SECTOR FOCUS:
AUTOMOTIVE HDDO
Total Base Number: how low can YOU go? Rich Becker, Customer Technical Service Specialist, Afton Chemical
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.
Challenge the numbers The difficulty inherent in agreeing on a one-size-fitsall number is possibly why minimum TBN does not feature in international lubricant standards.
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.
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.
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.
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LUBE MAGAZINE NO.157 JUNE 2020
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. Measurement matters 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.