ok - it's a bit quiet, so - here goes ...
HISTORY OF ENGINE OIL Part One
Black and Slippery
Engine oil is slippery, it gets dirty, it drips on the garage floor, it often appears out of the exhaust pipe as black smoke, it is not welcome in the kitchen. Even non-drivers know that its presence has to be checked now and again, with dire consequences if it is forgotten. Oil is thought of as the life blood of the engine. We certainly know that without it, our engine becomes a heavy lump of scrap iron. But what exactly is engine oil? Is there any difference between supermarket GT-LeMans-F1 Supermulti-grade at £3.00 for 5 litres and Red Line® Race Oil at £15.00 for 1 litre? Are either of these suitable for my car engine? So much money is spent advertising certain brands of oil, they surely must be better?
We will examine the development of engine lubrication to attempt to discover if what we need for our car engine is actually what we are getting.
Very shortly after the wheel was invented, it was discovered that a smear of cooked animal fat on the axle made pushing a whole lot easier. But, which was better - roast deer fat or boiled pig? Tests were run - lubrication technology was born! Vegetable oils were also used in early times for lubrication, heating and lighting, but the lucky few were those who lived near the black gold deposits - crude oil. Mankind has been using mineral oil for thousands of years, but only in the last two hundred has it been so widely exploited.
Early cars naturally used the slippery by-products of crude oil from which their fuel was obtained to protect the sliding and rotating metal parts of the early internal combustion engines. Adjacent moving metal parts require an oil film between them to prevent seizure and as speed increases, a medium to carry away heat. Originally, each new vehicle designer had his own method and type of fuel, lubrication and control layout, as well as number of wheels and engine size. However as automobile numbers grew, standardisation set in. A brand new manufacturing industry was born, not just in the building of vehicles, but in support of the automobile: oil and petrol in particular were consumables that were soon to be in demand in every corner of the world. By the early nineteen thirties, the vehicle manufacturers had recognised that there was a need for fixed standards of performance of lubricants and fuel so that cars and lorries could be sold anywhere world-wide, without major modifications or embarrassing failures. Prior to that, you took with you what you needed.
The Society of Automobile Engineers in the USA took on the task of setting the standards for engine oil. They made the decision to compare and define lubricating oils by viscosity. Viscosity, in lay terms, is how easily a liquid pours. Now this also reflects on the internal shear strength of the liquid, so for light mineral oils we can state categorically that the higher the viscosity of the oil, then the stronger it is. Your engine rattles? Put some thicker oil in it!
The SAE decided to compare the viscosity of oils at 100 degrees centigrade, albeit they were originally working in Fahrenheit. This is around the temperature of oil in a big end bearing - the most highly stressed part of an ordinary car engine. Viscosity decreases with temperature increase and at around 100oC, mineral oils start to become very thin and thus weak. Tests at this temperature are thus a useful oil strength indicator. Viscosity is measured by the remarkably accurate method of pouring oil through a known size hole and measuring how long it takes to come out. The result of this is known as kinematic viscosity. Units of this measurement are mm2/second, or, after the chap who pioneered viscosity measurement, centi-Stokes (cSt). The SAE then set down numbers to define ranges of viscosities, as shown in table (1) below.
Table 1 SAE Kinematic Viscosity of Engine Oil
Viscosity @ 100oC
cSt SAE Rating
16.3 - 21.9 50 12.5 - 16.3 40 9.3 - 12.5 30 5.6 - 9.3 20less than 5.6 10
This system worked very well and is still in use today. SAE 30 became accepted as the standard for engine oil, giving reasonable film strength for the white metal bearings of the day. This weight oil was usable down to around minus 10o centigrade before it became too thick to move round the engine, which covered use in the majority of situations around the world. Lighter and heavier weight oils were available for extreme climate use. Racing engines, needing a greater film strength due to higher bearing loads, generally called for the 40 and 50 weight oils.
During the war, engine development was accelerated, with far higher engine speeds and bearing loads being introduced by advances in aeroplane engine technology. This was reflected in post war car engines: standard saloon car engines having the capability of providing pre-war racing engine power outputs. However higher film strengths and therefore heavier weight oils were necessary. Unfortunately the SAE 50 weight oil needed in these engines when hot, has thickened sufficiently by zero centigrade to prevent an engine being cranked. Engine oils needed to be changed from summer to winter, with the attendant problems of variable weather conditions and climates where both extremes were found daily.
Long chain viscosity modifying polymers came to the rescue. The plastics industry was developing from petro-chemical research carried out during the war years. One of the discoveries made was the capability of manufacturing long chain hydrocarbon molecules or polymers. Properties of some these polymers included the capability of thickening engine oil at high temperature, without affecting the viscosity at lower temperatures. An SAE
30 weight oil can thus be transformed into an SAE 50 by means of a simple additive package, without affecting the lower temperature usability.To differentiate between 'straight' oils and those which had viscosity modifiers added, the winter or 'w' rating test was introduced. Oils were originally tested by the floating of a needle on the surface of oil in an open vessel. The oil was cooled in 5 degree centigrade increments until there was no movement of the needle when the vessel was tipped. The oil was then rated as to be usable at the previous higher temperature. Although today's testing is slightly more sophisticated, the results are the same, leading to oils being classified for cold temperature use from the table (2) below.
Table 2 SAE Winter Rating of Engine Oil
Low temperature rating Temperature at which
oil is usable
25w -5oC 20w -10oC 15w -15oC 10w -20oC 5w -25oC 0w -30oC and belowStraight SAE 30 oil tested in such a fashion shows it is useable down to minus 10 degrees, thus this oil can be called an SAE 20w30. By adding viscosity modifiers to thicken the oil to an SAE 50 viscosity at high temperature then the oil becomes an SAE 20w50. A 50 weight oil, only good for operation at 0 centigrade, can be called an SAE 30w50. Two oils, both SAE 50, identical under the old definition, are thus now easily distinguishable. This became the world wide accepted commercial method of identifying engine oils. To the benefit of both the oil producers and the motorist, the pre-war standard SAE 30 was converted by means of an easy additive into the beloved 20w50.
Technically, it is not acceptable to look at the cold weather performance of an oil and its 100 degree SAE rating and guess from this what the performance might be like at other temperatures. For that reason kinematic viscosity is also measured at 40oC and the Viscosity Index calculated: the rate of change of viscosity with temperature. For oils of similar SAE rating, the higher the viscosity index the smaller the effect of temperature on its kinematic viscosity. This is particularly important when looking at lubricants for racing, high performance engines and those where high temperatures and loads are expected, particularly as it indicates how the oil will perform above 100oC. The Viscosity Index number for engine oil is not normally quoted on oil cans, for obvious reasons on certain oils, but is available from all genuine performance oil producers.
Advances in the 1950s and 60s in the petro-chemical industry also led to comprehensive synthetic detergent packages for oil, and very efficient anti-wear and anti-scuff additives. Combined with advances in filtration technology, the motor car engine in the late 1960s had never been better protected. However, it was needed! With the opening of high speed motorways around the world and metallurgy and manufacturing technology advances allowing higher revving and greater specific power outputs from engines, lubricants were still being challenged. Long chain viscosity modifiers and detergents are soon destroyed in high load conditions and the oil then reverts to its original lower viscosity grade. Fossil oils deteriorate with age, use and mixing with fuel residues, losing over a period of time their lubrication and protective properties. Mineral oils therefore need to be changed on a very regular basis. Vehicle manufacturers, on the other hand, want to cut down service intervals to make ownership as cheap and easy as possible. In addition, there is the requirement for as light an oil as possible to cut down friction losses, making the vehicle both quicker and more fuel efficient. Lighter mineral oils, under more stress, break down far more quickly.
Help came from the aircraft industry. Gas turbine engines had developed to the stage where the immense pressures and temperatures involved would fry mineral oils on contact. The stage was set for totally synthetic lubricants to enter the automobile market.
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