The internal combustion engine oil works under relatively severe conditions. For example, the frictional speed of the piston ring against the cylinder wall is very large (10 to 12 m / s at 3500 to 4000 r / min for large engines and 3800 to 4000 r / min for small engines (9 to 11 m / s), and the temperature range of the combustion chamber of the engine varies widely, from -35 ° C (winter start) to more than 2200 ° C (combustion). Therefore, it requires the following main properties:
Low temperature (-5 ～ -30 ℃) of engine oil generally requires starting viscosity in the range of 6000 ～ 3000 mPa · s; oil products can maintain the minimum kinematic viscosity of the oil film under high temperature and high speed (100 ℃), generally not less than 3.8mm2 / s . Otherwise, the oil film is easy to break and the sealing effect is not good. The viscosity of internal combustion engine oil should take into account both good high temperature and low temperature viscosity, that is, good viscosity and temperature. The viscosity-temperature performance of lubricating oil is expressed by the viscosity index (see Lubricating Oil). A high viscosity index indicates a small change in viscosity due to temperature changes. Generally, the single-stage oil viscosity index is between 90 and 105, and the multi-stage oil viscosity index is between 120 and 180.
② Oxidative stability
Dialkyl zinc dithiophosphate and other antioxidants are generally used in oil formulations to improve the antioxidant properties of oil products.
③ Purity and dispersion
On the one hand, it can inhibit the formation of oxidized colloids and carbon deposits; on the other hand, it must be able to wash the sediments and suspend them in the oil, so as to reduce the tendency of the piston coating film and carbon deposits. The detergency of internal combustion engine oil is mainly achieved by adding additives such as calcium sulfonate (magnesium sulfonate), calcium sulfide alkyl phenol, calcium hydrate and succinimide.
That is, the ability of the lubricant to maintain a tight and firm oil film on the metal surface. Generally adding oily agents such as sulfurized esters can improve the anti-wear properties of oil products.