Frictional Characteristics of Crystalline Calcium Sulfonate Detergent in Engine Oil by Mini-Traction Machine 2014-01-2789

Harsh emission control regulation restricted the sulfated ash, sulfur and phosphorus (SAPS) level in passenger car motor oil (PCMO), thus lubricant industry need to find new additive to partially or wholly replace Zinc dialkyldithiophosphate (ZDDP), which has been used as an antioxidant and anti-wear agent for several decades. Overbased crystalline calcium sulfonate (CCS) detergent comprises calcite calcium carbonate and this structure might be useful to improve the anti-wear property of engine oil in severe lubrication condition, especially for PCMO with lower SAPS level.

Frictional characteristics were studied between overbased amorphous calcium sulfonate (ACS) detergent and CCS and their interactions with dispersant and ZDDP by Mini-Traction Machine, which is often used to measure the Stribeck Curve of lubricant. In poly alpha-olefin base oil, both the two detergents showed lower traction coefficient in boundary lubrication (BL) regime, and higher traction coefficient in mixed lubrication (ML) regime than that of the base oil itself, and the traction coefficient of CCS was higher than that of the ACS. With 0.5% high molecular weight dispersant (HMWD) existing in the oil, the traction coefficient of ACS increased significantly in BL regime, while that of CCS decreased dramatically in ML and elasto-hydrodynamic lubrication (EHL) regimes. The traction coefficient of CCS in BL regime raised gradually by increasing the treat level of dispersant, and the same phenomenon was also shown for ACS, which indicated that HMWD has some antagonistic effect on the anti-wear property in engine oil. In addition, introducing 1% ZDDP in the 5% dispersant and 1% detergent system, the traction coefficient of CCS reduced remarkably in BL regime and increased evidently in ML and EHL regimes, but the coefficient of ACS changed little comparatively. In fully formulated engine oil, the traction coefficient of the two detergents was similar to the results when they blended with HMWD and ZDDP. In summary, the traction coefficient of oil with CCS can be reduced in BL regime, but it also induced the increase of traction coefficient in ML and EHL regimes. It should be carefully balanced in fully formulated engine oil.