High Temperature High Shear Viscosity of Engine Oils
What it Means to Your Engine
High temperature high shear (HTHS) viscosity of engine oils is a critical property that relates to the fuel economy and durability of a running engine. The drivers behind lowering HTHS viscosity are new global governmental regulations to improve fuel economy (FE) and lower greenhouse gases (GHG) in new vehicles. Lower HTHS viscosity tends to improve FE and lower GHG but higher HTHS viscosity affords better wear protection so a careful balance must be found when formulating an engine oil. Sufficient HTHS viscosity is critical in preventing engine wear in the critical ring/liner interface area by maintaining a protective oil film between moving parts. One method used to measure HTHS viscosity is ASTM D4683. Oil is introduced between a rotor and a stator at the test temperature of 150°C. The rotor experiences a reactive torque to the oils resistance to flow (viscose friction) and this torque response level is used to determine the HTHS viscosity. HTHS viscosity by ASTM D4683 has been found to relate to the viscosity providing hydrodynamic lubrication in light duty and heavy duty engines. HTHS viscosity has also been found to relate to fuel economy. Think of the protective oil film as if you are trying to swim. If the film is too thick like molasses you can barely move and have to expend a lot of energy; too thin and you sink to the bottom. What you want is the right balance of support and ease of movement. The oil has to be thick enough to maintain separation of the critical moving parts but thin enough to allow for fuel efficient operation.
A new heavy-duty engine oil category, PC-11, is currently under development. PC-11 may be split into two categories one with HTHS viscosity less than 3.5 and one equal to or greater than 3.5. PC-11 oils with HTHS viscosity <3.5 would offer FE benefits but would be restricted to engines designed to run on lower HTHS viscosity oils. This may exclude many older engines. Engine manufacturers are evaluating their hardware to see if engine durability, especially for ring and liner scuffing is an issue with low HTHS viscosity oils so they can take advantage of the engine lubricant delivering improved FE to meet the new US Environmental Protection Agency (EPA) and National Highway Traffic Safety Administration (NHTSA) FE and GHG requirements. Engine manufacturers may have to redesign their engines to take advantage of the potential fuel savings of low HTHS viscosity oils. PC-11 oils with HTHS viscosity equal to or greater than 3.5 would cover the heritage fleet and new engine requiring higher HTHS viscosity for wear protection.