Lubricant Additives

196 Lubricant Additives: Chemistry and Applications

Reduction of fuel consumption and emissions can be achieved through [3] engine design changes
and modifi cations, such as

Application of roller followers

Use of coatings

Surface modifi cations

Material selection

Fuel quality

The engine lubricant

All these aspects are looked at and applied in the automotive industry. This chapter concentrates on
the engine lubricant.
The need to measure fuel savings has led to the development of American Petroleum Institute
(API) test sequences such as VI and VIA in the United States. Sequence VIB will be used for
International Lubricant Standards Approval Committee GF-3. In Europe, a fuel economy test has
been developed by Conseil Européen de Co-ordination pour le Dévelopments des Essais de Per-
formance des Lubrifiants et der Combustible pour Moteurs (CEC) (test number CEC L-54-T-96)
for the Association des Constructeurs Européens d’automobiles A1 and B1 specifi cations using the
DBM 111 engine. Both tests require that the candidate lubricant shows decreased fuel consumption
relative to reference oil.

7.2 FRICTION AND LUBRICATION REGIMES

Friction is defi ned as the resistance a body meets while moving over another body in respect of
transmitting motion. The friction coeffi cient is defi ned as

F

F

W

n

(7.1)

where FW is the frictional force and Fn the normal force or load.
For a lubricated surface, the coeffi cient of friction is determined by the lubrication regime. In
simple terms, the following three lubricant regimes can be distinguished:

1. Elasto-hydrodynamic lubrication (EHL) regime characterized by a (relatively) thick lubri-
   cant fi lm [4]. The mating surfaces are far enough from one another to prevent metal-to-metal
   contact. The load on the system is completely carried by the lubricant fi lm, and the viscosity
   of the lubricant determines the friction coeffi cient. Viscosity depends on temperature and
   pressure/viscosity coeffi cient.


2. Boundary lubrication (BL) regime characterized by a thin lubricant fi lm [5]. Under high
   loads, high temperature, or with low viscosity oils, most of the lubricating fi lm is squeezed
   out between the metal surfaces, and metal-to-metal contact occurs. The load is entirely car-
   ried by the metal asperities. A thin layer of absorbed or otherwise deposited molecules is
   necessary to prevent the two surfaces and their asperities from plowing into one another.


3. Mixed lubrication (ML) regime characterized by a lubricant fi lm of intermediate thickness
   [6]. The two metal surfaces have come closer compared to hydrodynamic lubrication, and
   metal-to-metal contact occasionally occurs. The load is carried by both the lubricant and
   the asperities.

These regimes are related to the friction coeffi cient f by a lubricant parameter defi ned as

su

F

su

p

or

(7.2)

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