OIL ANALYSIS OF AN ENGINE OIL – PHYSICOCHEMICAL VALUES

Kinematic viscosity is the measure of a fluid's resistance to flow under gravitational forces. It is determined by measuring the time (seconds) it takes for a given volume of fluid to travel under gravity, and through a calibrated viscometer, a known distance. The most common units of measurement are centistokes (cSt) and it should be noted that the viscosity value given for a lubricant is only valid if the test temperature is reported.

Viscosity affects lubricant performance and stability, and an unwanted change in viscosity can lead to metal-to-metal contact therefore, to wear problems. The most common viscosity changes in a motor lubricant can be caused by:

• INCREASED VISCOSITY: Mixing with a higher viscosity lubricant, severe chemical degradation by oxidation, presence of water, presence of glycol
• DECREASED VISCOSITY: Fuel dilution, mixing with a lower viscosity lubricant, oil shear, solvent or coolant contamination

Nitration is the degradation of the oil due to the presence of nitrogenous compounds. It is a common degradation mode in lubricants for 4-stroke internal combustion engines that use gas (natural or LP) as fuel. Nitrogen oxides are formed during the gas combustion process. Nitric oxide reacts with water to form nitric acid and this can cause corrosion on engine surfaces, as well as contributing to increased viscosity and the formation of sludge.

Some factors that increase nitration are:

• Low operating temperatures
• Wrong stoichiometric ratio
• Fuel filtration

Oxidation is the main reason lubricant degrades over time under normal operating conditions. The oxidation process can occur almost spontaneously, even in oils stored in their container. However, the rate at which additives are depleted and the base oil oxidized generally correlates with the intensity of pro-oxidants in the oil.

The consequences of this chemical process include an increase in oil viscosity and organic acids; the formation of sludge, varnish and deposits; additive depletion (including antiwear, dispersants, corrosion inhibitors, etc.); and the loss of other properties vital to the oil's performance. Once these undesirable conditions occur in the oil, the engine is no longer effectively protected against friction, wear and corrosion.

Some factors that contribute to accelerate oxidation are:

• High working temperatures of the oil
• Presence of water in the oil or moisture
• Presence of metallic particles
• Presence of oxygen

For more information on oxidation, we recommend this article:

The Total Basic Number measures the alkaline reserve of a motor oil, whose function is to neutralize the acids generated during combustion. When this alkaline reserve is depleted, the oil can become highly corrosive and that is why it is a value that must be considered when carrying out an oil change. This analysis is performed by potentiometric titration and its usual units of measurement are lo mgKOH/g.

The tendency of the Basic Number of a motor oil is to decrease over time, even so, there are factors that can accelerate this change:

• Use of fuels with high sulfur content
• Bad combustion
• Severe oil degradation
• Soot contamination
• Lengthen change intervals
• Dilution with glycol
• Mixing with wrong oil

The Total Acid Number is the measure of the concentration of acids in the oil. This analysis is performed by potentiometric titration and its usual units of measurement are lo mgKOH/g.

As the oil oxidizes and ages, small amounts of acidic by-products are formed which cause an increase in TAN. The rate at which the AN increase, with respect to the original value of the new oil, is indicative of the extent to which the oil has degraded. A high TAN commonly indicates that the oil's useful life has been exhausted.

Some factors that can accelerate the increase in the AN are:

• Severe oil oxidation
• Depletion of some additives
• Fuel with a high sulfur content.
• Oil change intervals not respected.