Why are Low HTHS Viscosity Specifications Being Introduced?

Why are Low HTHS Viscosity Specifications Being Introduced?

May 29, 2019

Original Equipment Manufacturers (OEMs) are moving to lower high temperature high shear (HTHS) viscosity engine lubricants for their latest engine hardware designs to deliver greater fuel efficiency.

The formal requirement for more fuel-efficient heavy-duty engine lubricants has been on the horizon as far back as 2011 when OEMs requested a new category be introduced by the American Petroleum Institute (API). This resulted in the introduction of API FA-4 as the first fuel efficiency category with first licensing from December 2016.

How Industry Specifications Have Evolved

During the 2016 category upgrade, API CK-4 was introduced to reflect the upgraded performance benefits beyond API CJ-4 for engine lubricants with a minimum HTHS viscosity of 3.5cP. New API CK-4 lubricants must pass more stringent oxidation and aeration limits with increased shear stability, providing greater protection for heavy-duty diesel engines.

The API FA-4 fuel efficiency category requires lubricants to pass the same performance tests as API CK-4, with the addition of a HTHS viscosity limit of between 2.9 and 3.2cP, to deliver increased fuel efficiency benefits whilst maintaining engine protection.

Given the increasing drive to lower HTHS viscosity lubricants by European OEMs, two future ACEA fuel efficiency categories have been proposed, potentially called ACEA F8 and ACEA F11. These new categories will be low HTHS viscosity variants of the upgraded ACEA E6 and ACEA E9 categories respectively and will have limits between 2.9 and 3.2 cP in line with API FA-4. The ACEA introduction date is expected to be in 2020.

Evolution of OEM Specifications

Mercedes-Benz was one of the first European OEMs to introduce a low HTHS service-fill specification, MB 228.61. As well as having increased durability protection requirements, the specification has more stringent fuel economy limits along with the requirement to meet the API FA-4 approval with a HTHS viscosity of 2.9-3.2 cP. Backward compatibility exists on specific engines, allowing fleets the opportunity to benefit from fuel efficiency gains from their existing vehicles.

Other OEMs have also introduced low HTHS viscosity specifications to cover specific on-highway diesel engines including Cummins CES 20087 and Detroit Diesel DFS 93K223.

Our View

Moving to high performing low HTHS viscosity engine lubricants is complex. New additive technology in combination with performance polymers and performance base oils is essential in enabling the move to low HTHS viscosity formulations to meet the latest OEM performance requirements and provide value added fuel saving opportunities to the modern commercial vehicle fleets.

Given the focus on greater fuel efficiency and reduced greenhouse gas emissions, it is realistic to expect OEMs to continue to target the introduction of new performance specifications below 2.9cP without compromising engine protection.

For more information on HTHS engine lubricants, contact your Lubrizol representative.


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