The article is written by Riya Veluri, an editorial team member of Industrial Lubricants. After her graduation, Riya works as a website developer & SEO specialist in Lubrication & Tribology Industry & writes technical articles on Lubricants, Lubrication, Reliability & sustainability.
Lubricity Theory and Definition
Table of Contents
When talking about Lubricity, understanding the working principle of a lubricant becomes a must.
Dry Contact or Dry Lubrication
No matter how smooth a metal surface might appear microscopic, the surface is a collection of irregular ridges and valleys. When two metal surfaces of a journal bearing try to slide against each other, the interaction between the two surfaces causes resistance to motion or friction. If sufficient energy is supplied, the two surfaces will move relative to one another.
Inevitably, particles from the highest ridges on the surfaces will break off. The initial breaking and the damage done by the resulting loose particles moving between the surfaces translate into wear. The energy put into this motion also causes the surfaces to heat up. The frictional heat will accelerate the wear and can eventually lead to the failure of the parts.
What is Lubrication & How to achieve it
Lubrication is the control of friction and wear by introducing a “friction-reducing” film between moving surfaces in contact. This film, the lubricant, can be a solid or fluid, with oil and grease being the most common ones.
So, what is a Lubricant?
A lubricant is a substance, usually organic, introduced to reduce friction between surfaces in contact, which consequently reduces the heat generated at the interface. The property of reducing friction is known as Lubricity.
Normally, lubricants are made of 90% of base oil (mostly petroleum fractions, called mineral oils or synthetic oils) and less than 10% additives & some solid lubricants (as per requirement 5 to 10%). Vegetable oils or synthetic oils such as hydrogenated polyolefin, esters, silicones, fluorocarbons are the most used base oils.
Disregarding the dry lubrication regime, three characteristic lubrication regimes can be distinguished: Boundary Lubrication, Mixed Film Lubrication, and Full Film Lubrication.
We generally want to avoid boundary lubrication where possible. Lubrication specialists observe that friction may be at its highest level during the boundary lubrication regime. Boundary lubrication is inevitable when starting and stopping, shock-loading or when continuous high loading conditions are present. By increasing the lubricant viscosity the timespan under boundary lubrication can be reduced.
Mixed Film Lubrication
Mixed lubrication is a combination between boundary and full film or hydrodynamic lubrication. Mixed lubrication occurs when a lubricating layer carries part of the contacting load but some asperities still contact each other.
Full Film Lubrication
Full film lubrication exists in two forms: hydrodynamic and elastohydrodynamic. Hydrodynamic lubrication (HL) occurs when a fluid film fully separates two surfaces in a sliding motion. Elastohydrodynamic lubrication (EHL) is very similar to HL but occurs when the surfaces undergo a significant deformation.
What is Lubricity
We discussed the basics of lubricants and lubrication; Now, let’s come to the main point, “lubricity”. Lubrication is a measure of friction or wear reduction by a lubricant. Lubricity is not an intrinsic fluid property and depends on its composition, the mechanical forces present at the point of contact and the material characteristics of the surfaces in relative motion. Consequently, superlubricity is a law of motion where friction is (almost) null. As an ad hoc definition, a kinemetic coefficient of friction below 0.01 is considered superlubricity.
Lubricity of diesel fuels
Today’s diesel fuels must meet strict regulations for sulfur content, this must be lower than fifteen parts per million in most markets. As sulphur was removed over the years to achieve today’s ultra-low sulfur diesel fuel, components that provided natural lubrication were also removed. While cutting the sulfur content provides environmental benefits, diesel engine parts are prone to premature wear, particularly the fuel pump and fuel injectors.
Lubricity improver additives restore lost wear protection to ultra-low sulfur diesel fuels. Due to their chemical structure, the molecules in these additives form a lubricating film between the moving parts in the fuel pump and injectors. As a result, the friction between surfaces is reduced, and the life of the fuel pump and other components extended. The use of these additives has become an economical solution leading to ultra-low sulfur fuels. Lubricity improvers are often combined with deposit control and other additives that help to enhance the engine performance.
Methods to Measure Fuel Lubricity
The lubricity or oiliness of a substance is not a material property, and its cannot be directly measured. Tests are performed to measure the lubricity in each system. In short, governing variables such as surface size, temperature and pressure must be specified. For two liquids with the same viscosity, the one leading to a lower wear are considered to have the higher oiliness or lubricity. For this reason, oiliness is also referred as the anti-wear property of the material.
Different Tribometers or lubricity test rigs are shown below:
Vehicle tests: In this lubrication test, the car is operated on fuel for a certain time or a certain distance. The components of the fuel system are then isolated and tested for wear. Through such tests, we can get the most representative results of real-world conditions. We can measure all wear-related failures, not just associated with boundary lubrication. Such lubrication tests are expensive and time-consuming.
Pump rig tests: This is an alternative to vehicle testing. In this lubrication test method, a fuel injection pump is mounted on a test stand, and an electric motor drives the pump in a pump rig test. Fuel is distributed through the pump for a certain period. After the test is over, the pump and any other connected equipment can be disconnected and tested for wear and other harmful effects. The main advantage of this test is that it is less expensive than a complete vehicle test, but it is still time-consuming to conduct. A testing time can go up to 500-1000 hours.
Bench tests: We are now discussing various bench tests that attempt to recreate boundary lubrication conditions, such as fuel injection pumps that allow for quick and relatively inexpensive measurements of fuel Lubricity.
The Ball-on-Cylinder Lubricity Evaluator: BOCLE was made for aviation jet fuel. It continues to be used for this application. It is especially useful for measuring the effect of fuels and additives on oxidative wear.
The Scuffing Load Ball-on-Cylinder Lubricity Evaluator: SLBOCLE was created to detect the failure of the diesel fuel system as a result of the introduction of low sulphur diesel fuel. It is similar to the BOCLE test, but it is less sensitive to oxidative wear and more sensitive to adhesive scuffing with changes.
The High-Frequency Reciprocating Rig: The HFRRO was designed to be suitable for assessing the Lubricity of diesel fuel. This can create a broader process of wear depending on the fuel being tested.
The Ball on Three Disks: The BOTD method is fairly recent and still evolving. It is a compact and more economical version of the ball in three-seater equipment.