As engine technology and fuel specs advance, so too will the choices for engine lubrication and drain intervals become more complex. The TBN (Total Base Number) of a lubricant will be one aspect that will play a major role in the future and must be considered when assessing the situation on the ground writes FleetWatch technical correspondent Dave Scott.
In the November/December 2001 edition of FleetWatch, an article was featured headed TBN Killing the Acid Attack. Relating to our diesel fuel specs at the time, the article stated: “Local road transport diesel fuel has a sulphur level not supposed to exceed 0,55% by mass but in the past few months, this has been at 0,75% due to a national fuel shortage in South Africa’. So what’s changed since 2001?
For a start, our diesel spec in those days allowed for 5 500ppm (parts-per-million) sulphur content. That subsequently dropped down to 3 000ppm and then in 2006, to 500ppm with 50ppm also now available. Ultra-low sulphur diesel (ULSD) at 10ppm is gazetted , see Govt. Gazette dated 8 March 2011 Volume 549 No. 34089 for comment. Its scheduled introduction is 2017.
So with much lower sulphur in diesel, one would think the acid attack has gone away , but has it? Remember that TBN is: ‘˜A lubricant’s ability to neutralise combustion and oxidation-derived acids. It is, in other words, a measure of lubricant alkalinity and used to estimate lubricant capability of being able to neutralise acid formations in an engine.’
If you are a private motorist and doing around 20 000kms per annum with 15 000km service intervals, then TBN is irrelevant to you. But if you are operating a fleet that runs at 20 000kms per month and you are advised that you may have to shorten your fleet oil-drain intervals due to premature depletion of the engine lubricant additive package, you will have a serious productivity and cost implication on your hands.
Bear in mind that it’s not only TBN that determines oil-drain periods: the degree of degradation in a lubricant as a whole must be assessed.
The advent of biodiesel
The attraction to biodiesel is growing with the increasing price of oil. If biodiesel is being blended with standard mineral oil diesel up to the permissible maximum of 5% by volume – matching both SANS 1935 in conjunction with SANS 833 for specification – then there are impacts on the ability of lubricant TBN to handle this in extended oil drain intervals.
Failure analysis expert, Patrick Swan, explains: “Biodiesels have a similar boiling range to the base oils used in crankcase oils but their oxidation stability ranges from poor to worse. Diesel engines never burn all the fuel passing through the combustion chamber so there is always some raw fuel that dilutes the oil, or fuel dilution.
“Because of the difference in boiling ranges between normal diesel and the base oils, much of the fuel dilution is vaporised (boiled off) and disappears out the breather. However, biodiesel cannot boil off and with its poor oxidation stability, readily degrades into sludge and mild acids that cannot be neutralised by the oil’s TBN.
“When biodiesels are used, it is therefore recommended that oil drain periods are shortened to allow the oil to remove the added sludge load and to prevent accelerated corrosive damage, particularly to the engine’s bearings and other softer metals.’
TBN does not define a lubricant’s quality , it is one of many parameters. Employing ‘˜cheap’ lubricants with inadequate TBN values undoes the promise of extended drain intervals for diesel engines on low sulphur and ultra-low sulphur diesel fuel.
Modern lubes are complex, balanced blends and a lubricant can be basic and acidic at the same time – that is it can have a TBN and Total Acid Number (TAN). Simply dosing lubricants with more additives is out of the question , never mind claims offered by snake oil purveyors.
The characteristics of high-quality lubrication performance can be thrown completely off-balance by simply increasing additives that provide alkalinity. Swan observes that: “A lubricant only works when the additives remain in solution in the base oil – and the TBN is a relatively weak alkali buffer solution. If TBN were a strong alkali, it would be just as corrosive to the engine as the acids it must neutralise. But, being a weak alkali also means that it can only neutralise strong acids such as those of sulphur (from the fuel and base oil) and nitrogen (nitric acid from combustion), and other strong acids that may be formed, depending on the chemicals present in the combustion chamber.
“However,’ adds Swan, “TBN has to be a strong buffer solution so that it retains its alkalinity for a long period. This is TBN retention.’
It’s better to use a new lubricant with even slightly lower TBN but with excellent retention qualities over a longer time-frame than having an extremely high initial TBN that experiences a sharp decline in engine-protection ability.
TBN values are listed in lubricant product handbooks but do not disclose TBN retention characteristics for a specific lubricant. A high-quality lubricant should stabilise the TBN depletion which is dependent on a number of factors but is critical in determining drain intervals.
EGR and nitric acid
Sulphur is not the only acid-forming element in an engine sump. Modern diesel engines have been forced to adopt hardware changes to match escalating increases in exhaust emission regulations and one of these is exhaust gas recirculation , EGR.
EGR works by re-circulating a portion of an engine’s exhaust gas back to the engine cylinders. In a diesel engine, the exhaust gas replaces some of the excess oxygen in the pre-combustion mixture. And as we move to Euro 5 exhaust emission standards – as planned per the Govt. Gazette of 18 March 2011 – so EGR will become a fairly widespread technology.
Because NOx forms primarily when a mixture of nitrogen and oxygen is subjected to high temperature, the lower combustion chamber temperatures caused by EGR reduces the amount of NOx the combustion generates. Re-circulating exhaust into the intake tract of a medium or heavy-duty diesel engine can reduce power output, increase fuel consumption and add abrasive contaminants and increase engine oil acidity which, in turn, can reduce engine longevity. EGR takes care of nitrous oxides formed during combustion in a diesel engine , this means that TBN must deal with increased nitric acid levels.
The forward view
As engine technology and fuel specs advance, so the choices for engine lubrication and drain intervals become more complex. TBN will be one aspect that will play a major role in the future and must be considered when assessing the situation on the ground. If a fleet is crossing borders and using unknown specced diesel, this will be yet another factor.
Take an integrated view beyond the spec on lubricant containers because sound lubrication policies and practices are not just about price and additives.