
In this centenary issue of our newsletter the discussion topic is volatility – an important characteristic of lubricating oil that is often overlooked. Volatility is the tendency of a substance to evaporate since volatile matter has the capability to go into the vapor phase. In the case of oil this usually happens at elevated temperatures. Volatility has a critical impact on the performance of lubricating oils – low viscosity engine oils in particular – that are operating at high temperatures.
The base oil used to manufacture lubricating oil is a complex mixture of lighter (low viscosity) molecules and heavier (high viscosity) molecules. Low viscosity multigrade engine oil contains a larger percentage of lighter fractions. These lighter molecules are more volatile and vaporise (evaporate) first when the oil is heated, leaving the heavier molecules behind and the viscosity of the remaining oil is increased.
Many motorists do not realise that when they notice that their vehicle is “using” oil, they are often witnessing the effects of volatilisation. Motorists usually just buy extra oil to replace what they assume the engine has “used”, but is adding more oil necessarily the best solution to the problem? What they are doing by adding more oil, is burning up their money. Extra oil is not the only expense motorists face when dealing with volatilisation. As motor oil goes through the process of volatising, the chemically lighter (or more volatile) portions are always the first to “boil off”. This leaves the heavier, less pumpable portions behind. This heavier oil cannot be relied upon to flow easily and quickly to all engine components. The result is decreased fuel efficiency, premature component wear and deposit formation within the engine. The expense to the motorist can be quite substantial. It is therefore no surprise that concern about engine oil volatility has increased over the last number of years.
The following factors have a major influence on the performance of engine oil:
LOWER VISCOSITY ENGINE OILS
Vehicle manufacturers are very concerned about fuel economy. Thinner, low viscosity oils are being recommended to reduce fuel consumption. Traditionally SAE 20W-50 and SAE 15W-40 multigrade engine oils were used, but today many manufacturers recommend oils as thin as SAE 5W-30 and even SAE 0W-20 to improve fuel efficiency.
HIGHER ENGINE TEMPERATURES
Many modern internal combustion engines are turbocharged and have higher compression ratios. Oil sump capacities are reduced to minimise engine size and weight. To aggravate matters airflow around the engine is reduced due to better aerodynamics. All these lead to increased engine oil operating temperatures.
EXTENDED DRAIN INTERVALS
Engine oil volatility strongly effects drain intervals. Many manufacturers have introduced oil service intervals of 30,000 km or two years (whichever comes first) for petrol engines and 50,000 km for light duty diesel engines. Drain intervals for heavy duty diesel are typically between 80,000 and 120,000 km. These extended drain intervals lead to increased stresses on the oil.
ADDITIVE VOLATILISATION
The most widely used and effective anti-wear and anti-oxidation additives in engine oil contain phosphorus that can partially volatilize during high temperature engine operation. Volatile phosphorus in the exhaust stream degrades the function of the exhaust catalyst in reducing air pollution. Early studies have indicated that phosphorus volatility is not related to base oil volatility or to phosphorus content in the unused engine oil. More recent research has shown that phosphorus additive chemistry is the main contributor to catalyst poisoning and major additive suppliers have since come up with improved phosphorus technology to minimise additive volatilisation.
Based on the above it is evident that the volatility characteristics of the base oils used to manufacture low viscosity engine oils are key to the performance and longevity of the final product. The Noack Volatility Test is a critical measure of lubricating oil quality. There are several variants of the test, but in principle they all determine the evaporation loss of oil at high temperatures.
In the most commonly used Noack Volatility Test (CEC L-40-A-93, ASTM D5800 and DIN 51581) a 65 gram sample of oil is heated for one hour at 250°C while a controlled stream of air carries the volatile components away. Results are reported as a percentage of the mass an oil has lost during the test – the lower the percentage, the better, as this demonstrates resistance to oil volatility and breakdown. There are no formal Noack Volatility limits or specifications for lubricant base oils, but experience has confirmed that a maximum limit of 15% allow engine oils to perform satisfactorily for long periods at elevated temperatures.
Q8Oils offer a comprehensive range of multigrade engine oils blended with high quality base oils and the latest additive technology to minimise volatilisation. For more information about the complete range of Q8 lubricants, phone 011 462 1829, email us at info@bcl.co.za or visit www.bcl.q8oils.co.za.