Engine oil does much more than lubricate moving engine components. It also prevents wear, regulates temperature, and protects critical surfaces under harsh operating conditions, while minimizing friction. A key component of this performance is friction modifiers, special chemical compounds that reduce friction between moving metal surfaces. However, what do they do and why are they relevant to today’s engine oils?

Friction-modifier additives help form a thin, protective, low-friction film between moving engine parts, especially where hydrodynamic lubrication is inadequate. This helps enhance engine efficiency, minimize energy losses, and facilitate smooth operation. Understanding these compounds is critical for lubricant manufacturers and formulators to create higher-performance engine oils.

What Are Friction Modifiers in Engine Oil?

Friction modifiers are chemical compounds added to engine oil formulations to reduce friction between surfaces in contact. Friction modifiers aim to minimize resistance to movement between engine components, whereas anti-wear additives help prevent surface damage.

During normal operation, the engine oil forms a lubricating layer between the various components, including pistons, cylinder walls, camshafts, and bearings. But the film may be reduced in thickness during low-speed, high-load, and cold start.

Thus, metal surfaces can experience boundary or mixed lubrication, leading to higher friction and energy losses.

This is where a ‘friction modifier’ additive is especially useful. These interact with metal surfaces, forming a low-shear protective coating. Thus, engine components can move more easily, even with a relatively thin film of oil.

How Do Friction Modifiers Actually Work?

A key role of friction modifiers is to minimize friction as one surface slides over another. But there are different ways that different chemical compounds do this.

Some compounds physically adsorb onto metal surfaces, and others form protective tribochemical films during operation.

1. Formation of a Low-Friction Surface Film

For many organic friction modifiers, the molecules carry polar chemical groups that adhere to metal surfaces. These molecules adhere to these surfaces, forming a very thin boundary layer.

This makes direct metal-to-metal contact less likely, and the surfaces can slide past one another with less resistance.

2. Reduction of Surface Interaction

Even though metal surfaces look smooth on the macro level, they have microscopic peaks and valleys. As motion occurs, these irregularities can interact and create friction.

But friction-modifier additives reduce the shear strength between interacting surfaces. This means the components’ movement requires less energy.

3. Protection During Boundary Lubrication

In certain severe operating conditions such as engine starting, the lubricating film may not completely separate the moving parts.

Hence, surface-active compounds play an important role. They offer extra protection during periods of inadequate conventional oil-film lubrication.

Four Common Types of Friction Modifier Additives

Different engine oil formulations use different friction-reducing chemistries depending on performance requirements, compatibility, and application. There are four main categories: organic, molybdenum-based, boron-based, and solid friction modifiers.

1. Organic Friction Modifiers

It is common to incorporate organic friction modifiers into lubricants. Typically, they contain fatty acids, fatty acid esters, fatty amines and glycerol derivatives.

They are molecules with polar groups that bind to the metal surface, and hydrocarbon chains that form a low-shear layer.

In addition, they can enhance boundary lubrication performance without requiring high treat rates. Therefore, organic compounds are useful in several formulations of car engine oils.

2. Molybdenum-Based Friction Modifiers

The molybdenum-based compounds, especially molybdenum dithiocarbamate (MoDTC), are key friction reduction additives in the formulation of today’s engine oils.

MoDTC can form tribofilms of molybdenum disulfide (MoS₂) rich on the rubbing surfaces under proper operating conditions.

These films have low shear properties and help minimize boundary-lubrication friction.

In addition, molybdenum-based lubricant additives can help improve fuel efficiency and lubrication performance. But they can be effective depending on the formulation chemistry, operating temperature, and interactions with other additives.

3. Boron-Based Friction Modifiers

Another class of boron-containing compounds is used in some lubricant formulations.

Boron-based chemistries can help reduce friction, enhance surface protection, and improve tribological performance.

For instance, borated dispersants (and other boron-containing chemicals) can have multifunctional applications depending on their chemical makeup.

But they may have a very different primary function. Hence, their compatibility with detergents, dispersants, anti-wear agents, and other engine oil additives needs to be assessed by formulators.

4. Solid Friction Modifiers

Compounds like graphite and molybdenum disulfide are solid lubricating materials.

These materials can be used to minimize friction by creating low-shear surfaces between moving parts. However, when used in passenger car engine oils, their use is more specialized, and care must be taken regarding particle stability, filtration, deposits, and compatibility.

Therefore, they cannot be used as substitutes for a conventional dissolved organic/organometallic friction modifier.

What Benefits Do Friction Modifiers Provide?

Adding the proper friction modifiers can enhance several characteristics of engine oil. But their actual benefits will depend on the engine’s design, operating conditions, and the full additive package.

Improved Fuel Economy

The engine’s moving parts use energy to overcome friction. Friction modifiers can help lower this resistance, resulting in less mechanical energy lost.

This means the engine can do the same work with less energy, helping reduce fuel consumption.

Reduced Engine Wear

Although friction modifiers are not replacements for dedicated anti-wear additives, they can help reduce damaging surface interactions.

In addition, their boundary films can protect components under difficult operating conditions, such as start-up conditions.

Smoother Engine Operation

Reduced friction can help to provide smoother operation between moving engine parts. Therefore, under proper conditions, the lubrication system can function more efficiently.

Better Lubrication at Low Speeds

At low engine speeds, some components may operate under mixed or boundary lubrication conditions.

For this reason, friction-modifier additives are extremely helpful, as they protect surfaces when the hydrodynamic oil film is insufficient.

Friction Modifiers vs. Anti-Wear Additives: What Is the Difference?

Both additive categories are beneficial for protecting engine parts, but they serve different primary roles.

The main function of friction modifiers is to decrease the friction between moving surfaces. Anti-wear additives, on the other hand, are used to minimize surface wear when under load.

One such additive is zinc dialkyldithiophosphate (ZDDP), a commonly used anti-wear additive. Produces protective tribofilms under severe circumstances.

On the other hand, organic and molybdenum-based friction modifiers act primarily by different surface interactions to reduce friction.

But these functions may overlap. Depending on the compound’s chemistry and formulation, some compounds can provide both friction-reduction and anti-wear benefits.

Why Are Friction Modifiers Important in Modern Engine Oil Formulations?

The new generation of engines aims to achieve better fuel consumption, lower emissions, and higher performance. Hence, lubricant manufacturers need to formulate products to meet several competing requirements.

This underscores the need for the appropriate choice of lubricant additives.

A good motor oil additive blend will maintain the oil’s viscosity, control deposits, reduce wear, prevent oxidation, and reduce friction.

In addition, friction modifiers must be compatible with other formulation components. For instance, some detergents, dispersants, and anti-wear compounds can affect their surface activity.

Therefore, the formulation of friction modifiers can’t be done independently. Rather, they will have to assess the whole additive system in laboratory testing and engine performance evaluation.

How to Select the Right Friction Modifier for Engine Oil

When choosing a friction modifier, the type of engine to be used and performance requirements must be taken into account.

Formulators should first consider the required viscosity grade and the lubrication conditions. They then need to consider whether the compound they have chosen is chemically compatible with the current additive package.

Also, friction and wear testing can be used to make sure that the specific formulation achieves the desired results.

Lastly, industry specifications and OEM requirements should be considered before commercialization of an engine oil formulation.

Conclusion

Friction modifiers are an important component in modern engine lubrication and serve several functions in the lubrication system, including reducing surface resistance, assisting boundary lubrication, and improving mechanical efficiency. Various friction modifier additives, including organic compounds, molybdenum-based and boron-containing compounds, provide certain performance benefits.

But if they’re formulated correctly and tested appropriately, they can be very effective! In summary, the use of appropriate friction modifiers and other engine oil additives assists engine oil manufacturers in formulating reliable, efficient, and high-performance engine oils.