Important fuel specifications for Diesel Used In IC Engine
In IC engines, the chemical energy contained in the fuel is converted into mechanical power by burning (oxidizing) the fuel inside the combustion chamber of the engine.
Fuels suitable for fast chemical reaction have to be used in IC engines, they are following types-
(a) Hydrocarbons fuels derived from the crude petroleum by proper refining process such as thermal and catalytic cracking method, polymerisation, alkylation, isomerisation, reforming and blending.
(b) Alternative fuels such as-Alcohols (methanol, ethanol)
- Natural gas (methane)
- LPG (propane, butane)
General Fuel Specifications:
Different properties of fuels have,
- Relative density (specific gravity)
- Fuel composition
- Specific heating value
- Flash point
- Surface tension
- Freezing point
1) Flash point:
- Flash point is the lowest temperature of a sample at which the fuel vapour starts to ignite when in contact with a flame (ignition source).
- Marcusson method – fuel container is slowly heated, while the fuel vapour is in contact with an open flame – T is measured
- For gasoline it is 25 oC, diesel fuel 35 oC and heavy diesel 65 oC
2) Specific heating value:
- Specific heating value, Hu is a measure of the energy content of the fuel per unit mass (kJ/kg or kcal/kg)
- Gaseous fuels sp heating value is given in terms of energy content per unit volume (kJ/liter or kJ/m3, kcal/m3)
- In IC engines lower heating value is given as the combustion products contain water in vapour form.
- For gasoline and diesel fuel, Hu=42000-44000 kJ/kg or Hu=10200-10500 kcal/kg
- Heating value of the combustible air-fuel mixture is a decisive factor for engine performance.
- Viscosity is an important parameter for CI engines, also influences fuel metering orifices since Re is an inverse function of fuel viscosity lower the viscosity, smaller the diameter of the droplets in the spray.
- Below certain limits, low viscosity increases the leaks in the fuel system. It is a strong function of T – must be given at certain T values at 50 oC, 1.5 – 5.0 Engler or 0.5 to 0.6 centistokes.
4) Surface tension:
- Surface tension is a parameter which effects the formation of fuel droplets in sprays
- increasing the surface tension will reduce mass flow and air-fuel ratio in gasoline engines lower the value, smaller the droplet diameter diesel fuel value is in the range of 0.023 – 0.032 N/m and for gasoline it is 0.019 – 0.023 N/m
5) Freezing point:
- The precipitation of paraffin crystals in winter can lead to clogged filters. It can be prevented by either removing paraffins from the fuel or adding flow improvers (additives).
- Antifreezing properties are determined by its filterability.
- For gasoline freezing point is –65 oC and for diesel fuel –10 oC
6) Cetane number:
Cetane number is used to specify the ignition quality of diesel fuel.
- Running on low Cetane number will produce cold start problems. Peak cylinder pressure, combustion noise and HC emissions will increase -more fuel will be injected before ignition, less time for combustion.
- Higher CN results in a sooner ignition – extremely high CN may ignite before adequate Fuel-Air mixing can take place – higher emissions. Power output can be reduced if burning starts too early.
- Measurement of cetane number:
Cetane number is in the range of,Cetane number is measured by comparing the “ignition delay time” of the sample fuel with a mixture of cetane (C16H34) and alphamethyl naptane (C10H7CH3). The Cetane percentage in the mixture gives the CN of the sample fuel.
- CN of the reference fuel cetane is arbitraryly set at 100, and of alphamethyl naptane at 0.
- CFR engine is used to measure the compression ratio at which ignition starts. CR is gradually increased while the engine is driven by an electric motor – a curve of CN vs critical CR is obtained.
- Inlet air temp is 30 oC and cooling water temp is at 100 oC
- An easier and practical method to obtain Cetane Number is by calculating the Diesel Index.
- Increasing the DI, increases the tendency to ignite.
- AP is obtained by heating equal amounts of annilin and diesel fuel. While cooling down, the temp at which the annilin separates from the mixture is the AP
- 50 – 60 for high speed Diesel engines
- 25 – 45 for low speed Diesel engines
- Normal Diesel fuel CN is 40 – 55
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