The formation of varnishes can occur both in oils that should already have been replaced and those that in principle are still suitable for service.
Varnish is an oxidised or carbonaceous adhesive material that covers the internal surfaces of the engine. With time and/or temperature these compounds mature until they form a consistent, hard and shiny substance.
In engine oil, the appearance of these compounds is even more complex than in other systems, because here, in addition to the temperature, catalytic converters, etc. combustion products such as fuel, soot, water, sump gases, etc. also play a role.
The most frequent causes of the formation of these varnishes are:
1. Formation of varnishes due to air intake
When the air enters the engine, it can cause the oil to fail. Ageing and contamination with water cause a loss of surface tension over the course of time, and air bubbles become trapped. When these are large enough they tend to condense and adhere to the surfaces. Initially, these sooty compounds, formed by contact with air, are similar to sticky rubber, but with time they mature thermally and become hard films of enamel.
2. Formation of varnishes due to oxidation
When the oil oxidises, numerous decomposition compounds, including acids are formed. The primary products form polymers and other condensation compounds with a high molecular weight. The viscosity begins to increase and the oxidised compounds in suspension begin to be insoluble in the oil. It is at this point when deposits and lacquers start to form.
3. Formation of varnishes by coking of the lubricant film
In an internal combustion engine there are different areas where the temperatures of the metal surfaces can be transferred to the oil. There can also be hot spots due to the high friction generated between two surfaces in contact, inadequate lubrication and inappropriate loads/speeds. These causes a potential risk of ignition and coking. This results in the formation of sludge and sooty waste.
The deposits formed on the surfaces of the engines interfere during normal operation of the moving parts of the engine. In addition they also contribute to wear and corrosion, or simply to the seizing of the surfaces.
There may also be other types of problems such as the soiling of filters, damage to gaskets, failure of the main crankshaft bearings and wear in the combustion areas in diesel engines.
Oil analysis plays a significant role when monitoring the cleaning and drying of the oil. Regularly performing a number of duly selected tests is very important for monitoring the sludge and varnishes.