The amphiphilic qualities of plant oils that emerge from their fatty acid makeup lead to improved lubricity. To produce diesel that is friendlier to the environment requires severe hydrotreating, unfortunately the same process also reduces the lubricity of the fuel. Adding petroleum based lubricity additives to the fuel may improve diesel lubricity but also present the challenge of being environmentally unfriendly or expensive. Plant oils such as castor oil, moringa oil and canola oil offer a green and relatively inexpensive approach to lubricating hydrotreated diesel fuels. This work reports on lubricity tests carried out on diesel fuel treated with castor oil, moringa oil and canola oil as lubricity enhancers to investigate lubricating abilities of the resulting mixtures. The high frequency reciprocating rig (HFRR) which is generally accepted as a universal test apparatus for determining the lubricity of diesel fuels was used. The diesel fuel was treated with these plant oils up to a concentration of 1 % (w/w). The friction coefficient (COF) and the amount of wear that occurred was measured and the wear scar surfaces were also evaluated. Factors that influence wear were also taken into consideration. The following findings were made from this study: • Castor oil, moringa oil and canola oil can function as diesel lubricity enhancers. Increase in plant oil concentration in diesel stimulated a stable film formation and improved the tribo-characteristics of the fuel. • A concentration of less than 1 % (w/w) of any of the three plant oils investigated in diesel (i.e. castor oil, moringa oil or canola oil) was enough to cause the wear scar diameter (WSD) of the fuel to reduce appreciably. As reference, the wear scar diameter (WSD) of the untreated diesel fuel after HFRR tests was 897 μm. However, at 1 % (w/w) of each plant oil in the diesel fuel, the observed reduction in WSD was 221.2 μm (castor oil), 339.06 μm (moringa oil), and 281.84 μm (canola oil). • Castor oil has comparatively better lubrication in diesel than moringa oil and canola oil owing to its hydroxyl functional group that increases both the viscosity and polarity of the oil. A concentration of less than 0.2 % (w/w) of castor oil in diesel was enough to bring the WSD of the fuel down to below the maximum allowable limit of 460 µm as specified in ISO 12156. • Viscosities of castor oil, moringa oil and canola oil have a significant effect on performance as a lubricity enhancers. • Friction coefficient for diesel treated with castor oil, moringa oil and canola oil increases with increasing temperature because at high temperatures, the lubricant film formed by fatty acids tends to be less stable and breaks down more easily. As expected, increase in temperature also causes an increase in wear, for all three mixtures. • Lubricating ability of moringa oil is much lower compared to castor oil and canola oil. Mild to severe abrasive wear was observed for diesel treated with moringa oil. • Moisture in the atmosphere affects the repeatability of the friction and wear tests. This occurs because of the formation of an oxide layer on the metal surface that involves water. Upon keeping a close humidity range of 50 % - 55 % in the test chamber an improvement in repeatability of the test results was observed. Keywords: plant oil, biodegradability, diesel fuel, lubricity enhancer. / Dissertation (MEng (Chemical Engineering))--University of Pretoria, 2021. / Chemical Engineering / MEng (Chemical Engineering) / Unrestricted