the dominant viscosity grades for engine oils have traditionally been above 3.5 centipoises (cP) in terms of high-temperature high-shear (HTHS) viscosity. This includes premium grades like 5W-40 and mid-tier grades like 10W-40. However, in recent times, there has been a shift towards lighter viscosity oils. The viscosity grade 5W-30, with an HTHS viscosity of 2.9 cP, has gained traction and witnessed significant growth in the European market. This shift can be attributed to the increasing pressure to reduce greenhouse gas emissions. In response to this demand, original equipment manufacturers (OEMs) are now moving towards even lighter lubricants with an HTHS viscosity of 2.6 cP or lower. The transition to lighter lubricants is driven by the aim to improve fuel efficiency and reduce emissions. By using lower-viscosity oils, engines can experience reduced internal friction, resulting in better overall performance and lower fuel consumption. This trend reflects the industry’s ongoing efforts to meet stricter environmental regulations and promote sustainability in automotive applications.
However, there is a shift expected among German carmakers, including Daimler, BMW, and Volkswagen, who are anticipated to introduce original equipment manufacturer (OEM) specifications that include SAE 0W-16 and 0W-12 oils. SAE 0W-20 engine oils that meet ILSAC (International Lubricant Standardization and Approval Committee) standards can be formulated using conventional Group III base stocks with Noack volatility of up to 15%. However, as the industry moves towards lubricants that meet the performance requirements of both passenger car and diesel engine oils, such as those specified by the European Automobile Manufacturers Association (ACEA) or GM’s dexos1, the allowed Noack volatility becomes more restricted. Both ACEA and dexos1 standards require oils to have a Noack volatility no higher than 13 percent. In some cases, original equipment manufacturer (OEM) specifications can be even more stringent, imposing stricter limits on oil volatility.
To improve the sustainability of commercial fuels and reduce friction, researchers have suggested decreasing their viscosity. However, research over the years shows that there is an inverse relation between oil viscosity and volatility which results in loss of oil during operation. Henceforth, to maintain steady performance, it is essential that both base oil and blend components have strong volatile properties, which can be assured through various volatility tests that meet
API oil guidelines. Noack Volatility Test indicates that volatility >15% is too high and will most likely not pass crucial oxidation tests, including the IIIG engine test. Additionally, both General Motors and European Automobile Manufacturer’s Association (ACEA) have their own specifications for the maximum Noack volatility, which should be followed. It is also worth noting that the volatility of engine oils is generally lower than the volatility of their base oils, and thus the Noack volatility of base oil is rated at 2% higher than the engine oil in Europe.
The addition of Polyalphaolefin (PAO) and esters in the original oil blend might be a viable option. PAO is a synthetic hydrocarbon (SHC) that is able to mimic the best hydrocarbon (branched) structure found in mineral oils. Due to its controlled structure, there is no small, volatile hydrocarbon present which decreases volatility and creates less hydrocarbon tailpipe emissions [8]. Dodecene-based PAOs, in particular, offer exceptional Noack volatilities; dodecane-based PAOs contain a 36-carbon atom component as the lightest material, which is about 6 carbon atoms heavier than traditional decene-based PAOs. Using dodecane-based PAOs or other low-volatility base oils can reduce oil loss due to volatilisation and maintain ideal viscosity, ultimately improving engine performance and efficiency.
Figure 2: Stability of PAO
One of the other proposed solutions includes esters which are a monolithic class of Group V base oils with exceptional properties like high oxidative stability and low volatility. Group V base oils (esters) can be formed from byproducts of Group IV base oils, with esters having dynamic viscosities ranging from around 5 Pa∙s to around 0.001 Pa∙s as the temperature increases. However, only esters derived from an oct-1-ene dimer could serve as potential Group V base oils due to volatility concerns with other esters at certain temperatures. Esters also aid additive solubility and elastomer seal swell, which
Continued on page 20 LUBE MAGAZINE NO.175 JUNE 2023 19
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