I’m sure we have all heard it at some stage, when someone has changed their engine oil and they tell you that the used oil came out “as clean as a whistle”, “like it was new oil”, or words to that extent. Truth be known, if you have completed a service period and the oil comes out with no discolouration, clean and translucent like it went into the engine, then you probably need to change the type or brand of oil you are using.
Modern engine oils are highly detergent to keep engine internals clean and free from build ups of combustion by-products such as -
Detergents used in engine oil are oil soluble bases that are derived from the organic soaps or salts of calcium, magnesium, sodium or barium. They are polar in nature, which allows them to cling to the surfaces of particles. They serve two principal functions -
The number of particles that can be contained by the detergents is limited. When the number of particles exceed the capacity of the type of detergent chemistry being used, deposits can form. Therefore, it is necessary that the engine oil be drained before this happens if engine cleanliness is to be maintained.
Another additive used in engine oil is “Dispersant”. Dispersants help to prevent sludge, varnish and other deposits from forming on critical surfaces. They
Dispersants are used in petrol and both light and heavy duty diesel engine oils. Dispersant's are polar additives that are used to disperse sludge and soot particles for the purpose of preventing agglomeration, settling and deposits. Dispersant's form a micelle enveloping particles and keep them finely divided.
These “Micelles” can trap deposit precursors up to 0.05 microns in size by providing a thick absorbed barrier film or they can also hold larger particles up to 0.1 micron in size by electrical charge repulsion. In this state, the acids and deposits cannot see the engine's metallic surfaces.
A good way to highlight how dispersants work is to watch dirty clothes washing in a top loading washing machine. The detergents in the washing powder/liquid release the dirt and then it is suspended in the water by the dispersants, so when the machine drains the washing water, the dirt and grime are washed away in the rinse. If this process did not happen, the clothes would come out of the machine dirty, not clean.
A modern engine oil is a combination of base oil and additive pack. The additive pack can be anywhere from 8% to 25% of the volume of the product. Approximately 80% of the additive pack is detergent and dispersant. This shows that modern specification oils have a high performance emphasis of cleanliness inside the engine. API SN grade oils have 40% better sludge prevention and 30% better piston cleanliness than the previous grade (SM) engine oils. With this emphasis on cleanliness, an engine oil meeting the latest international specifications should not be coming out of an engine clean after a service period.
No engine is 100% efficient at combustion. All engines, no matter how modern, produce by-products of combustion. Some of these by-products get circulated back through the engine via exhaust gas recirculation (EGR) and come into contact with the engine oil in the crankcase of the motor. Therefore, the oil needs to prevent these contaminants from attaching themselves to the engine internals preventing corrosion and sludge build up inside the engine. Sludge occurs when the engines oil can no longer carry or effectively remove particle matter inside the engine, or when the oils detergents and dispersants are saturated.
So apart from lubrication, heat dissipation and acid neutralising, engine oil is also expected to clean, prevent and suspend contaminants and particle matter until it is flushed at an oil change. So of course, oil should come out of the engine a much darker colour than when it went in. In diesel engines, discolouration happens very quickly and is generally darker than that of a petrol engined vehicle due to soot produced by combustion. An engine running on LPG will burn fuel more completely but they still produce unwanted acids that are corrosive to internal engine parts. The used oil though, will appear a lighter colour than petrol or diesel following a service interval.
So next time someone tells you that their oil came out of the engine following a service period and it was clean and new, you will know that the oil they used, was not doing the job it was meant to do. Consequently, if it comes out dark and dirty, you will at least know it has done part of the job that is was supposed to do inside the engine.
