FUEL PROPERTIES

Engine needs to run on fuel of high quality to produce maximum work output. Several fuel properties have been identified and defined as following.

· Heat value or Calorific value

· Specific gravity

· Volatility

· Flash point

· Fire point

· Pour point

· Viscosity

· Octane number

· Cetane number

· Carbon residue

· Sulphur content

· Gum content

Heat value or Calorific value

It is the indicative of heat energy being produced by the fuel when it is burnt inside the cylinder/combustion chamber of an engine. It is expressed in J/kg of fuel and is measured in the device which is known as calorimeter. The impurities in the fuel leads to decrease in its heat value.

Specific gravity

It is expressed as the ratio of the density of fuel to the density of water. The specific gravity affects the fuel atomization in the nozzles and spray penetration/injection in the engine cylinder/combustion chamber. Fuels which are relatively heavier have usually greater heat value. The specific gravity is measured by the hydrometer.

Volatility

It is the property of the fuel to get converted into vapours on burning at a specific temperature. The volatility is measured by means of distillation. In diesel fuel, volatility is indicated by 90%  distillation temperature (temperature at which 90% of the fuel is distilled off). Lower volatility in fuels leads to increase in carbon deposits, smoke content and also wear of engine components.

Flash point

It is the temperature at which the fuel must be heated to get flammable vapours and is driven off to ignite when brought into contact with the flame.

Fire point

It is the higher temperature at which the vapours will continue to burn after being ignitied. Generally, the fire point is 10⁰ to 21⁰ C higher than the flash point and it is the indicator of fire hazards. The lower the flash point, the greater is the fire hazard. In general, the flash point should be high enough to avoid producing flammable vapours.

Pour point

It is the temperature  at which the fuel becomes insoluble to pevent flow under specified conditions. In cold weather conditions, this becomes very important parameter as wax crystals start forming even when temperature is slightly over the pour point. A higher pour point implies that in cold weather the fuel will not flow easily through the filters and fuel system and also the atomization/spray characteristics are affected.

Viscosity

It is the property of fluid/liquid that resists the force which makes the liquid/fluid to flow. It is measured by the instrument known as viscometer in which the time required by certain volume of fluid to flow is measured under stated conditions. It affects the spray pattern of fuel in the combustion chamber. Low viscosity produces a fine mist, whereas high viscosity leads to coarse atomization.

Octane number

It is a standard used for determining the knock characteristics of fuels (petrol) and it refers to the percentage by volume of iso-octane (C₈H₁₈) in a mixture of iso-octane and normal heptane (C₇H₁₆). Fuel knock is prevented by the fuel’s ability not to self ignite in the combustion process.

Cetane number

It is the measure of fuel property in which it is measured that how easily and fast the fuel (diesel) is ignited when reaches into the engine combustion chamber/cylinder. A cetane number rating is obtained by comparing the fuel with cetane, a colourless liquid hydrocarbon, which has excellent ignition qualities and is rated as 100. The percentage of cetane in a mixture of cetane (C₁₆H₃₄) and α (alpha) methyl naphthalene (C₁₁H₁₆) is called cetane number. The higher the cetane number the shorter the lag between the instant fuel enters the combustion chamber and the instant it begins to burn. The commercial diesel fuels have got cetane varying from 30 to 60.

Carbon residue

Carbon residue refers to matter left after combustion is an indication of the amount of combustion chamber deposits when the fuel is burnt in the engine combustion chamber/cylinder. It varies from 0.15 to 0.35% on weight basis and its permissible limit depends upon the engine characteristics. This property is more critical in small high speed engines than in large, low speed engines.

Sulphur content

The presence of sulphur in high quantity in the fuel is not desirable as it increases the wear of engine components specifically, the piston rings and the cylinder walls. It also causes the formation of hard coatings on the piston and oil sludge in the engine crankcase. The sulphur in fuel after burning combines with the water to form corrosive acids which further damages the finished surfaces.  Sulphur content varies from 0.5 to 2% on weight basis.

Gum content

Gum in the fuels is formed by the polymerization of some unsaturated hydrocarbons. To have good quality of fuel, the gum content should be minimum.