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Total Base Number
It is a widely held belief that the Total Base Number (TBN also called Base Number) of an oil is required to prevent acidic corrosion within the combustion chamber of a running engine. This implies that it protects the piston rings, cylinder liner and piston crown from damage by sulphur or nitrogen containing acids. If you believe that then you are quite right, full marks, but what else does it do?
Oils within an engine tend to deteriorate due to reasons of temperature, fuel ingress or other sources of contaminants forming harmful deposits. These deposits can build up behind piston rings, on ring lands, under the piston crown or on sliding surfaces. The net result is sticking and increased wear. Lubricating oils contain detergents and dispersants to delay the formation of these deposits and reduce the rate at which they form. Detergents are generally considered as those compounds that neutralise the deposit precursors that form under high temperature and pressure conditions or as the results of using high sulphur fuel. Dispersants on the other hand are those compounds that disperse or suspend the deposit forming contaminants. However in reality there is no sharp line of demarcation between the two.
The principal detergents are soaps and salts of alkaline metals, usually calcium in the case of marine oils; they are often referred to as matallo-organic compounds such as sulphates, phenates and salicylates. These compounds are usually "over based". They contain more alkaline metal than is required to neutralise the acidic components used in manufacture of the additive. These are usually ash forming and spent additive will contribute to the insolubles loading in a used oil.
Typical dispersants are usually "ashless". They do not form ash on combustion. They have fairly unpronounceable names such as polymeric succinimides, polyesters and benzylamides. The molecules have a polar or charged end that will attract and hold on to potential deposit forming compounds keeping them in suspension in the oil.
The TBN additive will neutralise acidic combustion products, they will also neutralise deposit forming compounds within the oil, hold them in suspension preventing deposition and with luck also remove any deposits that are formed. Have you ever seen the results of a lubricant trial described in the technical press where you have a before and after picture of a nice clean piston? That is a direct benefit of these TBN additives.
Conversely what would happen without this additive? The illustration below shows acidic corrosion of a lead bronze shell bearing. This type of bearing is now uncommon, partially because of its susceptibility to corrosion. However it does serve to illustrate just how useful is the protection offered by TBN additives. The lead phase of the bearing is very susceptible to acid attack resulting in the formation of large pits on the bearing surface, breakdown of the oil film and the beginnings of a bearing seizure.
Oils within an engine tend to deteriorate due to reasons of temperature, fuel ingress or other sources of contaminants forming harmful deposits. These deposits can build up behind piston rings, on ring lands, under the piston crown or on sliding surfaces. The net result is sticking and increased wear. Lubricating oils contain detergents and dispersants to delay the formation of these deposits and reduce the rate at which they form. Detergents are generally considered as those compounds that neutralise the deposit precursors that form under high temperature and pressure conditions or as the results of using high sulphur fuel. Dispersants on the other hand are those compounds that disperse or suspend the deposit forming contaminants. However in reality there is no sharp line of demarcation between the two.
The principal detergents are soaps and salts of alkaline metals, usually calcium in the case of marine oils; they are often referred to as matallo-organic compounds such as sulphates, phenates and salicylates. These compounds are usually "over based". They contain more alkaline metal than is required to neutralise the acidic components used in manufacture of the additive. These are usually ash forming and spent additive will contribute to the insolubles loading in a used oil.
Typical dispersants are usually "ashless". They do not form ash on combustion. They have fairly unpronounceable names such as polymeric succinimides, polyesters and benzylamides. The molecules have a polar or charged end that will attract and hold on to potential deposit forming compounds keeping them in suspension in the oil.
The TBN additive will neutralise acidic combustion products, they will also neutralise deposit forming compounds within the oil, hold them in suspension preventing deposition and with luck also remove any deposits that are formed. Have you ever seen the results of a lubricant trial described in the technical press where you have a before and after picture of a nice clean piston? That is a direct benefit of these TBN additives.
Conversely what would happen without this additive? The illustration below shows acidic corrosion of a lead bronze shell bearing. This type of bearing is now uncommon, partially because of its susceptibility to corrosion. However it does serve to illustrate just how useful is the protection offered by TBN additives. The lead phase of the bearing is very susceptible to acid attack resulting in the formation of large pits on the bearing surface, breakdown of the oil film and the beginnings of a bearing seizure.
