When selecting oil for an engine it is important that the one chosen is suitable for the engine itself and the conditions under which it will be used. Two important factors determining the choice of oil are:
.
that the oil meets the quality requirements;.
that the oil has the right ‘thickness’ or viscosity. The term viscosity refers to the relative thick- ness of a liquid. A thin free-flowing liquid has a low viscosity, and a thick, slow-flowing liquidhas a high viscosity. It will be seen that the viscosity of a liquid like oil changes as the tem- perature changes. At high temperatures the oil becomes thinner (giving a low viscosity) and at low temperatures the oil becomes thicker (giving a high viscosity). The correct oil viscosity is essential for the efficient operation of the engine because when the oil is too thick (high viscosity) it causes resistance and too much power is needed to turn the engine, making it difficult to start when cold.
Thick oil does not circulate freely enough dur- ing the starting period, causing insufficient lubri- cation of the bearings and thus increasing wear. Oil which is too thin (low viscosity) combined with a high temperature and a heavy load has the risk of oil being pressed out from between the bearings or other engine components. This would cause the oil film, which must be present to keep the components apart, to break down.
The most significant characteristic of lubri- cating oil is its viscosity. This can be measured in different ways. One way, shown in Fig. 1.44, is to check the quantity of oil which passes through a hole in a tube of standard size during a certain length of time at a given temperature. A widely used system of grading oil, based upon viscosity, has been worked out by the American Authorities for Standardization (SAE – Society of Automotive Engineers). Var- ious oils are grouped into viscosity grades marked with SAE numbers. These indicate the viscosity index of the oil (see Fig. 1.45).
For engine oils the SAE grades are numbered: 5 W, 10 W, 15 W, 20 W, 30, 40, 50. The lower numbers indicate thin oils and the higher num- bers indicate thicker, or higher-viscosity, oils. For the SAE grades 20, 30, 40 and 50, the viscosity is measured at an oil temperature of 100C which is the normal oil temperature when the engine is running. The grading can be read on the right-hand side of the graph in Fig. 1.46.
Fig. 1.43 Mixed friction occurs in this ball and roller bearing
For the SAE grades 5 W, 10 W, 15 W and
20 W, the viscosity is measured at 18C
(0F). This can be seen on the left-hand side of the graph in Fig. 1.47.
Now we have a very wide range of oil viscos- ity from very thin oil (SAE5W) up to thick oil (SAE50). Every SAE grade represents an oil suitable for use within a specified range of tem- peratures and for a certain type of engine, for example, oils within the viscosity range SAE 5 W, 10 W and 20 W are suitable for use in cli- mates with temperatures ranging from very low to moderate. SAE 20 and 30 are suitable for use in moderate to hot climates. SAE 40 and 50 oils are mostly used in old engines designed for
rather thick oil or in badly worn engines with high oil consumption.
However, many engines operate in climates where the temperature varies considerably from season to season. In addition it is always prefer- able for any engine to run on low-viscosity oil during the starting period when the engine is cold, and on high-viscosity oil when the engine is hot and fully loaded. For practical reasons it is not possible to use a low-viscosity oil for starting and then change to a high-viscosity oil when the engine is hot. Therefore, we need an oil which is thin enough at low temperatures, but will also have a sufficiently high viscosity at high temperatures – that is a multi-viscosity or multi-grade oil.
We cannot change the fact that oil becomes thinner as it is heated, but it is possible, by the use of additives to the base oil, to reduce the extent to which this happens. For example, an oil having a viscosity of 10 W at18C can be improved so that it also has a viscosity of SAE30 at 100C.
As the oil now has a viscosity equivalent to SAE10 W at 18C and a viscosity equivalent to SAE30 at 100C, it is marked with both num- bers, that is SAE 10 W/30. Oils are available with a very wide viscosity range, e.g. 10 W/50 which may be used in any climate enables the cold engine to start on a thin oil (SAE10 W) and, when hot, to run on a sufficiently thick oil (SAE50).
Oils that meet the specification for more than one SAE grade are often referred to as multi- viscosity, all season or multi-grade oils. Remem- ber that the SAE grades only tell you the viscosity of the oil and not the quality. The correct method
Fig. 1.45 SAE grading of engine oil
Fig. 1.46 SAE 30 single grade oil
for selecting the oil with the right SAE number corresponding to the temperature and running conditions would be to check in the specifications for the engine. Oil producers have worked out, together with the engine manufacturers, recom- mendations for the oil viscosity most suitable for given conditions.
It is important that only the correct grade of oil is used in a given vehicle and the infor- mation about that is given in the driver’s handbook or the workshop manual. In most modern engines multi-grade oils are recom-
mended regardless of temperature. An
SAE10 W/40 oil, for instance, can be used in most climates thus simplifying the selection of oil. It is important to know that the SAE grades only give information on the viscosity of the oil and not the quality. This can be found by mak- ing reference to the API (American Petroleum Institute) classification.