The major purpose of lubrication oil is to provide low bearing friction, to transfer heat and to protect components from corrosion. Contaminants will affect the oil’s ability to meet these requirements. Typical types of oil contamination include air, water, fuel, soot deposits and debris such as engine component wear particles. Each of these can lead to the degradation of the engine lubricant.
There are two basic categories of air contamination. The first of these is dissolved air, which is a natural process where molecules of air are distributed throughout the oil. Dissolved air may affect the ability of the air to transfer heat when exposed to excessive oil temperature. However, in normal operating conditions it is not harmful to the lubrication system as it does not significantly affect oil properties such as viscosity. It’s a different situation with entrained air. Entrained air is unstable, suspended microscopic air bubbles that cloud the oil. It can significantly affect the lubricant and lead to severe engine degradation and ultimately to engine failure. In extreme conditions, if the entrained air reaches 30 percent, foaming can occur which exacerbates the potential for damage of the lubricating system.
In an automobile engine, excessive air can be introduced into the oil sump from churning of the crankshaft. A more typical scenario is when air is sucked into the oil pump when the entry to the pick-up pipe becomes partially exposed during vehicle acceleration, braking or cornering or when the vehicle is on a significant incline. Having excessive entrained air in the lubrication system will allow the oil to become compressible. This situation becomes critical when the hydrodynamic boundary layer thickness, that normally maintains separation between the crankshaft and its bearing surface, becomes so thin that a touch condition is possible. This will lead to scuffing, wear, and an increase in heat transfer and ultimately engine failure.
On some engines, the clearance between the cam shaft cams and the intake and exhaust valves are controlled hydraulically. If aeration becomes excessive, the oil will become “spongy” and lead to delayed opening and reduced stroke of the valves leading to a significant drop in power. Entrained air will also affect cavitation and oxidation. Cavitation damage typically shows up in the form of erosion pockets on the surfaces of the oil pump gears. Eventually the output of the oil pump will reduce, cavitation noise will be audible, and in extreme situations the pump will break.
Enjoyed this article? We have another eBook coming that will continue the conversation. Topics to look forward to are ones such as ‘Tools for Measuring Air Concentration‘, ‘Diesel Fuel Dilution of Engine Oil‘, ‘Water Contamination‘ and more. Stay tuned for more information and make sure to sign up for our newsletter.
Learn more about aeration measurement with ATA Aerify™.
4. Bureček, A, Hružík, L, Vašina, M. “Determination of Undissolved Air Content in Oil by Means of a
Compression Method, - Journal of Mechanical Engineering”. 61(2015)7-8, 477-485. 2015
5. Bregent, R., et al, “The SMAC, Under Pressure oil Aeration Measurement System in Running
Engines”, SAE 2000-01-1818, 2000.
11. Jim Fitch. “The Enduring Flash Point Test,” Machinery Lubrication.