HTHS: High-Temperature, High-Shear
Within highly loaded bearings, engine oil has a temporary loss in viscosity and ASTM D-4683 is test method which is thought to be representative of the condition encountered in the bearings of automotive engines in severe service. Standard SAE J300 places minimum requirements for HTHS for each viscosity grade of engine oil. Generally, the higher the HTHS rating of an engine oil, the better the protection of journal bearings. HTHS can be thought of as a measure of the Viscosity Index Improver (VII) capability of an engine oil.
VIIs are polymer additives in modern engines oils that raise the Viscosity Index of the base oil. These viscosity modifier polymers are relatively large molecules compared to the base oil molecules. VIIs are coiled up when cold but uncurl with heat and the VII polymer expansion causes the oil to have a higher viscosity.
Under hydraulic shear conditions (as found in journal bearings in connecting rods or behind piston rings) the polymers tend to contract and/or align with the flow, thereby temporarily deforming and losing some of their viscosity improving effect. This is known as Temporary Viscosity Loss (TVL).
VIIs can experience Permanent Viscosity Loss (PVL) due to mechanical shear because the polymers are permanently damaged. This happens when the oil passes through cavitation zones in the engine or through highly loaded gears. PVL can also occur from thermal degradation or oxidation. As the molecular weight of the VII polymers decrease, they become more shear-stable and its PVL stabilizes. However, after reaching a viscosity minimum from VII PVL, oxidation of the engine oil tends to cause its viscosity to increase.
Generally, the less VIIs added to engine oil, the less permanent viscosity loss can occur, which is why straight grade engine oils (eg, SAE 30, SAE 40) were used in high temperature and/or high shear applications. Synthetic engine oils tend to have naturally higher viscosity indexes so they require less and sometimes no VIIs to achieve their multi-grade viscosity ratings.
The minimum SAE J300 -2015 HTHS ratings for engine oils are:
|SAE Viscosity Grade
||High Temperature, High Shear Rate
@ 150°C (cP)
|40 - Winter
(0W-40, 5W-40, 10W-40 grades)
|40 - Heavy Duty
(15W-40, 20W-40, 25W-40, 40 grades)