In this article we will discuss about the testing of lubricants for IC engines. The selection of a suitable lubricant for a particular purpose requires knowledge of its properties in the working condition as well as the performance of these properties under conditions of service.
Experimental Determination of Lubricant Properties:
Properties can be experimentally determined by the following tests:
Tests made on the lubricants may be divided under two heads:
(1) Physical Tests:
Main properties of these tests are listed below:
(ii) Flash point and. fire point
(iii) Volatility i.e. loss by evaporation
(iv) Cold test and chill point test
(vi) Density and specific gravity
(vii) Colour, fluorescence, specific heats, etc.
The instrument used for the experimental determination of the numerical value of the viscosity of a fluid is called a viscometer. Viscometers are of two types. One measures directly the absolute viscosity, e.g. Rotation viscometer.
The other type measures relative viscosity, from which the kinematic viscosity can be calculated. Kinematic viscosity is the ratio of absolute viscosity to density. If density is separately found out experimentally, the absolute viscosity can be calculated from the kinematic viscosity.
Some of the important viscometers used for measuring the relative viscosity of the fluid are as follows:
(a) Saybolt universal viscometer commonly used in U.S.A.
(b) Engler viscometer, commonly used in Europe.
(c) Redwood viscometer is adopted in England, as well as in India.
In these viscometers, a given volume of liquid under controlled pressure and at constant temperature is allowed to flow through a capillary of known dimensions and the time of flow is noted. The time taken for the flow is a function of viscosity.
Fig. 13-61 shows the Redwood viscometer. It consists of a silvered copper oil cylinder of 48 mm diameter and about 89 mm in depth and has an agate jet fixed at the bottom. This cylinder is placed in a copper bath. The bath has a revolving stirrer to distribute the heat which is supplied by an electric heating coil disposed in the wall of the bath. The temperature of the bath and that of the oil in the cylinder are read by means of thermometers placed in the corresponding positions.
The agate jet in the oil cylinder can be closed by a small brass ball valve resting in a hemispherical cavity formed in it. The instrument is supported on a tripod with levelling screws. After levelling, the cylinder is filled with the test oil upto the pointed gauge.
The bath temperature is adjusted and with it the temperature of the oil as indicated by the thermometers. When desired condition is reached, the 50 ml measuring flask is placed in position, the ball valve is raised and the stop watch is started.
The end of the experiment is marked by the bottom of the miniscus passing the mark on the neck of the flask. The efflux time and temperature are noted. The latter should not vary more than 0.2°C during the run.
From this value of the time of efflux, the kinematic viscosity and absolute viscosity can be determined if the density of the oil is known.
The standard liquid used in England is rape oil, the viscosity of which at 15°C is taken to be 100; time t2 for a given viscometer is supplied with the literature.
(ii) Flash and Fire Point Tests:
Flash point is the lowest temperature, at which the oil gives off vapours that form with air an explosive mixture which ignites into a momentary, flash, on application of a flame to it.
Fire point is the lowest temperature at which oil just begins to burn continuously. Fire point is the lowest temperature at which oil just begins to burn continuously.
Flash point determination is one of the important tests, the omission of which may cause a serious fire hazard.
It may be suggested here that while the flash point and fire point of gasoline represent the formation of combustible mixtures of the oil with air, the case is different with the lubricating oils. Usually with lubricating oils the vapour of the oil does not make a combustible mixture found at the flash point is due to decomposition products of oxidation and heating of the oil. The temperature at which this occurs is not so important as the nature and the rate of the decomposition indicated and its effect on the residual oil.
The determination of the flash-and fire point is carried out by the help of the Pensky-Martens flash point apparatus. There are different instruments used in different countries. The Pensky-Martens apparatus is mostly used in England as well as in India.
Fig. 13-62 shows the Pensky-Martens flash-and fire point apparatus. It is a closed cup type of instrument i.e. the oil is kept in a closed vessel and is heated to form vapour. This type gives more reliable results. It consists of a flanged brass cup 50 mm in diameter and 55 mm deep with walls 3 mm thick. Oil is to be filled upto a marking inside the cup.
The cup is supported on an annular iron casting, which is the body heated by a flame
from below. A brass jacket is fitted on the casting and the cup is supported on it by its flange. The cup has a brass cover which closely fits on it and is located in position by means of guide screws so as to fit always in the same position.
The stirrer mechanism passes through the centre of the cover. It consists of two propellers mounted on a shaft. One of the propellers is located in the oil while the second one is placed in the vapour space. The stirrer shaft is rotated by means of a flexible shaft which is turned by hand. The cover has four openings.
One is for a thermometer while the other three are trapezoidal in shape. The middle one is for admitting the test flame and the other two are for admitting air. These three openings are covered by means of a slide which can be operated by means of a single screw. The same screw operates the mechanism of the test flame and dips it in the vapour space of the cup. A burner is maintained near the test flame for automatically lighting it when the slide is closed.
To carry out the test the cup is filled with the necessary amount of oil upto the mark and then it is heated. The rate of heating shall be 5°C per minute. Beginning at 10°C below the flash the stirrer is worked at the rate of two revolutions per second. The test flame is applied at every 2°C rise in temperature at about 10°C below the flash point. The flash is a miniature explosion clearly indicated. This method cannot give flash point closer than 2°C.
After the flash point is determined, heating is continued at the same rate and the fire point temperature, at which the vapours first burn continuously for at least five seconds, is recorded. Then the flame is put out by closing it with a cover plate.
Mineral oils, unlike fixed oils, have the property of volatility in the same way as the water evaporation. The steam engine cylinder oil should possess as low a volatility possible, otherwise the lubrication becomes a costly affair. Similarly lubricants used for special purpose must have low volatility at its working conditions. In normal working conditions lubricants have very low volatility.
There is no standard apparatus for determining the volatility of the lubricant. Fig. 13-63 shows one type of a vaporimeter. It is used for determining the volatility of the oil at the particular condition of working. It consists of a horizontal copper tube placed in an air bath, the temperature of which can be adjusted by changing the temperature of the oven in which the tube is kept.
The oven is heated by means of a gas supply. This tube is surrounded by a small copper tube helix. The air is admitted through one end of the small tube and the other end is connected to the larger tube. The oil to be tested is placed in the platinum dish which is pushed upto the middle of the bigger copper tube.
Air at a measured rate of 2 litres per minute is passed first through the helix tube and thereby heated to the necessary temperature and is then passed through the bigger tube and is exhausted from its one end. Glass or porcelain tube is placed in the bigger tube so as not to expose the oil to the heat. At the end of an hour the loss in mass is determined. This loss gives the measure of the volatility of the oil.
(iv) Cold Test:
All oils increase in viscosity as the temperature is lowered and if lowered sufficiently the flow of oil ceases completely. This test is of importance when oil has to work at low temperatures as in cold countries. But in a country like India this test is not of much importance.
This test is carried out with the oil which comes in intimate contact with water during its service, as for example turbine circulation oil, steam engine cylinder oil, etc.
Animal and vegetable oils form emulsions with water very quickly and so they are not used for the purpose. Mineral oil is resistant to it but its property of emulsification depends on its refinement. The emulsification test is made to find out the degree of emulsification and to determine the rapidity of separation from emulsification.
There is no standard test for emulsification. But oil to be tested is shaken with water and the degree of separation after a definite interval of time is noted.
(vi) Specific Gravity:
The determination of specific gravity of a lubricant does not give any properties of it as a lubricant. But it is possible to check the grade of the oil supplied. If a given oil has been satisfactory in a given case and its density is known then from the second lot of the same oil showing the same density will mostly give the same satisfactory service.
The specific gravity is determined by specific gravity bottle or by a hydrometer. Since the density of the liquid is also a function of temperature so in determination of the specific gravity the temperature of the substance and that of the water must be stated.
(vii) Colour and Fluorescence:
There is no direct relation between the lubricating properties and the colour. It only indicates the presence of certain complex hydrocarbons.
(2) Chemical Tests:
Chemical properties of these tests are listed below:
Free acids are harmful in lubricants. They attack the metal surface of the bearing. Soft metals are more affected by it.
These acids may come mainly from three sources:
(a) Mineral acid which is a residue of the acid used in refining the oil
(b) Petroleum acids coming from crude oil
(c) Free fatty acids which only occur if mixed with mineral oil.
The mineral acids and petroleum acids have not much detrimental effect on the bearing while free fatty acids have the most detrimental effect on it.
The acid value of oil is determined by a titration with alcoholic potash (KOH) and is expressed in terms of the number of milligrams of KOH required to neutralize one gram of the oil.
(ii) Oxidation and Gumming:
When exposed surfaces of oil come in contact with air the oil gets oxidized. The rate of oxidation increases with increase in temperature. Oxidation makes the oil unsuitable as a lubricant because the oil, due to this effect, becomes gummy.
This effect is more in some of the animal and vegetable oils. The mineral oil is only slightly affected by it. Oxidized oil when spread into thin films on cloth starts spontaneous combustion and so there is a big fire risk with such oils.
There is no standard test for it but iodine value which is the number of gram of iodine absorbed by 100 grimace of the oil gives a good indication of this effect.
(iii) Insoluble Residue:
This is one of the important tests which should be carried out for lubricating oils used for automobile parts. Some of the oils contain free carbon in them which is very harmful in the lubricant because parts to be lubricated will be worn out earlier due to this; also some of the hydrocarbons in oil decompose into free carbon when heated by coming in contact with very hot parts.
A test to determine this residue is carried out by heating a known quantity of oil in a closed crucible and the oil is partially evaporated and partially decomposed and finally a residue is left in the crucible. This residue consists of carbon.
Technically this is a vague term though it can be defined as the power of oil to maintain a continuous film and to absorb with the metal parts when used as lubricant. Till today no means has been found out to measure the oiliness.
This property is different from viscosity because out of two oils having the same viscosity one gives good service as lubricant while the second one does not do so. The reason is attributed to the property of oiliness of the two oils.
Oiliness seems to be function of the molecular composition of the oil. Animal and vegetable oils have greater oiliness than the mineral oils, Oiliness plays an important part in the lubrication of such parts where incomplete film is formed in a bearing. With complete film lubrication oiliness is not of much importance.
Not all of the physical and chemical tests are necessary to bring out the fitness of a lubricant for a given purpose. Over and above these tests one more test is carried out on the oil for guidance of the engineers which is the mechanical test of the oils for finding the coefficient of friction.
This test is carried out with the help of friction test machines. Coefficient of friction found out with these machines is the coefficient in the ideal conditions provided in the machines which are very difficult to get in practice and so some engineers think this test to be not of much importance. But an order of merit of a series of oils can be established by this machine. This order will also hold good in practice.
Many friction test machines are in market.
They are used for:
(1) Comparing oiliness
(2) Testing effects due to viscosity
(3) Testing the different methods of supplying oils at suitable points.
(4) Testing the effect of bearing metal of different composition upon friction loss
(5) Testing the effects upon friction loss of variation in temperature, pressure and speed.
The same machine will be useful for all the above tests if it has means to change pressure and speed.
This is one of the widely used machines for the mechanical testing of lubricants. Fig. 13-64 shows the Thurston oil testing machine.
It consists of a shaft supported in the bearings. The shaft has an overhung extension and is driven by a variable speed motor and a cone pulley. The extension has a sleeve which works as a test journal. The sleeve can be easily replaced and is chosen of the material to be tested. The bearing block, subtending an are of contact of about 60°C and placed on top and bottom of the shaft sleeve, is mounted in the housing.
The load is applied by means of a spring mounted inside the body of the pendulum which is a pipe fixed in the bottom of the housing, and having a bob fixed at the other end. The spring is calibrated to read the load directly by the help of the pointer and the scale in front of the pendulum. A wedge is provided to take off the load from the bearing. Oil is supplied to the bearing through the drop feed oilers. A thermometer is located to give the temperature of the bearing.
When the load is applied and the machine is started the pendulum swings and the reading is obtained on the scale which directly reads the frictional force in newtons.
To know the actual viscosity of the oil film in the bearing the temperature of the oil film is necessary. The temperature can be read by a thermocouple which is in contact with the oil film. A tachometer is provided to measure the speed of the journal.