Fuels & Alternate Fuels

Fuels:

A components used by engines to convert chemical energy into mechanical energy.

• The characteristic of fuel used have influence on design, efficiency, output, reliability and durability of an engine.
• But fuel play an important role in environmental pollution.
• Fuel should have high calorific value which gives high energy contents during burning.

There are 3 types of fuels, namely:

1. Solid Fuels
2. Liquid Fuels 
3. Gaseous Fuels 

Some key points for three types of fuels are:

• Solid fuel (like coal) can be used as fuel but after Gasification of Coal, but it has Ash & handling problems.
• Liquid fuel is converted into vapor form by injector to make combustion easy and uniform.
• Gaseous fuel can burn easily.
• For SI engines, aromatic hydrocarbons are best fuel because compact molecular structure (or greater number of carbon atoms) have best anti-knocking characteristics.
• For CI engines, paraffin hydrocarbons are best fuel because simple molecular structure (or lesser number of carbon atoms) have greater tendency to vaporize easily.

SI Engines Fuel Qualities:

1. Volatility   →   It is the evaporation of fuel at room temperature. This quality determines its suitability fro use in SI engines.
2. Crankcase Dilution   →   During combustion, fuel in combustion chamber change phase to liquid which passes through the spaces between piston and cylinder walls and cause crankcase oil to dilute. To reduce this, fuel should have low distillation temperature.
3. Vapor Lock Characteristics   →   High rate of vaporization of fuel will result in blocking of fuel lines.
4. Anti-Knock Characteristics   →   Abnormal burning of fuel in combustion chamber will result in greater heat energy which adversely effect the efficiency of an engine.
5. Gum Deposits   →     Hydrocarbons and impurities in fuel will result in the production of gummy deposits.

CI Engines Fuel Qualities:

1. Knock Characteristics   →   Fuel should have best knocking characteristics.
2. Volatility   →   Fuel should volatile in operating range to produce homogeneous mixture.
3. Starting Characteristics   →   Fuel should help in starting engines and have high cetane number and low self-ignition temperature.
4. Viscosity   →   Fuel have the ability to flow.
5. Corrosion & Wear   →   Fuel should not have impurities like sulphur content which can produce sulphuric acid which can cause corrosion.

Important Definition:

1. Cracking   →   Conversion of higher chain hydrocarbons into lower chain hydrocarbons at high temperature in the absence of air.
2. Hydrogenation   →   Conversion to Unsaturated hydrocarbons into saturated hydrocarbons at high temperature and pressure.
   
3. Polymerization   →   Combining of lower hydrocarbons into longer chain hydrocarbons.
4. Alkylation   →   Addition of Alkyl group to unsaturated hydrocarbons.
5. Isomerization   →   Conversion of Straight chain hydrocarbons (low octane number) into Branch chain hydrocarbons (high octane number).
6. Cyclization   →     Conversion of branch chain hydrocarbons into ring structure (Cyclo-alkane).
7. Aromatization   →   Formation of Benzene derivative.
8. Reforming   →   Converting low octane no. fuel into high octane fuel.
9. Blending   →   Mix additives to enhance properties of fuel.
10. Octane Number   →   The percentage by volume of iso-octane in a mixture of iso-octane and hexane having the same anti-knock properties as that of fuel used.
11. Cetane Number   →   The percentage by volume of cetane in a mixture of cetane and α-methyl naphthalene having the same ignition delay as that of fuel used.

Refining of Petroleum:

The process by which crude oil is distilled into different fractions in the fractional distillation chamber at high temperature is called Refining of Petroleum.

Alternate Fuels:

Firstly we will talk about gaseous fuel then we will just touch liquid fuel. We can use biological waste as fuel, we are not going in details but you can use take a bit knowledge about bio-fuels by looking the video below:

1. Natural Gas:

Components containing largely of methane 60-95% and is found in association with liquid petroleum is called Natural Gas.

• Octane Number is 110, that means good for SI engines.
• Refueling is a slow process.
• Because of the gaseous fuel, low volumetric efficiency.

2. Compressed Natural Gas (CNG):

If natural gas is compressed at 16-25 bar pressure then it is called Compressed Natural Gas.

• The CNG fuel feed system is similar to LPG fuel feed system.
• CNG conversion kits are used to convert petrol driven cars into CNG driven cars.

3. Liquid Petroleum Gas (LPG):

• For automobiles, three types of LPG is used, separate Propane and Butane or mixture of Propane and Butane.
• Contains less number of carbon of atoms, so less emissions of hydrocarbons.
• Since fuel is in vapor form, so no crankcase dilution.
• 50% longer life of engine.

4. Hydrogen:

• Hydrogen is considered to be the future of fuel.
• It is pressurized and stored in tank and used as fuel.
• It is a gas, so low volumetric efficiency.
• Nearly zero emissions because hydrogen is used individual in the molecular or atomic form.

Gasoline v/s Liquid Petroleum Gas (LPG):

LPG Fuel Feed System:

• LPG is a high rated SI engine fuel.
• Produce less emissions because of less number of carbon atoms than gasoline.
• It is stored under pressure & delivered through a high pressure line to engine where it is vaporized.
• Having low volumetric efficiency because of gaseous fuel.
• Fuel feed system is placed in rear portion of vehicles.

References:

• University notes + my knowledge.
• Videos from YouTube.
• Images from Google Images.
• Book: IC Engines by V Ganesan (4th Edition).
• Class Lectures.