2 Properties of fuels 1.Knock Rating 2.Volatility 3.Calorific Value 4.Gum Content5.Sulphur Content6.Aromatic Content
3 1.Knock RatingIf the temperature of the unburned mixture exceeds the self - ignition temperature and remains at or above this temperature during a period of time during the ignition lag , spontaneous ignition or auto ignition occurs at various 'pin point' locations .This phenomenon is called as knocking.One of the best qualities of fuel is that it should be used with higher compression ratio without knocking because high compression ratio provides high engine efficiency and lower fuel consumption.
4 Knock ratingThe fuel should have high anti-knock value to make use of the higher compression ratios.
5 2.VolatilityVolatility indicates the tendency of a fuel to change from liquid to vapour form.A high volatility is desirable from the point of view of starting ability and good distribution
6 Volatility affects the following engine/vehicle conditions:(In SI Engines or Petrol Engines):- 1.Less volatility means difficult starting and stumbling during warm-up . Excessive volatility would result vapour lock .2.Less volatility would cause the droplets of liquid fuel to break down the oil film on the cylinder wall , causing scuffing or scoring.3.Less volatility would also result in uneven distribution of fuel to various cylinders.
7 3.Calorific ValueThe calorific value of a fuel is the thermal energy released per unit quantity of the fuel when the fuel is burned completely and the products of combustion are cooled back to the initial temperature of combustible mixture.The fuel should have a high calorific valueGasoline has approximately the calorific value about MJ/kg.
8 4.Gum Content A good fuel should have the minimum gum content. All petroleum products oxidise in the presence of air . Gum is formed by the polymerization or oxidation reaction of certain unsaturated hydrocarbons.
9 5.Sulphur ContentAll hydrocarbon's contain free sulphur and Hydrogen sulphide.The presence of sulphur in the fuel is highly undesirable because of the formation of sulphur dioxide in presence of water vapour will have corrosive action on engine partsA high sulphur content is obviously undesirable.
10 6.Aromatic ContentAromatics ,though ,are high knock -rating hydrocarbons , yet have a corrosive action on rubber of the fuel system and have high freezing point .Therefore, their proportion in the engine fuel should not exceed beyond a certain limit.
11 7. Viscosity It is the measure of the resistance to flow. it affects the atomisation of fuel and operation of high pressure fuel pump.greater the number of carbon atoms ,greater is the viscosity of the fuel oil .the same the number of carbon atoms, a fuel will have higher viscosity with lower hydrogen content.Viscosity should be high enough to permit bulk flow and high enough to carry out lubrication.
12 8. Cloud PointThe temperature below which ,the wax content of the oil separates out in the form of solid is known as cloud point.This should be as small as possible so that this difficulty will not be experienced even in cold winter.
13 9.Anti-Knock QualityA best quality of a fuel used in S.I engine is ,it should be used with higher compression ratio without knocking because high compression ratio provide higher compression ratio and low fuel consumption .. The anti-knock characteristic of the gasoline is mostly dependent on the chemical composition and molecular structure of the fuel.
14 10.Specific gravity For Petrol It lies in the range 0.7 to .78 . In diesel, the value of the specific gravity is between 0.82 and 0.92.The specific gravity of the fuel should be higher for efficient combustionThe specific gravity of fuel can be determined by dividing the density of the fuel to the density of water.
15 DESIRABLE PROPERTIES OF GOOD IC ENGINE FUELS: A Good I.C engine fuel must possess the following properties:-1. It must have high energy density(KJ/kg).2.It should be easy to handle.3.It must possess good combustion qualities.4.It must have high thermal stability.5.It must have low deposit forming density.6.It should be easy to handle and store.7.It should be free from hazard.8.It should not have chemical reactions with engine components , through which it flows.9.It should easily mix with air and evaporate as quickly as possible.10.Products of combustion should not be corrosive to engine parts.11.It must possess low toxicity.12.Its effect on air-pollution should be the minimum.13.It should be economically available in very large quantities.The basic requirement of I.C engine fuel is , the combustion should be fast with maximum amount of heat release without forming any deposits and should not have destructive effects on the engine parts and atmospheric air by exhaust gases.
16 REQUIREMENTS OF FUELS FOR S.I ENGINES:- Gasoline is the major fuel used for S.I engine . It is a mixture of various hydrocarbons such as paraffin , olefins , napthene and aromatics . The composition depends upon the nature of crude available and method of refining used.A good fuel used in S.I engine must fulfil the following properties:-1. It must evaporate quickly when injected by carburettor in the current of air.2. It should not pre-ignite.3.It should be easy to handle.4.It should be easily available at reasonable cost.5.It should burn clean and should not deposit the residue.6.Its exhaust should not have any corrosion effect on the engine parts.7.It must have high Calorific value.8.It must be knock resistant.
17 Flash PointThe temperature at which the vapours of oil flash when subjected to a flame is known as the flash point . If container is closed ,it is called closed flash point and if open it is called open flash point.
18 Fire PointIt is the temperature at which the oil , if once lit with flame ,will burn steadily at least for 5 secondsThe flash point and fire points indicate the temperature below which , oil can be handled without any danger of fire.