Low HTHS oil: what it is and why it matters.

Oils & More

Read time: 3 min

Pubblicato il: 27 November 2023

Low HTHS oil: what it is and why it matters. How to maximize performance while reducing consumption

The new emission limits regulations have forced engine manufacturers to upgrade the technologies they use and introduce systems that reduce pollutant emissions. One of the parameters that has been worked on to achieve this result is the increased efficiency of the entire power train system, i.e. “FE - Fuel Economy” since, the less fuel is burned, the lower the exhaust emissions.

Of course, the lubricant has evolved to meet the new performance requirements and the upgrades have involved both the chemical composition of the lubricant and its physical characteristics and viscosity. Engine lubrication is mostly hydrodynamic, so the lubricant has a high viscosity; this ensures greater surface protection, but on the other hand results in a loss of power due to the internal friction of the lubricant's molecules. It is therefore necessary to optimise the viscosity of the oil to ensure proper performance.

Engine oil is subjected to high temperatures and high surface sliding speeds. Both of these conditions heavily affect the viscosity of the oil and it is not possible to rely solely on the viscosity class (e.g. SAE 5W-30 or SAE 10W-40). It is therefore necessary to have a measuring system that is able to simulate the real operating conditions in the best way possible. This includes the HT-HS (High Temperature- High Shear) parameter, which is measured using a specific internationally-standardised instrument, in accordance with Standard ASTM D4683.

Measurement of the viscometric profile to evaluate the HTHS

Until recently, hot viscosity was measured up to 100°C, an unrepresentative value when compared to the effective service temperature of current vehicles. It was therefore decided to measure the viscosity at 150°C and under high shear stress, in order to be able to evaluate the capacity of the lubricant in real operating conditions.
It is essentially a rotational viscometer at 3,500 rpm and at a temperature of 150°C.
These two parameters represent the typical service conditions. The value obtained has become an essential marker for the definition of the “Fuel Economy” of lubricants and it is included in the international API and ACEA specifications.

For traditional applications, especially HD (Heavy Duty), the threshold protection value is > 3.5cP, whereas for latest generation engines, manufacturers are recommending values < 2.9cP.

So, is it true that a lubricant with a low HTHS saves petrol? Yes, but if used in the right engine

LOW HT-HS = THIN FILM OF LUBRICANT = LESS VISCOUS FRICTION BETWEEN THE MECHANICAL PARTS = FUEL SAVINGS

Is the engine designed for low or very low viscosity fluids? If the answer is NO, choose products with standard viscosity otherwise the bushings may be damaged. If the answer is YES, choose Fuel Economy or Ultra Fuel Economy products based on the recommended specifications.

It is very important to point out that engines that require high HT-HS values may experience early wear and breakage if Fuel Economy oils are used, as a sufficient protective lubricating film is not guaranteed. Conversely, engines that require low HT-HS oils can also run with high HT-HS oils. It is always advisable to check the specifications in the vehicle's use and maintenance manual.

Low HTHS oil: what it is and why it matters.