The Effect of Alternative Fuels on Tribology

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As conventional fuels may run out within the next few decades, scientists and engineers have been looking to alternative fuels for many years, and many are now being used. There are a few different types of alternative fuel that have been created, with biodiesel and fuel alcohol being two of the most common.

However, how the tribology of these fuels interact with their surrounding environments can differ between fuels and differ to conventional fuels. In this article, we look at the effect that the tribology of these alternative fuels has on its surrounding environment.

What is Tribology?

Tribology is an area of science that is concerned with the interaction of surfaces in relative motion. Tribology is a multidisciplinary area that encompasses areas of friction, lubrication, contact mechanics, surface damage processes and surface optimisation. However the areas of friction, wear and lubrication are the most important. These areas are the most important because they are the properties which can be tailored to optimize the efficiency, performance and reliability of a tribological system.

Tribology comes into play when two or more surfaces come into contact with each other, and this can happen in a number of ways. When the two surfaces come together, the tribological interactions can take the form of frictional forces, surfaces adhering to each other, surfaces being kept apart from each other and stress or strain at a surface contact point. There are many applications, across many industries, that rely on the effects of tribology, and the tribology of fuels can play a big role in their usability and effectiveness.

Fuel Alcohol

Ethanol has established itself as a potential bio-based oxygenated renewable fuel, with a molecular weight that is roughly half of gasoline and a quarter of diesel. However, the much lower molecular weight and lower viscosity of ethanol presents very different tribological characteristics than conventional fuels.

Ever since its inception back in the 1980’s, fuel alcohol is known to be somewhat corrosive to combustion engines, but the level of corrosion is strongly dependant on the quality of ethanol used. The way in which the ethanol interacts with the internal components is very different to diesel and other commonly used fuels and can cause three types of corrosion – general corrosion, dry corrosion and wet corrosion.

These corrosions occur by different mechanisms based on how the ethanol interacts with the internal parts of an engine. General corrosion effects can often be seen when there are ionic impurities in the ethanol (which is why quality is important), such as chloride ions and acetic acid. On the other hand, dry corrosion is often caused by the polarization of ethanol when it is in contact with metal parts, especially those made of magnesium, lead and aluminum. Wet corrosion occurs when there is another liquid in the system (commonly water) and this azeotropic mixture causes oxidation of the metal components.

Whilst high quality ethanol shows some promise as a biofuel, efforts are needed to make sure that its corrosivity is minimized, and one way in which this has been achieved (with little to no corrosion) is by mixing ethanol with diesel. Whilst this is not a pure biofuel and still uses hydrocarbon fuels, it does mean that the amount of diesel used can be reduced.

Biodiesel

Unlike alcohol, biodiesel has many positive tribological effects. Biodiesel is a low-sulfur fuel, which is significant because many other low-sulfur fuels have caused engine parts to fail because of lubricity issues. On top of that, biodiesel is known to possess a higher lubricity than conventional fuels and can even increase the lubricity of other low-sulfur fuels when it is blended with them. The high amount of lubricity has been attributed to the high concentration of alkyl esters in biodiesel fuels.

However, there are still some negative tribological effects to biodiesel. Any components composed of copper and iron have been found to be more susceptible to wear, with statistics showing that wear on these parts could be increased by up to 67% and 272%, respectively. The reason for this increased wear is due to the biodiesel dissolving some of the other lubricants within the engine, which causes a higher friction coefficient on the moving parts, and therefore a greater amount of wear.

Sources:

  • Imperial College London: http://www.imperial.ac.uk/tribology
  • “Tribological Issues Related to the Use of Biofuels: A New Environmental Challenge”- Tandon A. et al, British Journal of Environment & Climate Change, 2011
  • “Lubricating Oil Tribology of a Biodiesel-Fuelled Compression Ignition Engine”- Agarwal A. K., ASME Internal Combustion Engine Division Spring Technical Conference , 2003, DOI: 10.1115/ICES2003-0609
  • “Experimental investigations of the effect of biodiesel utilization on lubricating oil tribology in diesel engines”- Agarwal A. K., J. Automobile Engineering, 2005, DOI: 10.1243/095440705X11239
  • “Biodiesel Performance within Internal Combustion Engine Fuel System - A Review”- Khan A. et al, Tribology in Industry, 2016

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