High Temperature High Shear Viscosity
High Temperature High Shear Viscosity of engine lubricants is a fluid property that relates to the viscosity requirements of an operating engine. The regions in an engine where HTHSV are of particular importance are crankshaft bearings, camshaft bearings, and piston ring – cylinder wall contact. The shear rate in the bearings is on the order of 106 s-1 while piston ring – cylinder wall contact shear rates are believed to be greater than 107 s-1. ASTM and SAE archives contain a many papers on the significance of HTHSV to engine operation, shear rate and temperature. Many of these papers were published in the 1980’s and early 1990’s. It should be noted that the relationship between HTHSV and engine performance have a dependency on engine design. Some of these design criteria are oil flow rate, bearing clearances, and finish of contact surfaces which have likely changed since the correlation work of the 1980’s.
The first instruments to measure HTHSV wvbnere research laboratory instruments. In a research laboratory, the easiest instrument to make is a capillary viscometer as it has very few moving parts. These research instruments were incorporated into ASTM D4624-93 (1998) Standard Test Method for Measuring Apparent Viscosity by Capillary Viscometer at High Temperature and High-Shear Rates which was discontinued in 2004. It is available from ASTM by special order and is an excellent reference for building a capillary HTHSV instrument.
ASTM D4624 was followed by the development of rotational instruments described in ASTM D4683, Standard Test Method for Measuring Viscosity at High Shear Rate and High Temperature by Tapered Bearing Simulator (TBS) and D4741 Standard Test Method for Measuring Viscosity at High Shear and High Temperature by Tapered Plug Viscometer (TPV). ASTM D4741 is technically equivalent to European Method CEC-L36-90. The TBS and TPV instruments were developed in parallel. The Tapered Bearing Simulator HTHSV was primarily developed by ASTM D02.07 members while the Tapered Plug HTHSV was primarily developed by members of CEC. There are some differences in the way the two instruments operate.
Why we need to know?
- 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.
- 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.