Detergents and dispersants in an engine oil work as follows.
Detergent and dispersant additives are used to keep the engine's metal surfaces clean, prevent the formation of deposits and to neutralize the harmful effects of corrosive acids that are formed by the combustion of diesel and gasoline fuels.
All engine oils, as they deteriorate either due to oxidation or by contamination, will form insoluble sludge and varnishes and resins that can become deposited on the surfaces of the engine. Once deposited, these sludge, varnish and resins can block oil lines and oil passages causing the flow of the engine oil from reaching the parts be lubricated. This in turn can result in increased wear, heat buildup and eventual engine failure.
Further engine oils can be exposed to fuel soot due to incomplete combustion of the fuel or carbon being introduced into the engine by various emission controls, acids formed by the combustion of the fuel, and the ingestion of moisture and dirt from the engine's air intake system. If these contaminants are allowed to buildup in the engine oil they can result in:
Increased engine wear, especially in the valve train area
Increased deposits, especially in the piston rings and crownland of the piston.
Increased cylinder bore polishing
Thickening of the engine oil's viscosity
The formation of oxidation precursors in the engine oil
Poor oil pumpability especially during cold weather conditions
Plugging of oil filters
Rapid depletion of the engine oil's additive system
Decreased engine durability and life.
In order to prevent the formation of these deposits, sludge and resins, it is important that the engine oil contain an effective detergent/dispersant additive system.
Detergents are oil soluble bases that are derived from the organic soaps or salts of calcium, magnesium or sodium or, barium. Primarily, today they are calcium or magnesium based. These materials are often referred to as organo-metallic compounds and they are polar in nature, which allows them to cling to the surfaces of particles. Detergents serve two principal functions.
First, they lift any deposits from the surfaces from the surfaces of the engine to which they adhere to and then chemically combine to form a barrier film, which keeps the deposits from coming out of suspension and coagulating. Detergents form two kinds of barrier films. On small particles, (generally less than 0.02 microns in size), detergents form an absorbed film which slows down coagulation of the particles. On much larger particles, (ranging from 0.5 to 1.5 microns in size), detergents cause the particle surfaces to acquire an electrical charge of the same sign so they can repel each other.
The polar metallic heads of detergents have a great affinity for each other. These molecules attract each other like magnets and form clusters called "micelles".
The deposit precursors being oil-insoluble have a greater affinity for the detergent molecule than the oil molecules. They are attracted to the detergent micelles (much like iron fillings are drawn to a magnet) and trapped within them. Thus, they are kept in solution in the engine oil and cannot settle out to form deposits in the engine.
The number of particles that can be contained in a micelle is limited. When a 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.
Secondly, detergents neutralize any acids formed by the combustion of the fuel by chemically reacting with the acids in order to form harmless neutralized chemicals.
Dispersants are polar additives that are used to disperse sludge and soot particles for the purpose of preventing agglomeration, settling and deposits. Dispersants envelops particles and keep them finely divided. Dispersants are polymeric and ashless compounds. These compounds are based on long chain hydrocarbons, which are acidified and then neutralized with a compound containing basic nitrogen.
The hydrocarbon portion provides oil solubility, while the nitrogen portion provides an active site that attracts and holds potential deposit forming materials to keep them suspended in the engine oil.
This dispersant molecule combines a compact, electrically polar head and a long, oil soluble tail, which might look like this.
In a succinimide dispersant, (which is the most widely used type of dispersant chemistry used), the piece on the left containing nitrogen (N) is the polar head; the piece both containing nitrogen (N) and oxygen (O) is the connecting link. "R" is the long, oil soluble tail. The polar heads attach themselves to any deposits or acids that may be formed by the combustion of the fuel to form micelles which are taken into solution in the oil by the R's. These micelles can trap deposit precursors up to 0.05 microns in size by proving 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.
As mentioned earlier in this discussion on detergents and dispersants, one of their functions is to help neutralize any acids that are formed by the combustion of the fuel. Each of these additives contributes to the neutralization of these acids by proving the engine oil with a Total Base Number (TBN).
TBN measures an engine oil's ability to neutralize acids that are formed by combustion. As long as an engine oils TBN stay above a certain limit during use (generally ½ of its original TBN number), during use the engine oil is still fit for service. In fact a new oil's TBN is less important than the TBN during service, which protects the engine. An engine oil must have the ability to retain its TBN reserve (alkalinity reserve) that is contributed by both the detergents and dispersants during its entire drain interval.
Of these two additives, it is detergents that offer the best alkalinity reserve. Though dispersants are a necessary additive for the formulation of engine oils, and engine oil's total TBN that is derived through the use of high dispersant chemistry does not offer adequate protection an engine needs against the corrosive attack of acidic combustion by-products. Dispersants are more rapidly depleted than detergents because of the way they chemically react with acids that are formed and by the way they react with other particulate contaminants.
Detergents, on the other hand, because they chemically react with the acids and any other particulate contaminants that are present in the engine oil, have the ability to retain their TBN reserve over longer periods of time, thus providing a more protective form of TBN over the entire drain period of the engine oil.