Understanding Oil 102
Welcome to the 102 section of understanding oil! Now that you have a basic understanding of oil's relationship to your motor as well as the basics of viscosity it's time to move onto the detailed properties of lubrication. We will first examine the various layers of lubrication.
The common belief is that your engine's internal surfaces only exist in two states. Those two states are lubricated and un-lubricated. The truth is that metal surfaces experience a variety of lubrication regions which occur because of lubrication viscosity, engine load and engine RPM. These regions have different effects on engine wear and performance. While there is no ability to tell which state your engine is at any particular time, understanding how lubrication works will better enable you to choose engine or transmission oils that fit your specific application.
The following are the regions of lubrication:
Hydrodynamic lubrication – two surfaces are separated by a fluid film
Elastohydrodynamic lubrication – two surfaces are separated by a very thin fluid film
Mixed lubrication – two surfaces are partly separated, partly in contact.
Boundary lubrication – two surfaces mostly are in contact with each other even though a fluid is present.
Osborne Reynolds (1842-1912) was the first person to research Hydrodynamic lubrication. He found that when a lubricant was applied to a shaft and bearing that the rotating shaft pulled a converging wedge of lubricant between the shaft and the bearing. He further discovered that as the shaft gained velocity that the lubricant flowed between the two surfaces at a greater rate. This "wedge" later became known as a hydrodynamic wedge which will be discussed in the next section.
The principle that allows hydrodynamic lubrication is the same that causes aquaplaning on a wet road surface. Aquaplaning occurs when water on the road is able to accumulate in front of your vehicle's tires faster than the weight of your vehicle and action of your tires can get it out of the way. The pressure created under the tire can cause the tire to separate from the road's surface causing a loss of grip. This process varies based on the condition of your tires and your vehicle speed.
Viscosity is important regarding hydrodynamic lubrication. The higher the viscosity the thicker the hydrodynamic film but also the greater the friction between the shaft and the bearing. Friction generates heat which will reduce the viscosity of the lubricant. The distance between the surfaces also effects this region. As loads increase the distance between two surfaces will reduce.
Hydrodynamic lubrication is the desired region when it comes to protecting surfaces. Having both surfaces separated by a fluid film ensures that surfaces do not touch and wear does not occur. Standard fluid film sizes are approx 10 to 40 microns.
This region of lubrication exists between hydrodynamic lubrication and mixed lubrication. In this situation you would have a very thin layer of fluid separating the surfaces.
Elastohydrodynamic lubrication will occur in situations where higher load causes the surface distance to shrink or situations where metals are non-rigid bodies that bend or flex under high pressure. Ideally, this is the area where drag will be the least because you have a solid film separation but it's not large enough to produce any serious drag on the bearing. This is associated with the Stribeck curve which is discussed in the next session.
During mixed lubrication you have operational conditions where two surfaces have intermediate contact. Mixed lubrication can occur due to a variety of circumstances such as thin viscosity, high load or low RPMs. The majority of boundary lubrication happens at low speeds such as engine starting and shutdown and high loads. Many lubricants will contain EP (Extreme Pressure) additives to prevent seizures caused by direct metal to metal contact.
This is the most damaging area of the lubrication regions. Boundary lubrication is when two surfaces are in constant contact and there is a higher coefficient of friction. This results in increased wear, loss of energy and a possible seizure of the bearing and journal materials.
A internal combustion engine will undergo various transitions between these lubrication regions depending on the operational conditions of the engine as well as the properties of the lubricant itself.