Back to top

Gerhard Horn: Diesel and Lubricity

7 July 2014

Lubricity. It is he main issue with current grades of diesel available in South Africa.

A lubricant has the capacity to reduce friction between two objects, which makes something with a high lubricity value rather useful in an internal combustion engine. Lubricity is thus a key factor in the current debate about diesel fuels.

The other big issue of the moment is environmental legislation. Motor vehicle and fuel manufacturers worldwide are trying their best to decrease the environmental footprint of their products, and this is making the debate more relevant than ever before.

There is no denying the validity of the worldwide concern about the environment, and reducing the levels of sulphur in diesel fuel is part of the overall effort to improve air quality.


Lowering sulphur levels has many benefits. In SA, we can choose between three grades of diesel with a sulphur rating measured in parts per million. The most common fuel is 500ppm, but cleaner 50ppm is becoming available at more and more outlets.

In November last year, Sasol became the first fuel supplier to offer 10ppm diesel in SA, but it is currently available at only 78 forecourts, in Gauteng and Mpumalanga. Sasol says a wider roll-out will take place during the year and in 2015.

Low-sulphur diesel, such as 50ppm and 10ppm, burns cleaner and emits less harmful gases into the environment, which makes it the obvious choice in a caring society. It also allows manufacturers to develop and introduce more fuel efficient and environmentally friendly cars. The dirty 500ppm fuel we are accustomed to would rip these finely tuned engines to shreds, but 50ppm has made it possible for manufacturers such as BMW, Mercedes and Audi to introduce their latest and greatest diesel engines to a market that has been demanding them for years.

Unfortunately, reducing the sulphur levels of diesel also reduces its natural lubricity, which has a drastic effect on engines that rely on the fuel itself to lubricate parts of the engine, such as the fuel pump and injectors. Modern diesel engines are designed around this problem, but what about the older diesel models out there?

That’s where the big diesel debate starts. As publishing editor Jannie Herbst wrote in his opinion piece in the November 2013 edition, there are a growing number of diesel vehicle owners who advocate adding a small amount of two-stroke oil to every tank of fuel, or at varying intervals. Doing so increases the lubricity of the diesel, countering the reduction of lubricity that happens when the sulphur is removed.

But the opposing argument is that those who make it should determine the composition of the fuel. If additives were necessary, surely the fuel refineries would add them at source? Perhaps it’s a matter of cost? Are we simply waiting for the first refinery to take the plunge and pay for the upgrades necessary to increase the quality of fuel to a point where no customer, no matter how old his/her car, has to worry about lubricity?

Yes, some additives are added at source, but in certain cases the fuel is contaminated somewhere between leaving the refinery and the point where it is pumped into a vehicle’s fuel tank. This happens when the fuel is transferred from one mode of transport to another.

There are also reports of unscrupulous service station owners who add paraffin to the diesel fuel in an attempt to make some extra cash.

Adding paraffin to diesel is one of the oldest tricks in the book, but it has to be said that it’s rarely done in SA these days. A modern diesel engine could theoretically run on this blend for a while, but it would eventually be damaged.

But we don’t want to focus on the modern diesel engine, simply because its future in SA is assured. From late 2015 or early 2016, cleaner diesel fuel blended with biodiesel will become a common sight in major city forecourts. What we’re really interested in is how older diesel engines will fare with new low sulphur diesel.

As mentioned, older diesel engines rely on the natural lubricity of 500ppm diesel to lubricate vital parts in the engine. Putting 10ppm in a 10-year-old diesel engine, or an older model generator, is like activating the timer on a time bomb. Eventually, it will explode.

That’s where the two-stroke debate really gets into its stride. Without the oil you run the risk of damaging the injector needles on older diesel engines, but chuck a bottle in the tank and you start messing with the delicate composition the engineers at the refineries have developed.

In search of answers, we spoke to a lecturer at the school of chemical and minerals engineering at the Potchefstroom campus of the North West University, Prof Sanette Marx, who earned a PhD in chemical engineering in 2003 and was made co-chair in biofuel research by the South African Energy Resource Institute (Saneri) in 2007.

Marx says there’s no simple yes or no answer, but she gave us the background that makes the debate more understandable.

Thirty years ago there was no argument: owners of anything with a diesel engine knew and accepted that they had to add additives to the fuel. The two-stroke debate is a modern issue, but it’s easy enough to trace its roots.

As passenger vehicles with diesel engines became more popular, the demand for diesel increased, which turned out to be a blessing in disguise for forecourt owners. It took shady dealers about five minutes to work out that they could increase profits by adding paraffin to the diesel, which resulted in some engines failing. It could have been because of a lack of lubrication, but the fact is that we simply don’t know.

Marx is well aware of both sides of the debate and she has heard stories about people getting further by adding two-stroke oil to diesel fuel, but she says there’s no scientific proof that you can prolong a diesel engine’s life by regularly adding two-stroke oil to the tank. At this point in time, it’s becoming an irrelevant argument anyway.

The dastardly deeds of forecourt owners are largely a thing of the past, thanks to refineries adding dye to paraffin. It is, however, still a concern when you travel outside SA, in countries where low sulphur diesel is still a relatively rare commodity.

Marx says that 10ppm diesel fuel would be like poison to older diesel engines, so in certain cases some extra lubrication would be necessary if 500ppm fuel were to suddenly disappear from the market.

“Dirty diesel” (500ppm) isn’t about to go anywhere soon, but the overall quality of diesel is set to change. The Department of Energy has stated that fuel manufacturers will have to blend petrol and diesel with biofuels from October 2015. Biofuel is sulphur free, so there should be environmental benefits. And it has high lubricity, which means diesel engines of all ages will be more than happy to run on the stuff.

The two-stroke oil debate may be rendered irrelevant in 2015, but that still leaves more than a year’s worth of debating to go!

The question remains: is it a good idea to add some two-stroke oil every now and then? Well, it certainly won’t damage the car and in some cases it’s even said to improve fuel consumption. We guess it all comes down to whether you trust whoever supplies your diesel outside SA’s borders. If you do, there’s probably no need to worry, but then again, a bottle of two-stroke oil is a small price to pay for peace of mind.