is HTHS and measures how easily molecules within the fluid slide past each other under the HTHS conditions of a warm, working engine. Lower HTHS viscosity heavy duty engine oils lose less energy when shearing under these conditions and data demonstrates that HTHS is strongly correlated with fuel economy.
and also helps to reduce potential issues with base oil availability. Solubility and compatibility issues could also arise with the inclusion of PAO, necessitating the need for more expensive componentry, which could also drive up the cost of the SAE 0W-20 finished fluid. On a practical note, low HTHS SAE 5W-20 heavy duty engine oils may also be more likely to be used beyond the vehicle’s warranty period, to retain the proven fuel economy and drain interval benefits that will offset the higher cost of the engine oil. By contrast, the incremental fuel economy benefits of a similar HTHS SAE 0W-20 formulation may not outweigh its additional cost.
Figure 1: Afton data demonstrates a linear relationship between fuel economy improvement and HTHS viscosity.
To support new hardware designs, OEMs are recommending heavy duty engine oils with lower HTHS viscosity – typically 2.6 mPa.s – but some are looking even lower. An HTHS of 2.6 mPa.s can be achieved with different viscosity grades: either a SAE 5W-20 or a SAE 0W-20. The difference in fuel economy performance between these two grades will not be evident in operation, as they present the same HTHS with the impact at low temperature at cold engine start being a fraction of operating time.
Friction modifier additives have been hailed as a potential solution to reduce fuel consumption, and minimising energy lost through friction would indeed be expected to improve engine efficiency. However, friction modifiers can get consumed quite quickly over time and only show any clear benefit in fresh oil performance. With a greater focus on real driving emissions, it is important to formulate a resilient heavy duty engine oil with performance that does not diminish before the next planned oil change.
Afton’s research with Commercial Fleet Managers consistently shows the importance of cost-optimisation and efficiency being a key focus to drive down total cost of ownership. As they work with small margins, fleet managers are reliant on additive companies and oil marketers to drive heavy duty engine oil formulation in a pragmatic way. The pressure on OEMs to meet 2025 targets could see HTHS viscosity moving even lower, but that does not have to mean ever-lower kinematic viscosity grades and the challenges these bring for engine wear and total cost of ownership.
Figure 2: Afton data demonstrates no fuel economy improvement moving from SAE 5W-20 to 0W-20 with the same additive package and comparable HTHS viscosity.
The optimal solution for an end user will depend on a number of factors.
Firstly, SAE 0W-20 heavy duty engine oils are usually more costly than SAE 5W-20 oils as they usually require a higher polymer content and the inclusion of PAO or Group III+ basestocks to aid their low temperature flow properties. By contrast, a SAE 5W-20 oil can be formulated with less viscosity modifier and using only Group III basestocks, which keeps heavy duty engine oil formulation costs down
Is moving to a SAE 0W-20 low HTHS fluid good for fuel economy, or a false economy? Afton Chemical believes that the most efficient solution is a SAE 5W-20 low HTHS fluid, delivering credible fuel economy benefits without compromising engine durability, supporting longer OEM-recommended drain intervals, and striking the right balance between heavy duty engine oil performance and cost.
www.aftonchemical.com
LUBE MAGAZINE NO.177 OCTOBER 2023
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