant viscosity v0 low-shear viscosity
(from Vogel relation) v? high-shear viscosity
(less than low-shear) g shear rate in oil b
critical shear rate (temperature dependent) m
parameter controlling width of transition region
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Realistic cases Shear rate dependence

• Controlling high/low viscosity transition can
  create high viscosity near dead-center, like
  idealized high-DC case

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Realistic cases Shear rate dependence

• case 2 distribution shows slightly more friction
  reduction than baseline
• High-DC strategy more effective at low average
  viscosities
• Total ring-pack friction reduction 7 with case
  2 parameters

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Wear

• A wear parameter was used

Cw Wear parameter Pbdy Ring/liner pressure
due to asperity contact v piston
speed - integrated over an engine cycle
baseline case
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Wear

• Keeping viscosity high near dead-centers reduces
  the wear parameter in that region

- fmep is the same for the three cases shown
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Wear

• At a given friction loss, controlling viscosity
  variation can reduce wear
• At the baseline fmep, wear parameter is reduced
  by 25, over an engine cycle, when viscosity is
  held high near dead-centers

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Boundary friction coefficient

• Boundary friction coefficient affects friction in
  two ways
• Boundary friction reduced directly
• Hydrodynamic friction reduced indirectly,
  reduction in fb allows viscosity to be reduced
  without penalties

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Conclusions

• Total ring-pack friction can be reduced by 7
• Mid-stroke viscosity reduction can lead to an OCR
  friction reduction of 10
• Maintaining high viscosity near dead-centers
  leads to friction reduction of 11 7 total
  ring-pack reduction
• If boundary friction coefficient is reduced,
  greater friction reduction is possible
• Wear must also be considered
• controlling viscosity variation can reduce wear
  parameter by 25

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Thank you