195

7

Organic Friction Modifi ers

Dick Kenbeck and Thomas F. Bunemann

CONTENTS

7.1 Introduction ........................................................................................................................... 195
7.2 Friction and Lubrication Regimes ........................................................................................ 19 6
7.2.1 Friction Reduction through the Lubricant ................................................................. 197
7.3 Friction Modifi ers versus Antiwear/Extreme-Pressure Additives ........................................ 199
7.4 Chemistry of Organic Friction Modifi ers .............................................................................200
7.4.1 Friction Modifi er Mechanisms ..................................................................................200
7.4.1.1 Formation of Reacted Layers ......................................................................200
7.4.1.2 Formation of Absorbed Layers ................................................................... 201
7.4.1.3 Formation of In Situ Polymers .................................................................... 202
7.5 Chemistry of Other Friction Modifi ers ................................................................................. 202
7.5.1 Metallo-Organic Compounds .................................................................................... 202
7.5.2 Mechanical Types ...................................................................................................... 20 3
7.6 Factors Infl uencing Friction-Reduction Properties ............................................................... 203
7.7 Friction Modifi ers: Current Practice ..................................................................................... 203
7.8 Friction Modifi er Performance .............................................................................................204
7.8.1 Stribeck Curve Determinations ................................................................................. 205
7.8.2 Friction as a Function of Temperature ...................................................................... 207
7.9 Consequences of New Engine Oil Specifi cations and Outlook ............................................ 208
7.10 Bench Tests to Investigate Friction-Reducing Compounds .................................................. 209
References ...................................................................................................................................... 209

7.1 I N T R O D U C T I O N

Friction modifi ers (FMs) or friction reducers have been applied for several years. Originally, the applica-
tion was for limited slip gear oils, automatic transmission fl uids, slideway lubricants, and multipurpose
tractor fl uids. Such products made use of friction modifi cation to meet requirements for smooth transi-
tion from static to dynamic condition as well as reduced noise, frictional heat, and startup torque.
Since fuel economy became an international issue, initially to reduce crude oil consumption,
FMs have been introduced into automotive crankcase lubricants, as well, to improve fuel effi ciency
through the lubricant. In the United States, additional pressure is imposed on original equipment
manufacturers (OEMs) by the corporate average fuel economy (CAFE) regulation.
Following the introduction of vehicle exhaust emission regulations in various regions around
the world, emphasis on friction reduction further increased. This can be well understood if it is
realized that 20–25% of the energy generated in an engine by burning fuel is lost through friction
[1]. The biggest part is lost by friction on the piston liner/piston ring interface and a smaller part by
bearing and valve train friction. It is predicted that in future engines the contribution of the piston
group to engine friction will increase up to 50% [2].