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The Oil In Your Engine


This report was prepared especially for the Club by Nick Mason-Pearson of Biss Lancaster, and Ewen Auld who is Automotive Technical Manager for Shell Oils. We are extremely grateful for their contribution.Shell logo


December 1996

We all know that the engine in our car needs oil - that much we learned from watching our fathers tinkering around under the bonnet at the weekend - but how much do we know about it? In some respects life was easier then - not least because we did not have as many choices when it came to picking the right oil for our engine. Oil was oil. Most modern cars are different to the old classics still on the road and oil has come a long way, with engines needing far less maintenance in that department. Nowadays it is all more sophisticated and scientific. The chances are that the oil we put in our engines in a few years time will have been developed as a result of accumulated experience on the Ferrari Formula 1 racing car which Shell is sponsoring for the next five or so years! But how many of us actually know what we are putting into our engines? 10W- 40, synthetic, semi-synthetic, multigrade, viscosity index. What does it all mean and why do we even need to know?

Over the past twenty years, car engine performance and power has increased by 30 per cent or so as engineering and development techniques have improved. The technology, that has brought us these improvements has also helped us reduce oil consumption by up to 60 per cent, with much longer periods now between service intervals and oil changes The choice of engine oil has - broadened to meet the requirements of different types and ages of cars - and the conditions under which they are driven. Whereas twenty years ago we were putting relatively heavy mineral oil into our engines to lubricate them, today the choice is more likely to be a much lighter semi-synthetic or fully synthetic motor oil. Given the advances in engine technology and the environmental demands now placed upon motor manufacturers, the modern motor oil is required to do a lot more than just lubricate. It has to cool, clean and protect the engine under all driving conditions and over longer and longer oil change intervals. In addition it is now being called upon to help with increased fuel economy and lower emissions. To understand the difference between the old mineral oil and the modern synthetic oil we need to know how they are made up.

Conventional mineral oils are produced by refinery distillation of crude oil, coupled with extraction processes to remove undesirable components. Oils produced by this process give generally satisfactory results but do not have the ideal molecular composition for modern, high performance applications. Synthetic oils also derive from crude oil, but by carefully selecting the feedstock and using a hydrogen-based synthesis process, we can physically restructure the hydrocarbon molecules to create a more consistent and homogenous end product. This process gives synthetic oils performance characteristics which are very much better than those of traditional mineral oil products. For instance, synthetic oils display excellent low temperature pumpability at cold start, while still resisting volatilisation at engine operating temperatures. By reducing the time it takes to pump oil to critical parts of the engine at cold start, we can significantly reduce wear, whilst the reduced propensity to volatilise lowers both oil consumption and exhaust emissions. This ability to manufacture motor oils to lower viscosities using synthetics, while still offering full engine protection, has the added benefit of reducing the power necessary to pump the lighter oil around the engine, thereby also increasing fuel efficiency.

Many petrol and diesel passenger car engines now incorporate fuel injection, multi-valving and turbo-charging. They are hotter, faster, tighter and develop more power for a given engine size than in our Fathers' day. Consequently the lubricant is under much greater stress. Synthetic and semi-synthetic motor oils are formulated to withstand this stress over ever-increasing oil change intervals. So how do we know what viscosity of oil to choose? Up until the end of the 50's, motorists used different oils for winter and summer lubrication. These were called 'monogrades' and were recognised by the classification given by the Society of Automotive Engineers in the USA. For instance, SAE 20 (a relatively light oil) was used during the winter months to ensure that the oil could be pumped easily to critical engine parts, even in very cold start up conditions. In summer, SAE 30 or 40 would normally be used. In addition to the change in viscosity between winter and summer, the motorist had to change the oil far more regularly than is now the case. An oil change interval of 1,000 miles was fairly common in those days. Compare that with a normal 10,000 mile oil change interval today. In the late 1 950s the oil companies developed the multigrade oil. Such oils contained polymer additives which reduce the rate at which the oil thins out when heated. In other words, it was now possible to blend an oil which was thin enough to pump effectively at cold start, but which was thick enough to keep the engine parts sufficiently lubricated at normal engine operating temperature. In effect, multigrade oils could be run all the year round as there was no need to change the oil viscosity to suit the season. The classifications changed too, following the pattern SAE SW-40, SAE 10W-40, SAE 15W-50 etc. These usually tell us more about the oil itself and the conditions under which it has been tested, but they do need deciphering.

To explain, let us take the example of one of our oils, Shell Helix Ultra SW40. The "5" defines the oil's viscosity measured at low temperature (in this case -35°C) and the "W" denotes its suitability for winter use. The "40" defines its viscosity at 100°C. Note that the high number - ie. 40, 50 etc. is always measured at 100°C, whereas the lower number viscosity temperature measurement varies, ie 5W is measured at - 25°C, 10W is measured at -20°C and 15W is measured at -15°C. From this you will see that the SAE 5W-40, which is fully synthetic, is ideal for today's high performance engines. It offers full protection from very cold start to running at high temperatures on the motorway. In addition, the low viscosity of this particular oil significantly reduces fuel consumption when compared, say, with a conventional motor oil. On the other hand and SAE 15W-50 oil such as Shell Helix Standard - a mineral oil - is more viscous and this makes it suitable for older engines which are less tight due to wear and/or their design. Many car manufacturers today recommend an SAE 10W-40 oil such as Shell's Helix Plus which is a semi -synthetic oil and which offers many advantages of a fully synthetic product. So far we have looked at the base oil - whether it be mineral or synthetic - but that is only part of the picture. A good quality motor oil will typically consist of 80% base oil blended with 20% complex chemical additives, whose purpose it is to improve the performance of the base oil itself and to protect the metal surfaces within the engine. We have already mentioned the polymer additive which goes to make multigrade oils. This type of additive is called a Viscosity Index Improver (VII). The term VII derives from the scale used to measure the rate at which an oil thins out when it is heated - in other words, its Viscosity Index (VI). A monograde oil would typically have a VI of 100, whilst that of a mineral based multigrade would be 150. Synthetic oils are far superior in this respect, with the typical VI of a top synthetic motor oil such as Shell Helix Ultra 5W - 40, being 167. This property is of great value in maintaining the viscosity of the oil, and therefore the protection of the engine, throughout its oil change intervals.

In the days before additives, it was normal practice to remove the sump of the engine at regular intervals to clean out all the accumulated sludge and other contamination. Today's engine oils contain detergents which clean engine surfaces and neutralise the harmful acid products of combustion. They also contain dispersants which prevent the solid contaminants from forming into lumps - instead they are held in suspension until the oil is drained. This is why engine oil goes black so quickly - it is a good sign that the dispersant in the oil is working. any oversize particles are simply removed from the oil in circulation by the oil filter. Anti-wear agents are also a very important part of a modern motor oil. The most common of these - generally known as 'zincs' - work by adsorbing onto metal surfaces. Activated by the heat generated in the tighter parts of the engine, such as in the valve-train mechanism, they produce low shear metal compounds which prevent abrasive wear of the moving parts. Air entrainment and foaming can cause serious problems in engine oils. With motor oils being required to perform more and more hydraulic functions - such as with valve lifters - the resistance to forming foams becomes increasingly important. Anti-foam agents work by lowering the surface tension in the oil and thus reducing the tendency for bubble formation. Silicone at very low dosage rates is an effective anti-foam agent. Additives are very expensive and contribute largely to the cost of a motor oil. However, modern engine technology demands a combination of superior base oils and high quality additives, and any compromise in this area will do lasting damage to the engine. This goes some way to explaining why motor oil is so important and hopefully helps you decide which one is right for your engine. In future issues we will be looking at other essential fluids for the car - transmission oil. brake fluid etc.


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