HTHS Viscosity (ASTM D4683) is a critical metric for understanding engine lubricant performance under extreme conditions, with implications for formulation, wear protection, and efficiency.
The Criticality of Testing Conditions
– 150°C: Simulating Engine Reality: Modern engines often exceed operating temperatures of 100°C. The 150°C temperature in HTHS testing emulates real-world scenarios, ensuring lubricants maintain protection.
– 10^6 s-1 Shear Rate: Inducing Temporary Loss: This extreme shear rate induces two key phenomena:
* Shear-thinning: Lubricants, especially those with viscosity modifiers (VMs), see their viscosity temporarily decrease due to the alignment of polymer chains.
* Threshold for Potential Permanent Loss: This shear rate marks a point where oils can begin to experience irreversible viscosity reduction, impacting their long-term performance.
The Nuances of Shear-Dependent Behaviour
– Newtonian vs. Non-Newtonian: Under low shear, many lubricants act as Newtonian fluids (constant viscosity). HTHS focuses on the non-Newtonian behaviour exhibited at high shear:
* Shear-Thinning: The most common behaviour, where viscosity decreases with increasing shear rate, offering fuel economy benefits.
* Second Newtonian Plateau: At extremely high shear, viscosity may stabilise again, providing a boundary condition for modelling lubricant behaviour.
Essential Implications for Engine Oils
– The HTHS Balancing Act: Formulators must strike a delicate balance:
* Protection: HTHS must be sufficient to maintain a robust oil film, especially in critical areas like bearings and camshafts, to prevent wear.
* Efficiency: Lower HTHS can reduce viscous drag within the engine, contributing to improved fuel economy.
– Viscosity Modifiers in Focus: VMs are essential to achieving multi-grade oils. HTHS highlights their behaviour under high shear stress. It is imperative that VMs provide the desired viscosity profile while minimising the risk of permanent viscosity loss.
– Market Standards and Used Oils: HTHS plays a key role in industry specifications (e.g., ACEA, API). Meeting minimum HTHS thresholds, often between 2.6 and 3.5 mPa·s demonstrates a lubricant’s fitness for various engine applications. Additionally, assessing HTHS in used oils is crucial for evaluating lubricant health and the potential for viscosity degradation over time.
Advancing Lubrication Technology
At Valorem, we understand the complexities of optimising HTHS performance. Our expertise in lubricant formulation and insights from our valued partners like International Petroleum Products and Additives Company, Inc., allows us to create finished fluids that meet and exceed HTHS requirements. With our advanced solutions, we help manufacturers develop cutting-edge engine oils that deliver superior wear protection and enhanced fuel efficiency.
Menu