A few important tests are recommended to determine the suitability of fuels for an engine.
The following are the important tests:
1. Gravity Test:
The gravity of a fuel or oil may be expressed as specific gravity or as API gravity, a scale devised by American Petroleum Institute. The specific gravity of a liquid is the ratio of its weight to the weight of an equal volume of water at 60°F.
The relationship of the API gravity scale to specific gravity is expressed by the formula-
A.P.I degrees = 141.5/specific gravity of oil at 60°F – 131.5
The instrument used for testing the specific gravity of oil is known as hydrometer. The lighter fuel of lower specific gravity has higher API gravity expressed in degrees. Pure water has got API gravity as 10.
2. Distillation Test:
It is a measure of volatility of a fuel. In this test, a measured amount of oil is boiled in a flask, starting from a low temperature and gradually increasing it. The amount of condensed vapour corresponding to each temperature is collected and noted separately.
This process goes on operating till the evaporation ceases. Fig. 5.1 shows distillation curve for the common fuel oils. 10 percent point is of primary importance as a specification related to engine starting.
The lower this temperature, the better the starting characteristics of the fuel. 50 percent point is important as an index of the engine warm-up characteristics i.e. the lower the 50 percent point temperature the faster the warm-up of the engine. 90 percent point temperature provides a good indication of the general performance of the fuel.
3. Vapour Pressure Test:
A special instrument known as Reid vapour pressure test is commonly used to measure the vapour pressure of a fuel. The pressure is read on the Bourdon gauge at 100°F temperature. Vapour pressure is a measure of starting ability as well as vapour lock tendencies of the fuel.
4. Sulphur Test:
Sulphur test is done for the following purposes:
(a) To find out presence of corrosive sulphur compounds in the fuel oil.
(b) To find out quantity of free and combined sulphur present in the fuel oil.
The corrosive sulphur is indicated by immersing a polished strip of copper in the fuel for 3 hours at 122°F. A comparison is made with this strip and a freshly polished copper. If corrosive sulphur is present, the colour would be discoloured. For finding the total quantity of sulphur, a sample of fuel is burnt in test bombs or wick lamps and sulphur is measured.
5. Carbon Residue Test:
Carbon residue is an indication of the carbon forming properties of the oil under certain engine conditions. It is a means of measuring the amount of carbon residue left on evaporating oil under specified conditions.
6. Colour Test:
Colour test also indicates certain physical properties of oil to some extent but it is not a very reliable test.
7. Gum Test:
Gum test is a measure of the presence of gum in the oil. The test consists of evaporating samples of fuel in special apparatus. The residue is weighed in milligrams per 100cc of fuel. The evaporation takes place in a copper dish and is continued for a longer period of time, indicating gum-forming properties that may develop upon storage, exposure to air and light.
8. Flash Test:
The flash point of oil is the temperature at which inflammable vapours are given off. The oil sample is heated and the temperature at which distinct flash is obtained when flame is passed over the container is called flash point. The results are important for safety and fire protection measures.