Important fuel specifications for gasoline/ Petrol 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) Gasoline volatility:
Benzene for example has vapor pressure of 0.022 MPa at 38 0C in a closed container of 38 0C, benzene evaporates until the partial p has a value of 0.022 MPa, If T is raised to 80.5 OC, then saturation p will be 0.1 MPa and will be constant during the boiling
For gasoline it is not possible to indicate a single value of evaporation T or vapor pressure.
Gasoline contains large number of compounds – up to about 400
7) Cold starting:
- For SI engines to start, A/F ratio must be within the ignitable range, ie in general must be between 7:1 to 20:1 by weight.
- When the engine is cold, it is difficult to ignite lean mixtures, because fuel may not vaporize sufficiently – under these conditions the mixture is rich to bring it to ignitable range. This is done by the injection time or by the use of a choke with carburetted engines.
Antiknock quality of gasoline:
- Knock occurs when the unburnt gases ahead of flame front (the end gases) spontaneously ignite causing a sudden rise in pressure accompanied by a characteristic pinging sound – this result in a loss of power and can lead to damage the engine.
- Combustion chamber shape, spark plug location, ignition timing, end gas temperatures, in cylinder gas motion, air-fuel ratio of the mixture, fuel specifications etc. effects the occurrence of knock.
- Compression ratio of the engine also strongly effects knock. The higher the CR, the better the thermal efficiency – but the greater the tendency for knock to occur.
- Critical compression ratio – when knock starts. So higher fuel octane quality is required.
- Autoignition of the end gases causes a rapid increase of p, producing p waves which resonate in the combustion chamber at a frequency of between 5000 – 8000 Hz, depending on the geometry of the chamber
- Knock results in an increase of T in the cylinder and causes a severe damage to engine components like cylinder head gasket, piston, spark plugs etc.
-In 1929 Octane scale was proposed by Graham Edgar. In this scale two paraffinic HCs have been selected as standards (PRF, primary reference fuels)- iso-octane (2-2-4 trimethyl pentane) with very high resistance to knock (arbitrary assigned a value of 100) and n-heptane with extremely low knock resistance (assigned a value of 0).
-Test engine for determining Octane values, was developed by Cooperative Fuel Research Committee (CFR). It is a single cylinder, variable CR engine.-Octane number of the fuel is the volume percentage of iso-octane in a blend with n-heptane (PRF), that shows the same antiknock performance as test fuel tested in standard engine and standard conditions.
-Two different test conditions specifies the Research Octane Number (RON) and the Motor Octane Number (MON)
-Antiknock Index = (RON + MON) / 2
-TEL is added to the PRF to increase the ON above 100 or n-heptane is added to the sample to reduce ON below 100, then nonlinear extrapolation is applied
ON can be increased by antiknock agents – at less expense than modifying HC composition by refinery process.
Most effective agents are lead alkyls –
TEL – tetraethyl lead, (C2H5)4 Pb
TML – tetramethyl lead,
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