Lube-Tech
3.3 Volatility From the beginning of engine oil evolution, viscosity over volatility is seen as the determining characteristic during oil selection. This argument refers to how quickly engine oil vaporises. During the development of early engine oils, volatility did not pose an issue due to their extremely heavy weights. However, the introduction of reduced viscosity altered the relationship between viscosity and volatility quickly. Emission after treatment systems were also absent from early engines. As such, the emissions produced were determined by the lubricants utilised. Engine oils with reduced viscosity generated lower viscous resistance within the engine, leading to improved ICE efficiency and reduced fuel consumption. With the increased reduction of emissions and fuel consumption developing problems for engine oil the engine would eat per mile of operation, volatility had to be reduced to limit engine oil consumption loss from high temperature evaporation and prevent changes occurring in the properties of the oil. Although less viscosity helped to reduce fuel consumption, more vapour was found to be produced by the engine. This indicates an inverse relationship between low viscosity along with higher volatility - raising concerns about increased evaporation loss [23]. Henceforth, a method to determine optimal volatility for engine oil performance, given a lubricant’s viscosity, became a necessity.
3.4 Temperature: Viscosity vs. Volatility Due to this rapid and massive reduction in engine oil weights, base oil viscosities, and an increase in oil volatility, new methods were developed to discern oil volatility that departed from the traditional practice. One such method known as the NOACK volatility test, was adopted by the American Petroleum Institute’s engine oil category requirements for Original Equipment Manufacturers.
The NOACK test involves trials of heating engine oils in an evaporating crucible. As such, the crucible is
30 LUBE MAGAZINE NO.179 FEBRUARY 2024
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.150 page 4
closed by a corresponding screw cover. A small hole atop the crucible allows samples to volatilise freely. The crucible is then heated to 250 °C for an hour, and the final percent volatility loss is recorded using the weight of the empty crucible, crucible with sample, and crucible post-trial. Evaporation loss is obtained from the difference in weight of these variables: ((B - A) - (C - A)) x 100%; where A = empty crucible weight, B = crucible plus sample weight, and C = crucible plus sample after one hour of heating.
It is worth noting that the results obtained from the NOACK are only a modeling estimate of the evaporation component of oil consumption. It is only a reference which can be indicative of how the engine could act in given conditions but not necessarily as actual correlatable performance in an engine. In normal engine operation, high temperatures will typically drive off the lighter ends of a lubricant while in service - resulting in the increased viscosity and thickness of lubricants, leading to reduced fuel economy. A lower NOACK number means less oil consumption due to evaporation when the engine oil gets hot [24].
The industrial standard limit for percent volatility loss by the NOACK method is about 15%, as seen in Figure 2. However, this value has been lowered in recent decades to 13% and lower as various companies used differing procedures and instruments for this method, each of which has different specifications and challenges. Each synthetic oil passes the NOACK standard limit for percent evaporation loss (below 15%) with varying degrees of volatility conservation. As observed in Figure 3, multiple leading brands have differing limits when compared with each other, where the lower the limit is better as it grants the most wear protection [25]. In Figure 3, 0 - 15% represents the industrial standard range, with the green bar representing the ideal (the lowest) limit regarding the dataset. 16 - 20% gives a range of experimental error, and 21% onwards is too high and
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