96 N O V E M B E R 2 0 0 5 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y
il change intervals for gaso-line and diesel engineshave been increasing grad-
ually in many countries around theworld. AMSOIL has been marketinglong drain oils in the U.S. for manyyears and has now been joined byExxonMobil Corp. in offering oil drainintervals up to 15,000 miles (24,100km) for gasoline engines.
In Europe, for cars built in 2000and later, Volkswagen has introducedoil drain interval specifications of30,000 km or two years (whichevercomes first) for gasoline engines and50,000 km or two years for light-dutydiesel engines. Many European spec-ifications for heavy-duty dieselengines now have oil drain intervalsof between 80,000 and 120,000 km.
One of the main factors that allowengine oils to function satisfactorilyfor such long periods is oil volatility.The less oil that volatilizes out of anengine, past the piston rings and intothe exhaust gases, the more oil staysin the engine, and the longer the oilretains its initial viscosity. Of course,oxidation and thermal stability of theoil is also very important.
All the new European and U.S.specifications for engine oils nowinclude limits for oil volatility, mostusually using the NOACK test (CECL-40-A-93, ASTM D5800). In this test,a sample of oil is heated for one hourat 250 C, and a controlled stream ofair carries away the volatile compo-nents. The test conditions areintended to replicate the conditionsaround the pistons and cylinder
walls in an engine.Another test used to measure
volatility of oils is gas chromato-graphic (GC) simulated distillation(ASTM D6417). This test estimatesthe volatility of an engine oil from thepercentage of the oil’s simulatedboiling range below a specified tem-perature. In the case of the latest APISM specification, the temperature is371 C, and the percentage of oil thatboils below this temperature must bea maximum of 10%. Additionally, thesame specification requires the oil’sNoack volatility to be less than 15%.In the latest European ACEA A3/B3,A3/B4 and A5/B5 specifications, anengine oil’s Noackvolatility must be lessthan 13%.
Although thesevolatility tests wereoriginally intended forengine oils, they arenow also being used tomeasure the volatilityof base oils. This, how-ever, presents a dilemma to manufac-turers of base oils, because thevolatility of an engine oil is likely tobe lower than that of the base oil(s)from which it is made, whichever testis used to measure the volatility.
In the case of the Noack test, partof the oil’s volatility is the result ofoxidation and thermal cracking, sothe use of oxidation inhibitors in theengine oil will lower the measuredvolatility. In the case of the GC test,the use of additives in the engine oilwill lower the percentage that boils
below the specified temperature bybetween 10% and 20%.
Two problems that arise for baseoil manufacturers are which volatilitytest to use and what volatility limit toaim for with each viscosity grade.Although there is a general correla-tion between Noack and GC testresults, the correlation is not linearbecause the GC test does not havean oxidation and thermal stabilitycomponent.
For example, oils that give an 8%volatility at 371 C in the GC test canhave Noack volatilities ranging from16% to 20%. Most manufacturers ofbase oils in Europe use the Noack
test in preference to theGC test, reflecting the useof only the Noack test inEuropean engine oil spec-ifications.
Current industry prac-tice in Europe suggeststhat the Noack volatility ofa base oil can be around2% higher than the specifi-
cation for the engine oil to ensuremeeting a formulator’s requirements.But are base oil manufacturers givingaway too much quality to engine oilmarketers? There does not appear tobe much published evidence to tellone way or another. <<
David Whitby is chief executive of Pathmas-ter Marketing Ltd. in Surrey, England. Youcan contact him at [email protected].
Worldwide
O
By R. David Whitby
Two problems thatarise for base oil man-ufacturers are whichvolatility test to useand what volatilitylimit to aim for witheach viscosity grade.
Volatility of engine oils
96 tlt worldwide 11-05 10/24/05 3:44 PM Page 96
