Fuels | Mechanical Engineering SSC JE (Technical) PDF Download

Fuel is a substance that is used to produce energy. It can be in various forms, such as solid, liquid, or gas, and it undergoes a chemical reaction (usually combustion) to release energy that can be harnessed for various purposes, such as powering engines, generating electricity, or providing heat. 

Types of fuel:

• Fossil fuels: Coal, oil, natural gas
• Biofuels: Ethanol, biodiesel
• Nuclear fuel: Uranium, plutonium
• Alternative fuels: Hydrogen, solar fuels

Requirement for an IC Engine Fuel

• It should take very little time for combustion
• it should have high energy density
• Low deposit forming tendency.

The constituents of Crude Petroleum and their Properties

•  Paraffins (C_n H_2n+2) like methane, propane, Isooctane, n-Heptane. They are saturated and stable compound. Branch chain or isoparaffins are highly knock resistant in SI engine than straight chain paraffins
•  Olefins (C_n H_2n) like ethylene, propylene. They are unsaturated and unstable compound. They cause gummy deposit after oxidation.
• Napthenes (C_nH_2n) like cyclo butane, cyclo hexane. They are cyclic and saturated compound. They are more stable than olefins.
• Aromatics. They have ring structure of benzene (C₆H₆) as central structure. They are highly active or even explosive (Like Toluence) Important Products of Refining Process of Crude petroleum
• Natural Gas. They are paraffinic compound mainly methane.
• Liquified petroleum Gas (LPG). These are also paraffinic compound propane and butane. They can be liquified in ambient condition by applying pressure.
• Gasoline or petrol. It is the highest liquified petroleum fraction. All liquid fraction having boiling point upto 200ºC are gasoline. Its specific gravity is 0.70 to 0.78.
• Kerosene. These have boiling range 150ºC to 300ºC and specific gravity 0.78 to 0.85. These are heavier than petrol.
• Diesel. These have 200ºC-370ºC boiling range and wide range of specific gravity.    These are having more specific gravity than petrol

Effect of Volatility on Petrol Engine performance

• Volatility is the tendency of fuel to go from a liquid to a gaseous state on slow heating of fuel, Quantity of fuel evaporated with temperature is measure which is called distillation and this is a measure of volatility of the fuel.
• Front end volatility (0-20% evaporation)
  Cold starting, hot starting and vapour lock are three important performance characteristics which are affected by the front end volatility of the gasoline used.

1. Cold starting: High front end volatility is required for easy starting of engine.
2. Hot starting: if the front end volatility is very high, it will create problem in hotstarting as more and more vapours will be present in the combustion chamber making the mixture too rich to ignite.
3. Vapour lock: Low front end volatility is required so that sufficient amount of liquid fuel could be pumped as more vapourization makes air fuel mixture lean because of less quantity of vapour and liquid fuel.
 
• Mid range volatility (20%-80% evaporation)  
1. Engine warm up, acceleration smoothness and fuel economy: The mid range volatility should be sufficient enough to get all these performances of the engine .
2. Carburetor icing: A low mid range volatility is required to prevent carburettor icing. More volatile fuel evaporates rapidly, lowering the
temperature of carburettor body. With high humidity content, water vapour in the fuel condense and freezes.

• Tail end volatility (80%-100% evaporation)
High tail end volatility causes less crankcase dilution, less engine deposits, gum formation and spark fouling.

• Aldehydes and peroxides formed after oxidation of unsaturated hydrocarbon are knock inducing compounds so Alphanaphthol is used as antioxidant
• Sulphur in fuel causes corrosion, odour and poor explosion characteristic in petrol.

OCTANE NUMBER

•  It is percentage of iso-octane in the fuel containing iso-octane and n-Heptane that gives the same knocking intensity as that of the fuel whose octane number is calculated. Octane is 80% and n-Heptane is 20%.
•  Octane number of a fuel can be increases by adding Tetra ethyl lead (TEL)
•  TEL causes spark plug fouling so ethylene dibromide is used to avoid lead deposits for spark plug fouling.

Requirement of Diesel Fuel

• Flash point: It is the temperature at which a visible flame occurs for less than 5 seconds. It should be high.
• Fire point: It is the temperature at which the flame can sustain for more than, 5 seconds. It should be high.
• Cloud point: It is the temperature at which the wax content of the diesel separates out in the form of solid. it should be low.
• Pour point: It is the temperature below which the entire fuel freeze. It should also
  be very low.

CETANE NUMBER

• It is the percentage by volume of cetance (C₁₆ H₃₄)in a mixture of cetane and amethyl napthalene (C₁₀ H₇ CH₃) that has the same performance in the standard test engine as that of the fuel whose cetane number is calculated. So if a fuel is
  equivalent to 100% mixture of cetane and a-methyl naphthalene where cetane is 85% and a-pmethyl naphthalene is 15% then cetane number of the fuel is 85.
• High cetane number of Diesel engine fuel reduce its knocking tendency
•  Octane no and cetane no are related as
  
  

  Alternative Fuels for IC Engines

  
   LPG 
• It is a mixture of mainly propane and some butane and iso-butane.
• Cheaper than gasoline
• High knock resistant and do not preignite easily.
• Better manifold distribution.
• Crankcase oil dilution is small.
• Efficiency of engine is lower due to high heat of vapourization.
• It requires higher compression ration to have the above said advantages.

Methanol (CH₃OH)

• The octane number of methanol is greater than petrol so with methanol, 20% greater output than petrol engine can be obtained due to higher compression ratio.
• Methanol engine has greater thermal efficiency.
• The mass of methanol consumed is 10% more than petrol due to lower calorific value of methanol.

•  Methanol can also be obtained from municipal solid waste
•  Methanol is used in racing cars because of increased power due to high compression ratio

Ethanol (C₂H₅OH)

•  Its octane number is greater than petrol so more thermal efficiency than petrol engine.
•  The volumetric efficiency is also increased. So mean effective pressure and thus power output is more than petrol engine.
•  Ethanol can be obtained from any feed stock containing carbohydrate such as corn, Wheat, sugarcane, potatoes, starch in carbohydrate

•  Vegetable oil particularly sunflower oil can be blended with diesel to reduce the consumption of diesel.

CNG (Compressed Natural Gas)  

•  It is a mixture of 90% Methane and remaining Ethane.
•  It is a good SI engine fuel due to high Octane number.
•  Fuel availability is large.
•  it is cheap, odourless and safe in operation.
•  Low engine emissions.