Engine oil formulation might seem a far cry from limbo dancing, but doing either successfully requires three things: balance, expertise, and an accurate assessment of conditions.
Take balance. Acids are highly undesirable in any engine as they cause corrosion. These acids can come from several sources, including oil oxidation and exhaust gas recirculation, however sulphur in diesel fuel has historically been the biggest contributor. Sulphur’s acid-forming tendencies can be countered by over-based detergents – the first line of defence – which combine with sulphur to form neutral salts.
Acids left unneutralised not only attack metal engine parts but can also react with the oil, creating free radicals – the start of that dangerous chain reaction called oxidation. This is when the second line of defence, antioxidant additives, come into play.
Over-based detergents make up around 40% of a typical additive package for diesel engine oil, and provides almost all of the base present in the lubricant – its Total Base Number (TBN). TBN determines how much acid, from whatever source – sulphur or oxidation being the main ones – that the fluid can neutralise.
Diesel fuel used to contain higher levels of sulphur; resulting in engines needing more detergent protection, so the thinking was that lubricants needed a higher TBN. Modern diesel fuels contain at most half, and in many places just a tenth of their former sulphur levels, yet TBN has remained unchanged in many fluids – and users can fail to realise that such high levels are no longer required.
Excess TBN brings nothing to a lubricant except added cost; it could even contribute to ash formation. With insufficient TBN being even less desirable, getting the right balance is crucial.
Challenge the numbers
The difficulty inherent in agreeing on a one-size-fits-all number is possibly why minimum TBN does not feature in international lubricant standards.
Some Original Equipment Manufacturers (OEMs) specify minimum TBN: for the Motor (Engine) and Turbine Union’s (MTU) category 2 it is 8, MACK’s EO-N states 10. Other OEMs have removed TBN from specs or never included it.
Some governments, particularly in developing markets like Africa, legislate minimum TBN; Kenya specifies 10 while in Tanzania it is 7, for example. In these countries, testing TBN/detergent levels offers a quick way to identify lower quality or counterfeit oils.
The Standards Organisation of Nigeria used to mandate a minimum TBN of 6.5 for multigrade engine oil and 5 for monograde; it dropped TBN from its specifications in 2017.
This variation in TBN is why the second factor, expertise, is so important.
Afton’s testing has shown that it’s not the starting level of TBN that counts; it’s how much TBN remains just prior to the oil being changed.
Traditionally, TAN (Total Acid Number) was measured alongside TBN; the point where TBN crossed over with TAN (TBN number lower than TAN number) was deemed the end of a lubricant’s useful life, after which higher acidity was expected to start causing wear and oxidation.
Indirect measures are not great indicators of fluid performance though, as Afton’s extensive field trials in China have shown.
We have found that TAN/TBN crossover does not correlate closely with fluid condition or engine wear. Actual engine wear is best assessed by measuring the level of wear metals in the oil, whilst – as used in the MACK/Volvo T-13 engine test – spectroscopic analysis shows actual oil oxidation/nitration.
Real world efficiency
The third success factor is accounting for operating conditions, which are often overlooked with TBN. Just as a limbo dancer performs better on a dry than a slippery surface, external conditions will affect engine oil performance.
Driving conditions span everything from road quality – about 53% of the Africa’s roads are unpaved – to the likelihood of truck overloading, local climate, and levels of dust and/or water contamination.
Afton has run many millions of kilometres in field trials all over the world, including China.
China has proved to be a great location, offering a wide range of road conditions and climates, tendencies towards vehicle overloading, coupled with well-controlled testing facilities.
The more challenging the conditions, the sooner direct measurement of wear metals and oxidation shows that the oil needs changing. Lubricant is frequently replaced well before TBN is exhausted; much of the detergent is unnecessary and unused.
Therefore the key question changes from ‘how much TBN should the oil have’, to ‘how much TBN does the oil actually need before the next oil change?’