MYTH or magic? The merits of adding a little two-stroke oil to a tank of diesel is currently one of the hottest topics on 4×4 forums and around the braai fires. Protagonists argue that the practice is cost-effective and safe while detractors argue it’s a recipe for disaster. Such has been the controversy that Adrian Velaers, an engineer at Sasol’s Energy Technology division, decided to settle the argument once and for all. Velaers ran tests in Sasol’s laboratories and published a report titled Two-stroke oil in diesel – a technical study, which was presented at the 11th International Tribology Conference in Pretoria earlier this year. This is a summary of his findings.
Background information
Diesel has to pass stringent quality legislation (SANS 342:2014) that prescribes the minimum criteria the fuel must meet before it can be distributed from the refinery. This fuel is already saleable to the public and contains a general additive package (please see the article Are all fuels the same? in CAR’s October 2013 issue) that allows the fuel to meet the desired lubricity and cetane number, among other requirements. Major oil companies add their own proprietary additive packages to align the fuel’s quality with the brand’s performance goals.
Why two-stroke oil?
Two-stroke oil (2SO) has the lubricity properties required to lubricate a two-stroke petrol engine, which does not have an oil sump. The 2SO dissolves in petrol, but forms a residue on the metal parts inside the two-stroke engine when petrol evaporates before combustion. This residue layer of 2SO acts as a lubricant to protect the moving metal parts.
2SO in diesel?
The introduction of lower-sulphur diesel to our market, necessitated by the advancement of modern turbodiesel engine technology, has resulted in the decrease of diesel lubricity during the refinery process.
The reason for this is not that sulphur is a lubricant, but that the process to remove sulphur also tends to remove special “polar molecules” that stick to metal surfaces and improve lubricity. The theory is that if a motorists adds 2SO to a tank of diesel in a 1:200 ratio, that oil would replace the missing “polar molecules” and restore lubricity, which, in turn, should prolong the life of the diesel-injection components. This is a myth for the following reasons:
The theoretical conundrums
• As mentioned before, the diesel leaving the refinery already contains a basic additive package, including lubricity enhancement additives that more than replace all the lost “polar molecules” to meet the SANS 342:2014 lubricity requirement;
• Unlike petrol, diesel does not evaporate before combustion, so any dissolved 2SO cannot easily be deposited on metal parts to improve lubricity;
• Most 2SO is rich in zinc – a harmful contaminant and aknown cause of stubborn injector deposits (through a process known as injector fouling), which can cause a drop in injector flow rate leading to a drop in power. Zinc also causes ash build-up in diesel particulate filters, which blocks them over time.
The tests
Velaers concentrated on five key aspects and decided on these test methods:
1 Lubricity tested on a high frequency reciprocating rig (HFRR) and scuffing-load-ball-on-cylinder evaluator (SLBOCLE);
2 Cetane number measurement using an ignition quality tester;
3 Zinc content measurement;
4 Exhaust emissions by running an engine on a dynamo-meter simulating the new European drive cycle (NEDC);
5 An injector fouling test using a test bed with a 1,6-litre, four- cylinder turbodiesel engine with a common rail fuel system and piezo-electric injectors.
| Sample name | Description |
|---|---|
| Raw refinery diesel | Low sulphur refinery diesel prior to the addition of basic additive packages |
| Raw refinery diesel + 2SO | 2SO added to the raw refinery diesel at a 1:200 ratio |
| Market diesel | South African low-sulphur diesel from a Johannesburg forecourt |
| Market diesel + 2SO | 2SO added to market diesel at a 1:200 ratio |
The results
Many different fuel samples were tested for the purpose of compiling the 10-page technical report, but we will focus only on the most relevant examples.
Lubricity
The raw refinery diesel did not meet the requirement of both lubricity test methods. Adding the 2SO did not help because the diesel still failed the test; the difference in lubricity falls within the variability of test methods.
The market diesel with the added lubricity packages (ex-refinery and from the relevant oil company) easily passed both methods’ requirements. Although the addition of 2SO did make a slight difference, lubricity fell within the variance of both test methods.
The lubricity improver in the additive packages was particularly effective in bringing the raw refinery diesel into the required specification for market diesel. In summary, the addition of 2SO had almost no effect on lubricity.
| Lubricity measurements methods |
|---|
| The HFRR method utilises a steel ball that is submersed in the test fluid and pressed against a flat metal surface with a force of 200 g. The ball is rubbed against the surface at high frequency at a stroke of one millimetre for 75 minutes. The ball is removed and the diameter of the flat scar at the contact patch measured. A good lubricant will quickly provide hydrodynamic lubrication and prevent the flat scar from growing. The SANS limit for the scar when diesel is the working fluid is 460 µm. The SLBOCLE method presses a steel ball against a rotating ring partially immersed in the fluid that is undergoing the test. The load on the ball is gradually increased until the friction between the ring and the ball exceeds a set limit. This will also correspond to a scuff mark on the ring. This test is not used in any diesel specification, but a conservative ball load of 3 500 g is accepted for diesel as an industry norm. |
Cetane number
The addition of 2SO had a negligible effect on the cetane number of the two diesel samples, one being the market diesel described in the lubricity test.
Zinc content
Zinc contamination as low as 1ppm is known to cause severe injector fouling. As 2SO contains around 16 ppm zinc, it was clear that any blend with diesel that contains trace amounts of zinc will have a detrimental impact on the zinc content level. This was proven as the market diesel (with no zinc content) plus 2SO had a zinc level measured at 0,135 ppm. Therefore, the addition of 2SO does increase the zinc content of the diesel dramatically.
Exhaust emissions
The exhaust emission testing showed no marked increase in emission levels with the addition of 2SO at the 1:200 level. This can be attributed to the fact that 2SO is a light oil and produces similar by-products of combustion to diesel, which explains the negligible difference.
Injector fouling
The standard injector-fouling test procedure (involving a 1,6-litre four-cylinder turbodiesel engine with common rail injection) was carried out on a test dynamometer. The engine was fitted with new injectors and ran over a very high-load test cycle for 16 hours (two eight-hour phases with a cool-down period in-between). Every 30 minutes the engine’s full power and fuel flow were measured at 4 400 r/min (rated power at maximum fuel pressure and flow point). Any drop in fuel flow would indicate that injector fouling was taking place. A standard EN590 diesel with 2SO dosing was used.
From the graph it is clear that EN590 diesel without 2SO showed little drop-off in fuel-flow rate and power, whereas the EN590 with 2SO clearly showed a drop in fuel flow rate as deposits were starting to form in the injector holes. The reason for the recovery at eight hours is that the cool-down period induced thermal shock that loosened some of the debris.
The obstructions soon formed again and worsened towards the end of the 16-hour test. It was clear that 2SO does result in injector fouling which was totally predictable as zinc is one of the main contributors of injector fouling (it is purposefully added to diesel during specific injector-fouling tests).
Summary
The results of the tests that were carried out suggest there is no benefit in adding 2SO to diesel. In fact, the practice of 2SO dosing increases the likelihood of injector fouling, a loss of performance, and possibly damaging vehicles’ exhaust after-treatment systems. Velaers says, the oil companies would probably not mind if they sell additional 2SO with each tank of diesel, but from a consumer point of view it is a futile exercise. Even ignoring all the dangers, you are still wasting your hard-earned money by indulging in the ill-advised act of 2SO dosing.
