Motor Oil Breakdown Causes…
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Oil breakdown occurs when the major properties of the oil have changed due to oxidation, deposits, thermal degradation, corrosion, shearing and contamination.
This reduces the oil’s ability to perform its primary functions of reducing friction, dissipating heat, preventing corrosion and keeping the engine clean.
This article will discuss the some of the causes and effects of oil breakdown and how it affects the modern combustion engines in the vehicles we drive every day.
Oxidation and Deposits
Oxidation is the most important form of chemical breakdown of motor oil and its additives. The chemicals in motor oil are continuously reacting with oxygen inside an engine. The effects of oxidation, due to this reaction, as well as the by-products of combustion, produce very acidic compounds inside an engine.
The various acidic compounds cause corrosion of internal engine components, deposits, changes in oil viscosity, varnish, sludge and other insoluble oxidation products that can cause a performance and durability degradation of your engine over a period of time due to oil breakdown. The products of oxidation are less stable than the original base hydrocarbon molecular structure and, as they continue to be attacked by these acidic compounds, can produce varnish and sludge.
As an engine goes through multiple heating and cooling cycles, this sludge can harden and cause other problems such as restricted passageways and decreased component tolerances. Varnish can cause such things as piston ring and valve sticking. The deposits can also affect heat transfer from pistons to cylinder, and in extreme cases, can cause seizure of the piston in the cylinder. Pistons also have oil return slots machined into them that can become plugged and result in increased oil consumption and additional deposits created on top of the deposits that are already there, caused by oil breakdown.
Deposits also form on the tops of pistons which, over a period of time, can cause pre-ignition, increased fuel octane requirements, detonation/pinging and increased exhaust hydrocarbon emissions and an overall destructive effect on the engines internal parts. Deposits also form inside valve covers, timing gear covers, oil pump pickup screens, oil filters and oil passageways.
Part of the job of refining petroleum oil is to remove as many naturally occurring chemicals that can reduce the oxidation resistance of oil. Oxidation resistance can then be improved by the addition of additives engineered into the oil, such as anti-oxidants. Anti-oxidants include several different chemicals with the most common one being ZDDP (Zinc Diethyl Dithiophosphate). Anti-oxidants also become depleted with use and when that happens the oil starts to oxidize rapidly causing oil breakdown.
Refining an oil to reduce these naturally occurring chemicals that can lead to oxidation and oil breakdown, also, tends to inhibit the capability of oil to provide good boundary lubrication. Some of these chemicals that are refined out include aromatics, unsaturates and napthenes. Therefore, petroleum motor oil that has been highly refined using modern techniques will have good oxidation resistance but poor boundary lubrication.
Boundary lubrication can be improved and oil breakdown reduced by the use of engineered additives blended in by the oil manufacturer. This illustrates the case that refining petroleum oil is a compromise. The chemicals that come out of the ground cannot be controlled and have to be refined out. The extent of refining is usually selected to give the proper balance to meet the specifications for a particular grade of lubricant, whether it be for high performance, severe duty, average use or anywhere in between.
When motor oil is heated beyond a certain temperature, it will start to degrade, even if there is no oxygen present. This is called thermal degradation and causes the oil to change viscosity (a form of oil breakdown). The thermal stability of motor oil cannot be improved by use of additives but it can be improved by refining out the same compounds that decrease the oxidation resistance.
As temperatures increase, thermal degradation increases. In order for an oil to provide proper service and protection at high operating temperatures, highly refined oils with plenty of anti-oxidants should be used. For average service, less highly refined oils can be used. There is also a direct correlation between price of a particular oil and its performance under temperature extremes.
Less costly oils are generally refined less and have a lower capability to prevent/reduce thermal degradation. Petroleum oils have a much lower operating temperature range, while premium quality synthetic motor oils have a very high operating temperature range and are much more resistant to thermal degradation and oil breakdown.
Petroleum oil that is new or kept clean by proper filtration is generally non-corrosive and will provide good protection against corrosion caused by the atmosphere. However, inside an engine, oil oxidation by-products will attack internal engine steel and bearing materials that are typically manufactured from aluminum, copper, lead and tin (a lead-tin flashing is used for break-in purposes on the few engines that use aluminum rod and main bearings).
Most gasoline and diesel engines use copper-lead main and connecting rod bearings. Water present, due to condensation caused by temperature and humidity changes or short stop and go driving where the engine never reaches the proper operating temperature, although still hotter than the ambient temperature, can also cause corrosion. The hotter the oil is when water is present the more severe the chemical reaction is and corrosion related damage could definitely occur. In addition, water present in an oil for an extended period of time can emulsify the oil and form a mixture which is much more corrosive than the two components alone and can then form sludge which may block oil filters or small passages due to oil breakdown.
It is critical to operate an engine at normal operating temperature to prevent and burn off any water that is present by an evaporation process. The most severe type of driving an oil can be subjected to (as well as an engines internal components) is short drives and intermittent operation in which the engine and oil never have time to reach normal operating temperatures for an extended period of time. Another cause of oil breakdown.
Quality motor oils have a corrosion inhibitor added. Corrosion inhibitors also vary in terms of effectiveness, quality and quantity. Again, less costly oils may not protect against corrosion as well as more expensive motor oils that have a better corrosion additive package. Some common corrosion inhibitors include ZDDP (Zinc Diethyl Dithiophosphate), Calcium and Barium Sulphonates.
Many people use the term “oil breakdown” to represent what they think is a motor oil that has “broken down” and is in need of changing, when in reality the actual process of “ oil breakdown” is not properly understood. The correct word for this is oil breakdown due to shear forces. An internal combustion engine imparts high shear forces on motor oil, which is sandwiched between two rotating or sliding forces under load and heat.
The molecular structure is essentially torn apart by these mechanical shear forces. The component of the oil that is affected most by these shear forces is the viscosity improvers. These viscosity improvers allow the manufacturer of the oil to create multi-grade oils suitable for a wider temperature range of operation. The end result of these shear forces is a decrease in the viscosity of the oil, as well as a decrease in the viscosity index.
Once motor oil has sheared beyond a specific point it will not revert back to it’s base structure when it cools down and the shear forces have ceased. This applies to petroleum oils only, synthetic motor oils are extremely resistant to the detrimental effects of shear forces. Another way to explain this phenomenon is as follows: If you look at the molecular structure of motor oil under a microscope, you will see chains of molecules grouped together and linked together. The smaller molecular particles are attached to the larger ones. As oil shears, these smaller molecules break away and align in the chain.
As engine heat and shear forces continue and increase, these molecules break away from the base structure and, in the process, provide less and less resistance to wear. If this shearing and excessive continues over an extended period of time, engine damage can occur. If shearing is only mild, then when the oil cools down the structure will revert back to its original structure and still be capable of providing proper engine protection. Multi-viscosity petroleum motor oils are more susceptible to shearing than straight weight petroleum motor oils. As previously mentioned, synthetic oils are extremely shear resistant and less prone to oil breakdown.
Some Engines Require a Straight-Weight Oil
Manufacturers of small engines, such as used in lawn mowers, generators and even some larger diesel engines, require either a SAE 30 or SAE 40 oil. The reason is that, first and foremost, they are referring to petroleum motor oil. These types of engines operate under extreme RPM, load and heat and, in the case of small engines, are mainly air-cooled.
Multi-viscosity petroleum oils are full of viscosity improvers (VI’s). It is these viscosity improvers that are the weak link. As these agents are subjected to heat, load and high RPM, the oil cannot reach its intended high temperature viscosity, which results in shearing. As this shearing continues, the results can be loss of oil film strength, increased engine wear rates and temperatures and oil consumption issues. That is the reason straight weight petroleum oils are recommended for these types of applications to avoid oil breakdown.
When multi-viscosity synthetic oil, such as a 0W-30, 10W-40 or 15W-40 is used these concerns are non-existent. Synthetic oil is extremely shear resistant. The bottom line is that the manufacturers specifications are based on petroleum oils, yet your owner’s manual is not going to explain that, so you, in turn, go out looking for the “required” straight weight petroleum oil. You would be much better off using synthetic oil in these types of applications.
Motor oil contamination also causes deterioration of the oil. Some of the more common contaminant sources include dirt, sand and dust from the air, soot, unburned fuel in the oil, water from condensation of the combustion process, wear metal particulates that the oil filter cannot trap and hold, corrosion by-products and additive elements that have degraded. Also, dirt, sand and dust can continue to enter the engine and, in addition to creating more wear debris, combine with other contaminants and cause more damage than they would separately.
One of the many by-products of combustion and or blow-by is soot. Soot can be highly abrasive as well as cause filters to become filled and or plugged in extreme cases. Another contaminant is acidic by-products of combustion, which can produce a highly corrosive mixture and cause corrosion and pitting of internal engine components and additional generation of wear debris. These same acidic solutions can also mix with water inside the engine and form an emulsion that can cause problems with oil filters and passageways.
Another source of contamination is fuel. A charge of fuel is rarely 100% burned during the combustion process. This unburned fuel can mix with the oil present in the cylinders. Fuel contamination can also be caused by worn sealing components, such as excessive piston ring to cylinder clearances, allowing unburned fuel to blow-by the rings.
When motor oil is diluted with fuel, the effect is that the viscosity is lowered. If this reaches extremes of contamination, excessive wear and engine damage can take place do to oil breakdown. Operating an engine that is not sufficiently warmed up can also increase combustion blow-by. It is much better to let your engine sufficiently warm up before driving away, which can have a significant effect on preventing fuel blow by, as well as producing a much more efficient combustion cycle.
As we can see, motor
affects all of us who operate personal vehicles, commercial trucks, heavy equipment, ATVs, boats, airplanes, generators and a variety of other motor powered implements.
Basic Lubrication Principles
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