IC Engine Fuels

The subject of fuels for automobiles has been remained of interest for last many years as development basically depend upon the availability of good fuel. Lots of research have been done on this subject for last 100 years and still carried out.

The fundamental knowledge of types of fuels and their characteristics is essential for understanding combustion phenomenon. The properties and character of fuels will have profound effect on the design, power output, efficiency, fuel consumption and also the reliability and durability of engine.

The chemical reactions which are responsible for heat release are considerably fast but time taken in preparing the mixture of air and fuel depends upon the nature of fuel and method adopted to supply the same to the combustion chamber. Therefore, the fuels used in IC engines have to fulfill some basic requirements for better performance of the engine.   

Desirable Properties of good IC Engine fuels

The basic requirement of IC engine fuel is, the combustion should be fast with maximum amount of heat release without forming any deposits and should not have destructive effects on the engine parts and atmospheric air by exhaust gases.

A good IC engine fuel must possess the following properties:

·        It must have high energy density (kJ/Kg),

·        It should be easy to handle and store and free from hazard,

·        It must possess good combustion quality,

·        It must have thermal stability,

·        It must have low deposit forming tendency,

·        It should not have chemical reaction with engine components through which it flows,

·        It should easily mix with air and should evaporate as soon as possible (high h_fg),

·        Products after combustion should not create any corrosion to the engine parts,

·        It should have low toxicity,

·        Its effects on air pollution should be minimum,

·        It should be economically available in very large quantities.

Types of Fuels

The IC engines are mostly operated on liquid fuels or gaseous fuels. But 99% of the world IC engine uses liquid fuels and a few engines uses gaseous fuels where it is readily and economically available.

Different types of gaseous and liquid fuels are discussed below:

Gaseous Fuels:

The gaseous fuels are normally used in SI engines. The different gaseous fuels are listed below:

1.      Natural gas:

It is found in many parts of world but mainly in USA. It is also carried from the place of availability to the place of use through pipelines. Its composition varies with the source but mainly it contains CH₄ (75-95%) and remaining C₂H₆ and N₂.

The natural gas from some areas contains H₂S which is highly harmful to the engines.

2.      Manufactured gases:

Various methods are used to manufacture the gases. Coal gas is manufacture by heating soft coal in closed vessel. The contents of coal gas depend upon the type of coal and method of operation used in manufacturing. A clean coal contains 33% H₂ and 66% CH_4. Its energy content is about 50% of natural gas.

3.      By product gases:

The gases produced during the manufacture of other substances are known as by product gases. Blast furnace gas is by product gas of steel plants. It contains mainly CO and N_2. It contains large amount of dust particles. Therefore, more effective methods should be used to clean it before using it in engines.

4.      Sewage Sludge gas:

Sewage sludge gas is made available from present well developed sewage disposal plants. It mainly contains CH₄ and CO₂ with small presence of H₂S.

5.      Biogas Plants:

The biogas is produced from the cow dung which is available in large quantities. It can be easily manufactured with any chemical reaction and therefore it is easy to manufacture and use locally.

 Advantages of Gaseous fuel

·        It can be easily carried through pipes,

·        It can be easily compressed and stored,

·        The starting of engine is easy which uses gas as fuel,

·        The freezing of oil does not occur with gaseous fuel.

Disadvantages of Gaseous fuel

·        Storage volume per unit energy is very large compared to oil,

·        Cost of gases is high on the basis of energy content,

·        The size and weight of engine is considerably large compared to engine using liquid fuel,

·        The purifying cost of gaseous fuel is considerably high,

·        The capital and running cost for the plants used to manufacture gases is considerably high.

Liquid Fuels:

Liquid fuels carry lot of importance as 99% of the world IC engine use liquid fuel. All the liquid fuels which are used in IC engine are derived from crude petroleum which is naturally available in large quantities.

The liquid fuels are mainly classified into two groups. Liquid fuels which are vaporized easily are petrol and alcohol and are commonly used in SI engines. The other type which is directly injected in the combustion chamber is known as diesel or fuel oil.

All the liquid fuels have two basic elements, Carbon and Hydrogen and they are popularly known as hydrocarbons. The general chemical formula for hydrocarbon is C_nH_m. According to the values of n and m, the physical and chemical properties of hydrocarbon will vary. The hydrocarbon according to the structure of C and H are classified in four categories;

·        Paraffins (C_nH_2n+2)

·        Olefins (C_nH_2n)

·        Naphthenes (C_nH_2n)

·        Aromatics (C_nH_2n-6)

Within each group also, the physical properties of individual compound differ according to the number of carbon and hydrogen atoms in the molecule. The physical differences between compounds, even in any group, influence the way fuel evaporates and hence the formation of combustible mixture. The differences in chemical properties of hydrocarbon from different groups affect the combustion process and hence the properties of fuel and air requirements.