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Past Issues

November 2001

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TechnicalWATCH

TBN
killing the acid attack

TBN - is it a new computer virus, a disease or biological threat? Thank goodness it's none of those. What it is is a discussion factor when 'transport gurus' get together and appear very knowledgeable on lubrication, sulphur levels in diesel fuel, oil drain intervals and engine wear. When you first hear this, you nod your head and agree "it's an important issue!" Afterwards you wonder why everyone was waxing enthusiastically over an obscure acronym like TBN when, as commonly perceived, 'oil is just oil'. FleetWatch technical correspondent Dave Scott contends that operators should familiarise themselves with this term as it represents the factor that protects an engine from acid attack.

Limited research shows how little we all know about TBN as only one factor of high quality lubrication - and even more, how some people seem very expert when they bandy TBN around as a conversational club to beat the rest of us into submissive silence.

It all starts with a definition. Lubricant manufacturers' product manuals give Total Base Number (TBN) definitions in terms meant only for those who understand. Get this one description: 'The quantity of acid expressed in terms of the equivalent number of milligrams of potassium hydroxide that is required to neutralise all basic constituents present in one gram of sample.' Now you know. Phew! That's about as clear as mud isn't it?

Let's go down a notch or two and get to one that even I understand: "A lubricant's ability to neutralise combustion and oxidation-derived acids." It is, in other words, a measure of lubricant alkalinity and is used to estimate a lubricant's capability of being able to neutralise acid formations in an engine.

TBN values - useful but not absolute
In general terms, the higher the TBN, the greater a lubricant's ability to neutralise acid formations. There are, however, many other factors - such as TBN retention. For example, passenger car multi-grade 25W-50 lubricants will have a TBN around 7, while a diesel powered truck will use 15W-40 lubricants with a TBN of 13. A marine diesel engine, using bunker fuel with sulphur content over 2,5% by mass, will use lubricants at TBN of 55.

Local road transport diesel fuel has a sulphur level which is not supposed to exceed 0,55% by mass. However, in the past few months, this level has been at 0,75% due to a national fuel shortage in South Africa.

A lubricant can be basic and acidic at the same time. In other words, it can have a TBN and Total Acid Number (TAN). Modern lubricants are very complex indeed. A comment from long-time fuel and lubricant industry man, Eric Ambrose, encapsulates it all:

"Overall, I think it's important to distinguish what is an oil and what is a lubricant? My opinion is, that oil is the stuff you fry your chips in! A lubricant is the synergy of base oil selection coupled with researched and tested metallic and organic additive compounds, all formulated and blended to compliment each other in a product that is the lifeblood of a vehicle engine. An 'oil' it is not!"

Patrick Swan, failure analysis expert, endorses the fine balance played by modern lubricants in an engine. See his comment adjoining this article.

TBN results from additives
Modern day lubricant additives provide: 

• Surface protection against wear and corrosion of bearings, pistons and rings, cylinder liners, camshaft lobes, gears etc

• Protection for the lubricant itself so that its useful life is extended. 

• A lubricant for specific performance needs and customer applications that's environmentally safer, and TBN is the result of fine-tuning between base stock oils and well-chosen additives.

Beware the snake-oil additive man! The performance characteristics of high-quality lubrication can be thrown completely off-balance in the simplistic notion that TBN values can be raised by increasing additives that provide alkalinity.

TBN retention - a vital aspect
A lubricant's TBN has the most important job of protecting an engine from acidic attack and to provide this shield over a long period of time - not only for the first few hundred kilometres. A lubricant's ability to protect an engine over extended periods against acidic attack is called having good 'TBN retention'.

It is thus preferred to have an even slightly lower TBN for a new lubricant with better TBN retention over a longer period of time, rather than having an extremely high TBN to start off with but having a sharp decline in this ability to protect the engine. 

TBN values are available in lubricant product handbooks but these don't tell anything about TBN retention characteristics for specific lubricants. Note that the rate at which a TBN depletes is also related to an engine and the operating conditions under which it is applied. The only way of knowing which is the best lubricant for an engine under its current operating conditions is to evaluate it - in your engine, under your specific operating conditions.

A lubricant supplier should be able to give an indication of how strong a lubricant's TBN retention is. A high-quality lubricant should stabilise at a TBN number two or three below the value of a new lubricant and stay at that until draining, or possibly drop slightly as drain period approaches.

The total base number decreases as acid increases and the general policy is to drain when the number reaches 50% of its original value (this is Caterpillar's recommendation) or a minimum of 4 TBN. Any lower than this places engines under the risk of corrosion.

Of course there are other factors to take into consideration and that is the viscosity and insoluble matter which would have built up during operational service. So, it's not only TBN which determines the drain period but an assessment of the degree of degradation in a lubricant as a whole

No substitute for testing
There is, however, no substitute for testing by taking samples of a new lubricant - and then another just before draining used engine oil, analysing this and repeating the test for other lubricants in the market.

At the end of the test, the lubricant with the highest TBN value, the lowest overall wear metals and the least change in viscosity would, in general, be the best lubricant for an application. Technical advisers from lubricant suppliers should also be able to assist and sponsor the process.

Important to note is that while synthetic lubricants position themselves in promotional claims as being superior to mineral oils in every respect, a synthetic lubricant does not necessarily mean it has high TBN values. So how should TBN be treated between mineral and synthetic lubricants? Essentially the same way as one would go about comparing different mineral based lubricants.

Something else to remember is that there is a price premium for synthetic lubricants and all options must be weighed before deciding on going this route. Synthetics are, in general, more refined than the mineral base - thus also resulting in a better TBN retention characteristic. But this is by no means a generalisation. There are mineral based lubricants available on the market that can out perform synthetic based lubricants. Their performance is, however, dependant on the application.

Synthetic lubricants have proved themselves in the power train behind an engine - gearbox and differential. Where synthetic lubricants come into contact with combustion processes in diesel engines, they do not deliver as much value due to the sulphur levels of South African diesel fuel.

Point to ponder

• What can be expected of TBN performance in current lubricants when the sulphur content in diesel fuel decreases from 0,55% by mass to 0.30% at the beginning of 2002? Reduced amounts of fuel sulphur will result in less sulphur introduced into an engine - and eventually into lubrication. The amount of sulphur present in engine oil is directly related to the amount of sulphuric acid that forms in engine oil. It can be safely concluded therefore, that TBN retention will improve and so will engine life.

• It's time to focus on cost of ownership issues and go beyond the information and price tag on a can of oil. TBN is just one factor that needs serious consideration. As we have stated in this magazine before: What counts is the cost of lubrication, not the price of oil!

• Where road transport operations are involved in cross border transport, it is important to bear in mind that the quality of diesel fuel may change drastically. In such cases, TBN could be a vital issue where diesel with sulphur levels of 1% are introduced into fuel tanks.

• Modern multi-valve and electronically managed diesel engines - and there will be more of them introduced into South Africa - don't perform well using what are known as 'snake oils' and poor quality fuel - or fuel with high sulphur and spiked with paraffin. These engines need premium quality fuels and lubricants together with effective maintenance procedures if they are to provide fleet operators with optimum cost of ownership. 

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