The drive to reduce emissions and increase fuel efficiency has led to significant changes in heavy-duty engine hardware design, including downsizing, down speeding and stop/start technology to name a few. With such changes in engine hardware design, engine lubricants are increasingly having to perform in high temperature high shear (HTHS) environments within the engine—notably in the camshaft, bearings as well as the piston rings and cylinder liners.

Today, HTHS viscosity—or specifically “low” HTHS viscosity—is a phrase fast becoming associated with increased fuel efficiency. The focus on HTHS viscosity is gaining momentum as it is regarded as the parameter of the viscosity classification that more accurately mimics the engine lubricants behaviour in increasingly severe operating conditions.

HTHS dynamic viscosity is the current industry standard that best predicts fluid behavior in HTHS environments. It is designed to ensure engine lubricants maintain sufficient film strength to prevent excessive wear under engine operating conditions, while also allowing engine lubricants to have low enough viscosity to deliver fuel efficiency and emissions benefits. HTHS viscosity measures the temporary viscosity loss under these conditions:

The lower the measured number, the lower the HTHS viscosity of the oil and the higher the anticipated fuel efficiency benefits.

HTHS viscosity is measured in milliPascal.second (mPa.s) but is also commonly referred to in centipoise (cP).

Typically, heavy-duty diesel engine lubricants have had a minimum HTHS viscosity of 3.5 cP. Lower HTHS viscosity is being pursued by an increasing number of original equipment manufacturers (OEM) to move significantly below this long-established 3.5 cP HTHS limit in conjunction with modern engine hardware designs.

Lower HTHS viscosity engine lubricants are complex and require formulation changes over higher HTHS viscosity engine lubricants. The vital roles of the additive package, performance polymer and base oil mean they must work in harmony with each other, while maintaining durability and delivering improved fuel efficiency.

Learn more about HTHS viscosity in this video.