The effect of inorganic fullerene-like (IF) WS2 nanoparticles added to mineral oil which has a potential to reduce engine friction was investigated using a reciprocating piston ring - cylinder bore bench test system. The test system simulates actual engine conditions controlling oil temperature, speed, and normal load. Frictional characteristics of various IF-WS2 nanoparticle concentrations in mineral oil were experimentally analyzed. After a certain period of running in mixed lubrication regime, a significant reduction in friction coefficient was observed when the IF-WS2 formulated oil was used. This reduction remains to some extent with the reference mineral oil even after solvent cleaning. The results show that a thin tribofilm gradually forms on the piston ring and the cylinder bore surfaces that reduce the coefficient of friction in mixed lubrication regime. The effects of lubrication regime, load, speed, temperature and surface roughness on tribofilm formation were also discussed.

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