Diesel sulfur levels in developed countries have drastically diminished in recent years, reducing one of the chief sources of acid in engine oils. As a result, engine oil formulators could relax their attention to total base number, or TBN, the chief means of fending off acid.

But sulfur levels in diesel remain very high in Africa and this creates potential for much more acid in crankcases. Industry sources say there is little for operators to do but to shorten drain intervals.

Weapon Against Acidity

Since this problem stems from a relaxation of TBN levels, its worth pausing to clarify the meaning of TBN. It certainly is not something explained or even mentioned on lubricant product containers, despite the fact it is a critical factor when assessing diesel engine oil drain intervals.

Definitions can be found in lubricant manufacturer product manuals. Chevrons Caltex Product Guide for Fuels, Lubricants and Specialties in Southern and Central Africa defines TBN as the amount of hydrochloric or perchloric acid required to neutralize all the basic constituents of a one-gram sample of a petroleum product. So one can see that TBN is measured in milligrams of acid.

Of course, the engine oil formulator looks at the reaction from the opposite perspective. It is the acid that they want to neutralize, and TBN measures the amount that the oil can handle.

The higher the TBN, the more acid the lubricant can neutralize. Marine diesel engines, which run on bunker fuels with sulfur content above 2.5 percent by mass, need lubricants with a TBN of 55. A diesel-powered heavy-duty truck in Africa may call for a multigrade 20W-50 engine oil with TBN greater than 1,000, while a 15W-40 oil for petrol-powered passenger cars might have a TBN of around 8.

Acids can come from a variety of sources. Sulfur in fuel turns to sulfuric compounds during combustion and creeps into the crankcase. If those compounds encounter water there – especially likely in Africa – they combine to create sulfuric acid. Poor combustion, excessive blow-by, soot contamination, oxidation and overheating are other causes.

In Africa, all of these factors are significant contributors of acids in crankcases of diesel-powered vehicles. That includes a significant portion of light-duty passenger autos, as well as heavy-duty trucks. To start with, diesel fuel across Africa has sulfur levels that are orders of magnitude higher than in Europe. In Europe, regulations cap sulfur levels at 10 parts per million. African standards vary greatly from country to country, but sulfur levels range from 50 ppm to 3,500 ppm. Fuels here also tend to have much higher levels of water and dirt contamination, which aggravates the situation.

Adding to lubricant stress is the age of the vehicle park. The majority of Africas vehicles are more than 10 years old and are likely to have leaky seals that allow elevated levels of blow-by combustion gases to enter the crankcase. These gases include both nitrous oxides and sulfur dioxides that, in the presence of water, easily form nitric and sulfuric acid.

The small portion of newer vehicles on the continent may avoid the blow-by problem, but pollution control technologies become another source of acid for some of them. As additive supplier Lubrizol noted in a brochure titled Technology for Growth Markets, modern diesel engines have been forced to adopt hardware changes to match tightening regulations on exhaust emissions, and one of these is exhaust gas recirculation. Nitrous oxides are formed during combustion in a diesel engine. EGR keeps these from being emitted through the exhaust pipe by diverting them to the oil sump, where they can form nitric acid.

Pressure from Organic Acids

Then there is the issue of biodiesel. Use of biodiesel is expanding in Europe because of government mandates, but in Africa diesel made from plants actually becomes economically attractive when crude oil prices flirt with U.S. $100 per barrel, as they are doing once again. Europeans may be grappling with the performance variances between biodiesels made from different plants, but Africa has much larger variances due to spotty adherence to product standards.

South Africa has a standard for biodiesel – SANS 1935 – but it requires an investment of approximately 3.1 million rand (320,000) in specialized equipment to conduct 26 tests needed to certify batched production. The steepness of these costs, combined with lax enforcement, results in many producers not following guidelines and production of much substandard biodiesel.

Fuel industry sources estimate that the country has as many as 200 start-up biodiesel producers. Around 10 of these operate on a commercial scale, while the balance are small, private producers, mainly farmers.

Biodiesel enters crankcases as it is swept out of piston chambers following combustion. The same thing happens to fossil-based fuels, but biodiesels evaporate less readily and so have a much higher tendency to accumulate and decay in oil sumps. Its a reasonable conclusion that organic acids from biodiesel are also contributing to the problems in African crankcases.

So far Africa has avoided another looming factor that is raising crankcase acid levels in the developed world: diesel particulate filters. South Africa is trying to match Euro 2 emission standards. New trucks are being sold, some with and some without EGR. New Japanese models such as the Fuso FK and FM ranges are imported with EGR ports blanked off; they will be retrofitted for Euro 3 when this is legislated. The point is that Africa is not yet at the stage of Euro 4 that needs DPFs. DPFs require restrictions on the elemental packages in lubricants and this means a lower TBN, while biodiesel and EGR are raising acid levels. In the developed world, never mind Africa, this is placing major pressure on TBN.

Marketers of synthetic engine oils promote them as being superior to mineral oils in every respect, so some operators might wonder if synthetics would offer more help combating acid in the crankcase. In fact, being synthetic does not necessarily mean an oil has high TBN values. To determine an oils performance in this area, some advocate evaluating them the same way as mineral based lubricants. Comparing performance data is the only way to be certain.

An overlooked factor – TBN retention

As explained above, TBN is the main weapon against crankcase acidity. Formulators increase TBN through the addition of based or overbased detergents – primarily metal phenates, metal sulfonates and metal carboxylates, but also additives containing salicylates and thiophosphates.

In order to be effective protecting engine parts against acid, engine oils must have not only a high enough TBN but also good TBN retention. The latter characteristic represents the oils ability to shield against acid over a long period of time – not just when the oil is fresh.

While TBN values are available in lubricant product handbooks, they do not disclose TBN retention characteristics for specific lubricants. Rather than having an extremely high initial TBN that experiences a sharp decline in engine-protection ability, industry sources say its better to use a new lubricant with even slightly lower TBN but with excellent retention qualities over a longer time-frame. A high-quality lubricant should stabilize the TBN depletion, which is dependent on a number of factors but is critical in determining the drain intervals.

In an article in the September 2010 issue of Tribology & Lubrication Technology, which is published in North America, Jack Poley from Miami-based Condition Monitoring International advised, Base number isnt nearly as important as it was before the advent of desulfurization of diesel fuel…. Oxidation and fuel soot build up much before a base number begins to reflect a problem.

While it may be accurate for North America, Poleys assessment does not apply to current experience in Africa. Acidity in crankcases is a very real problem in Africa, partly because diesel sulfur levels have not fallen like they have in developed regions. But there are also other factors that are raising acid levels, and some of these are present in the developed world, too.

The problem may be bigger in Africa at the moment, but Africas experience may also foreshadow problems for the rest of the world. One cannot look at sulfur in fuel as the only reason for measuring TBN. In the bigger picture TBN is going to be a challenge for engine manufacturers who are seeking longer drain intervals with lower exhaust emissions, and for the lubricant producers who must fine-tune engine oils for the task.