Lower viscous lubricants for fuel economy

Nowadays, there is pushing demand in fuel economy for passenger vehicles in order to fulfill legislative requirements for CO₂ emissions. This in turn leads to the introduction of ultra-low viscosity lubricants which could be one efficient way for lubricants to contribute to the fuel economy performance of passenger cars by reducing shear forces. However, a decrease in lubricant viscosity will result in thinner oil films and thus it will be more difficult for the oil to keep the loaded contacts efficiently apart from each other. This may imply a transition from full film to mixed lubrication with potentially accelerated wear rates and locally increased friction. In order to avoid detrimental impacts on the engine and all its components it is necessary to add for example extreme pressure (EP) or anti wear (AW) additives which have the ability to form friction and wear reducing tribolayers. However, the prediction of the forming tribolayers is highly complicated by operating conditions such as load, speed, and temperature. For that reason, additives are used in high concentrations to guarantee the formation and durability. In this context, zinc dithiophosphates (ZDDPs) are widely used as lubricant additives. The triumphal procession of ZDDP in the automotive industry already started more than 70 years ago. ZDDP acts as an anti-wear agent, antioxidant, and corrosion inhibitor by decomposing peroxide and effectively destroying peroxy radicals.

Alternatives need to be researched

Apart from the abovementioned positive effects of ZDDP as a lubricant additive, it is well known that sulphur and phosphorous oxides as well as ash may negatively affect the effective life of exhaust catalysts. Additionally, due to phosphorous and sulphur limits in engine oil specifications, it is considered to progressively reduce the usage of ZDDP or even to replace it. A successful reduction or replacement certainly needs a thorough understanding of the governing mechanisms.