Lubricant Additives

Organic Friction Modifi ers 197

where
s = system speed
u = lubricant dynamic viscosity
F = load (Fn)
p = contact pressure

The so-called Stribeck curve gives the relationship between f and these lubricant parameters. The
shape of the Stribeck curve and the transitions from BL to ML and ML to EHL depend on a number
of parameters such as material roughness (microgeometry), contact pressure, and lubricant viscos-
ity. High contact pressure such as that present at point contacts leads to a different Stribeck curve as
at line contact (lower contact pressure) (see Figures 7.1 and 7.2).

7.2.1 FRICTION REDUCTION THROUGH THE LUBRICANT

Engine friction originates from several components, that operate at different conditions of load, speed,
and temperature. Hence, these components may experience various combinations of EHL, ML, and BL
during engine operation. For each of these regimes, a number of factors govern engine friction.

FIGURE 7.1 Stribeck curve at high contact pressure.

Boundary lubrication

BL

ML

Mixed lubrication

Elasto-hydrodynamic lubrication

EHL

Lubricant parameter

(viscosity × speed/load)

Friction coefficient

f

FIGURE 7.2 Stribeck curve at low contact pressure.

Boundary lubrication

BL

ML Mixed lubrication

Elasto-hydrodynamic lubrication

EHL

Lubricant parameter

(viscosity × speed/load)

Friction coefficient

f