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The practice of adding 2-stroke oil to diesel is a topic that is enthusiastically discussed and debated by diesel vehicle owners around the world. The consensus opinion on the ideal blending ratio as per some internet forums is reported to be a 200:1 volume mixture of JASO-FC grade 2-stroke oil in low sulphur diesel. The benefits of this are claimed to be better lubrication of injectors and fuel pumps, improved cetane number resulting in better combustion, and no detrimental effects. As these claims are based on anecdotal evidence, this study aimed to quantify any such effects under scientific laboratory conditions.
The motorist’s motivation for following this self-medication advice stems from a perception that low sulphur diesel has inadequate lubrication capabilities in the high-tech fuel pump and fuel injection hardware found in modern diesel engines. The basis for this is not that sulphur itself acts as a lubricant, but rather that trace amounts of polar molecules present in crude-oil give diesel good lubricity properties. It is true that the refinery process used to remove sulphur from diesel also tends to remove these polar molecules. However, it is quite simple to replace the lost polar molecules by adding a lubricity improver additive which is the universal norm for low sulphur diesel practiced by the oil industry throughout the world.
This study reviews the industry standard test method for diesel lubricity which is part of SANS 342:2014, the standard governing the sale of diesel in South Africa. A diesel fuel passing this test demonstrates a high level of lubricity and adequate protection of modern diesel injection equipment. A number of test fuels were blended with and without 2-stroke oil and tested according to this method. An additional diesel lubricity test method known to be representative of diesel fuel pump wear was also used to confirm the results. The study also tested the cetane number of the same fuels to quantify any cetane benefit derived from 2-stroke oil in diesel. The 2-stroke oils used in the study were also analysed for metal content and high levels of zinc and other metals were found in the oils tested.
The study also included engine dynamometer testing using a modern common rail passenger car diesel engine. Engine performance and emissions were compared under laboratory conditions. Common rail injector fouling tests were also run to compare low sulphur diesel to the same fuel dosed with 2-stroke oil.
The results of the study support a view that the practice of dosing diesel with 2-stroke oil is surprisingly ineffective in terms of lubricity and cetane improvements. Engine performance, fuel consumption and emissions were also unchanged; however the use 2-stroke oil in diesel is potentially harmful to modern diesel injection equipment. Trace amounts of zinc, an element which is found in most 2- stroke oils, are well known to cause injector nozzle fouling and the study measured high levels of injector fouling when the test engine was running on diesel dosed with 2-stroke oil. While the oil industry may not mind the additional revenue from the sale of 2-stroke oil with each tank of diesel, this study demonstrated that it is not in the best interest of the user to do so.