Lubricants containing 90% base oil are crucial for reducing engine wear. However, the performance of the lubricant is affected by the degeneration of the base oil by oxidation (or) the presence of contaminants, which affects the efficiency of engines. This article intends to raise awareness of the potential consequences of using degraded lubricating oil.
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Role of Lubricant Oil in Engine Functioning
Lubricant oil is a mixture of additives, essential oils, and products derived from petroleum refining. In an engine, the lubricant oil has the following five functions:
- It regulates the friction between the bearing surfaces.
- Prevents metal-to-metal contact and reduces engine wear.
- Helps in the dissipation of heat generated by fluid friction and limits the temperature.
- Flushes out debris between moving parts and prevents corrosion.
- Prevents mechanical shock in gears.
Properties of Degraded Lubricant Oil
Degraded lubricant oil refers to oil that undergoes physical and chemical changes over time, decreasing the quality and efficiency of the lubricant oil. The following are the causes of degradation and the properties of the degraded oil:
Oxidation
Oxidation may occur because of oxygen or heat. The heat generated from metal-metal friction due to improper lubrication or other conditions results in the separation of atoms in oxygen molecules. Oxidation occurs when individual oxygen atoms come into contact with lubricant oil, forming certain acids contributing to oil degradation.
Oxidation causes sludge, varnish, base oil breakdown, increased viscosity, additive depletion, and loss of the oil’s anti-foaming properties. Increased viscosity is an indicator of degraded lubricant.
Thermal Breakdown
According to the Arrhenius rule for temperature, increasing the oil temperature by 10 °C reduces the oil life by half. Excessive heating and the consequent cracking cause decreased viscosity, molecular shearing, and polymerization.
In most cases, decreased viscosity is the primary indicator of thermal degradation rather than increased viscosity during oxidation. The shearing of molecules volatilizes them and leaves no deposits, resulting in dehydrogenation and the production of coke deposits and lacquers.
Micro-Dieseling
It is a pressure-induced thermal breakdown caused by a sudden increase in temperature and adiabatic compression of air bubbles in the oil that circulates in pressurized systems, pumps, and others, leading to oil failure.
When the engine’s compressive pressure reaches 250 psi, owing to its operation, the temperature increases to greater than 200 °C, initiating thermal failure. Thermal degradation occurs when sufficient oxygen is unavailable. Consequently, different end products are formed during oxidation and thermal degradation.
Micro-dieseling generates tar, soot, and sludge owing to its high pressure. Additionally, the varnish is made from carbon-insoluble materials, such as coke, tars, and resins, with low implosion pressures.
Electrostatic Spark Discharge (ESD)
This induces temperatures above 10,000 °C, producing different deposits. In the advanced stages, the formation of sludge, varnish, insoluble materials, and elevated fluid degradation are observed. Many people hear cracking noises and observe the burnt membranes on oil filters during ESD.
Contamination
When lubricant oil comes into contact with dirt or metal particles, the lubricant becomes contaminated. Contaminated particles expedite lubricant degradation by acting as catalysts.
Additive Depletion
This occurs when additives swiftly drop out of the oil and react with the other components. For example, organic deposits (rust and oxidation additives) drop out and react to generate the major antioxidants that show up in oil analysis.
Adverse Effects of Degraded Lubricant Oil on Engine Performance
When the lubricant oil degrades, the ability of the oils to perform the above functions diminishes, resulting in the following negative consequences:
- The degradation of lubricant oil reduces its ability to form a protective film between metal parts, resulting in increased metal-metal friction, increasing the wear and tear of the engine components.
- Degraded oil has low lubrication and high viscosity, reducing friction less efficiently. This affects fuel economy and engine efficiency.
- Degraded lubricant oil has reduced heat dissipation capabilities, thus increasing the engine's temperature.
- The degraded oil restricts adequate lubrication of engine parts owing to the clogging of oil passages by combustion by-products.
- Degraded lubricant oil does not sufficiently protect the engine from corrosion or oxidation.
- The reduced efficiency, increased friction, and overheating of the engine reduce its performance.
Recent Studies
An article recently published in Tribology International evaluated how cleaning out different amounts of carbon black particles from degraded lubricant oil affects performance. The results revealed that the performance of the oil was reduced by three factors: consumption of additives in the oil, adherence of additives to soot particles, and breakdown of oil. Thus, despite cleaning the carbon black particles, the performance of the oil was not restored.
Another article published in AIAA Scitech 2023 Forum examined the reactivity of turbine oil, Mobil DTE 732, at different temperatures. They observed the breakdown of oil and the formation of solid deposits that were stuck on the metal surface at higher temperatures. The results revealed an exponential decrease in the oil life with increasing temperature.
Conclusion
Overall, degraded lubricant oil has a significant negative effect on the performance of the engine, mainly because of the formation of soot-like material owing to the breakdown of the lubricant oil that gets stuck to the engine parts.
Researchers have also observed that additives present in oil play a crucial role in preventing oil breakdown. Thus, it is imperative to occasionally change the lubricant oil in an engine to enhance its performance.
References and Further Reading
Degradation of Oil. [Online] Available at: https://www.mosil.com/blog/degradation-of-oil/
Breaking down lubricant degradation by failure mode for reliable oil analysis. [Online] Available at: https://www.accelix.com/breaking-down-lubricant-degradation-by-failure-mode-for-reliable-oil-analysis/
Rammohan, A. (2016). Engine’s lubrication oil degradation reasons and detection methods: A review. Journal of Chemical and Pharmaceutical Sciences, 9(4), pp.3363-3366. https://jchps.com/issues/Volume%209_Issue%204/jchps%209(4)%20320%200630816%203363-3366.pdf
Omar, A. A. S., et al. (2021). Chemical and physical assessment of engine oils degradation and additive depletion by soot. Tribology International, 160, p.107054. doi.org/10.1016/j.triboint.2021.107054.
Juarez, R., et al. (2023). High-Temperature Degradation and Coking of Aircraft Gas Turbine Engine Lubricants. AIAA SCITECH 2023 Forum. https://arc.aiaa.org/doi/abs/10.2514/6.2023-1252
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