Effects of castor oil as lubricity enhancer on friction and wear characteristics of piston engine running on diesohol

Recently, ethanol has become acceptable for blending with diesel, in the so called diesohol, and is known to reduce particulate matter exhausted from diesel engines significantly. However, its poor lubricity is a concern for long-term application. Many efforts have been made to improve the lubricity of diesohol by combining additives into the ethanol-diesel blend. Castor oil is one of the most effective additives for enhancing lubricity and reducing the wear of engine components. However, the lubricating mechanism of the diesohol blended with castor oil has not been clearly reported. Therefore, in this research, the role of castor oil in lubricity enhancement was studied by varying the castor oil content in diesel from 5% to 15% v/v while retaining the ethanol content of 10% v/v. According to the ASTM D6079, the standardized lubricity test was performed using the high frequency reciprocating rig (HFRR). The fuel holder of the HFRR was modified to minimize the vaporization of the ethanol. After lubricity tests, the wear of the tested specimens was examined using a scanning electron microscope. The increase in lubricity of the blended fuel was found only up to the castor oil content of 10% v/v. At the blending amount of 10% v/v, the friction coefficient at the contact of the tested specimens decreased by 31%, while the mean wear scar diameter decreased by 25%. SEM imaging revealed delamination of the scuffed surface under diesohol lubrication without the castor oil blend. As the blending amount of the castor oil increased, the wear mode changed, and delamination was not found in the cases of the castor oil content of 10% and 15% v/v. For a long-term 220-hour engine test, diesohol blended with 10% castor oil was inferior for soot deposition and wear of metals that reduce the engine torque.