Lube-Tech
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.155 page 1
Friction modifiers: Study of an inverse relationship between engine efficiency and wear
Dr. Raj Shah and Nicholas Douglas, Koehler Instrument Company, Holtsville, NY Introduction
In the pursuit of achieving improved fuel economy and/or increased engine power, several advancements with engine oils to improve engine efficiency have surfaced. The most prominent example is work done to reduce internal friction (or traction) of an engine lubricant by reducing apparent viscosity. While there is no theoretical limit to how thin an engine lubricant’s viscosity can be to maximise efficiency, an engine has a theoretical limit for lubricant film thickness. Beyond that point, or too thin of a fluid film; premature wear and engine failure can occur. To address this problem, friction modifiers (FMs) have been employed to provide additional boundary lubrication preventing engine parts from contact, and reducing wear.
The importance of FMs has increased dramatically as engine lubricant viscosity and film thickness decreased in recent years to meet governmental regulations on OEM fleet fuel efficiency. FMs can permit use of lower viscosity engine lubricants to meet the needed engine efficiency requirements while managing or even enhancing engine durability.
This review will discuss the current state of engine oil friction modifiers, advancements into different classes of modifiers, their current applications, outcomes of use, and future projections for these modifiers.
Classes of FMs and how they are applied Organic friction modifiers (OFMs) Friction modifiers can be divided into several classes; these include organic friction modifiers, inorganic friction modifiers (iOFMs), and functionalised polymers [2]. Structurally, organic friction modifiers (OFMs) have an amphiphilic molecular structure, like a cell membrane Polar heads of this structure allow preferential migration toward polar surfaces (i.e., metal). Figure 1 introduces typical OFM structure for a friction modifier. Depending on how densely packed the modifier is, variations in friction coefficients will exist [3]. Amphiphilic structure in additives is key to OFM structure since nonpolar forces allow the structure to retain shape, allowing the OFM to slide across surfaces. Primarily with metal surfaces, anti-friction properties work upon adsorption to the surface; in this case, engine surfaces [4]. OFMs
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