Your 4X4 is not a lawnmower…

Ever since it became a thing in the early 1980s, low- and ultra-low-sulphur diesel (ULSD) has been marked as the non-lubricating Public Enemy No 1 of diesel engines. To the point where some, well, lots of people actually, suggest tipping a snifter of 2-stroke oil into your fuel tank every time you fill up to return some lubrication to the diesel.

But this is one of those myths that can only have started because someone half heard the story. So, strap in, take off the tin foil hat and get ready to have your mind blown as the things you thought you knew are completely upended. Because, science.

It’s worth noting that this article applies mainly to modern diesel engines with sophisticated injection and pollution gear. Anyone with something from the 1980s can leave the room.

The truth about tipping 2-stroke oil into diesel

Ultra-Low-Sulphur Diesel ain’t slippery enough
Trawl the interweb and those mates you’ve never met but sound like they know what they’re talking about will tell you that with less sulphur in ULSD there’s less lubrication and that you need to add stuff to the fuel tank to improve the situation. Sort of. But it’s at this point where those mates of yours left the room when the talk on ULSD was being given.

See, diesel by its very nature is an oily, slippery substance but high levels of sulphur in fuel causes significant pollution (sulphur dioxide) being pumped out of the tailpipe. So, as countries began clamping down on tailpipe emissions, reducing the amount of sulphur in fuels was seen as a quick way of reducing emissions. Then along came the DPF and EGR treatment to further reduce the emission of fine particles and nitrogen oxides (NOx). Continuing with a high sulphur content in diesel would have killed the pollution gear and all of us too. Eventually.

But, back to the slipperiness of ultra-low-sulphur diesel. Right. The process by which sulphur is reduced in fuel is known as, and take a breath, hydrodesulphurization (HDS) – say that five times fast. This process doesn’t just reduce sulphur down to the current mandated level of 10ppm (parts per million) for ULSD but it also strips away the natural lubricity of diesel (but that’s not sulphur because that stuff only plays a very minor role in providing lubricity to diesel). And this is mainly due to the loss of nitrogen- and oxygen-based polar trace elements. So, you see, sulphur has slight lubricating properties but it’s not the lubricating component of diesel. My head's hurting.

ABOVE This is the engine test bed used by SASOL to compare the gunking effect of adding 2-stroke oil to diesel as opposed to running straight ultra-low-sulphur diesel.

“Think of this like multivitamins, they only really do anything if your body is lacking something, if it isn’t, they’re an expensive way to get yellow wee – ULSD isn’t lacking lubricity, so…”

Adding slipperiness back in…
Fuel companies, to meet stringent guidelines around diesel composition, must return the lost lubricity to the diesel once the sulphur level has been reduced, and this is usually done via a lubricity improver. And, in the market, there are two types widely used by fuel companies, and they are either neutral or acidic. Both types carry the necessary long hydrocarbon chains with polar end groups (the stuff that’s removed during the reduction of sulphur in diesel). Meaning the ULSD is, more or less, returned to its natural lubricity level.

Is your head hurting? Mine is.

Okay, so the polar head groups in the lubrication additive are what clings to the metal surface; they’re attracted to it, and the hydrocarbon chains are what forms the boundary protection (lubrication of, say, piston rings). Think of it as wrapping the metal bits in a soft blanket to keep them safe.

And it gets more interesting because bio-diesel has been identified as an excellent form of diesel-fuel lubrication – better indeed than anything else on the market. According to researchers at the School of Engineering and Technology, Central Queensland University, “Use of biodiesel with the diesel fuel, especially with the ULSD (ultra-low-sulphur diesel), can serve as a lubricity improver along with the proven qualities of emission and combustion performances […] one per cent addition of biodiesel can enhance the ULSD’s lubricity by 30 per cent,” the researchers wrote.

So, essentially, the off-the-shelf lubricant additives you can buy is essentially the same stuff the fuel companies already include in their mixture. So, if you want to add one of these commercial lubricants, go right ahead, it certainly won’t hurt your engine…might even help.

Time for a history lesson
Fuel companies have upped their game when it comes to lubricity additives in ULSD. Back in the late 1980s and early 1990s fuel companies were using dimer acid as a replacement lubricant. Why? Because it was being used in jet fuel and it worked very well.

But things didn’t go to plan. The introduction of dimer acid onto the market in Sweden (1991) saw an influx of diesel passenger vehicles suffering from fuel filter blockages, camshaft deposits and fuel pump failures. But why? Well, turns out dimer acids were reacting with the crankcase oils and becoming an insoluble sludge. Dimer acids were subsequently banned from use.

So, just how slippery is diesel?
All diesel sold in Australia, the US, Japan, and Europe must adhere to stringent guidelines on the chemical structure, if you like (for reference the diesel run here and in many other countries is known as EN590). For instance, in Australia (and plenty of other countries around the world) diesel must meet a friction/wear/flat spot ‘measurement’ of 460 microns maximum. So, in some cases, the scar might be less than that if the fuel supplier has added more lubricant.

How is lubricity measured? Via a High Frequency Reciprocating Rig (HFRR), of course. This is a steel ball pressed against a flat surface with a fixed load of 200g. The ball is then rapidly vibrated back and forth using a 1 mm stroke. After 75 minutes, the flat spot (wear scar) that has been worn on the steel ball is measured with a microscope. The size of the wear scar (in microns) is directly associated with the lubrication qualities of the fuel being tested. It’s suggested that if the HFRR wear scar diameter is less than 460 microns, the fuel will perform satisfactorily in an engine.

ABOVE All fuel sold in Australia must meet stringent lubricity measures and to test the lubricity of everything from fuel (petrol and diesel) to oil a High Frequency Reciprocating Rig (HFRR) is now the industry standard. By rapidly vibrating a ball back and forth across 75 minutes a wear scar is created on metal. If it's less than 460 microns the fuel is up to standard.

Any old engine oil will work, right?
Nope. The main reason that old-timers suggest 2-stroke oil tipped into the fuel tank is best is because unlike four-stroke engine oil, the 2-stroke stuff won’t turn diesel to sludge when mixed in.

What is it about 2-stroke oil that got this story started?
It’s not just to do with the belief that a lower level of sulphur in diesel makes the stuff less lubricating, but also because zinc dialkyldithiophosphate (if you can say that first time we’ll give you a free subscription to Unsealed 4X4) in mineral oils and zinc di-thiophosphate (ZDTP) for synthetic oils that was invented by Castrol (a fuel company) more than 70 years ago is an anti-wear ingredient. But, over the years the concentration levels of the stuff allowed in oil (both 2-stroke and four-stroke) has been reduced because of concerns about health and the environment. See, while the ingredient does afford good metal-to-metal lubricity high concentrations of zinc can foul injectors in modern diesel engines. This is less of an issue in older diesel engines (built before the mid-1980s).

But it’s not just the lubricating nature of zinc, that got this story started…see, in a petrol engine, 2-stroke oil is designed to adhere to metal engine parts once the petrol has evaporated away, keeping everything lubricated. Just be mindful that some pour-in lubricants are intended to be added once a tank and some only every 10,000km.