LIQUID FUELS

LIQUID FUELS

Petroleum (Crude oil):

  • An important primary liquid fuel.
  • It is a dark colored viscous oil found deep in the earth’s crust.
  • It is believed to have been formed millions of years ago by anaerobic decay of marine plant and animal life under the influence of high temperature and pressure.
  • It is mainly a complex mixture of hydrocarbons (like straight-chain paraffins, cycloparaffins, olefins and aromatics) with small amounts of other organic compounds containing N, O and S, and traces of inorganic compounds.
  • The average composition of crude oil is:

C: 83 – 87%; H: 11 – 15 %; S, N and O: 0.1- 5%.

Petroleum refining:

“It is the process of separation of crude oil into different useful fractions on the basis of their boiling points”

  • The crude oil, freed from water and sulphur, is first heated in a pre-heater (350-4000 C). The vapors are then fed into a specially designed large bubble cap fractionating column.
  • The volatile components condense on the upper plates of the fractionating column while the less volatile fraction is collected on the lower plates.

 

Cracking:

  • The objective of cracking is to obtain greater yields of improved gasoline by thermal decomposition of the surplus heavier fractions.
  • Gasoline obtained by cracking gives better engine performance (less knocking) than straight-run gasoline (obtained from fractional distillation of crude oil).

 Cracking is defined as the process of decomposition of higher molecular weight hydrocarbons (higher boiling) into lower molecular weight hydrocarbons (low boiling).

  • Cracking process involves breaking of C-C and C-H bonds.
  • It produces low boiling alkanes and alkenes.
  • A small amount of carbon and hydrogen are also produced.

 

Advantages of catalytic cracking:

 

  • The octane number of gasoline produced is high.
  • The yield of gasoline is also high.
  • The process can be better controlled.
  • The product contains a very little amount of undesirable sulphur.
  • There is a saving in production costs since high temper-atures and high pressures are not needed.
  • In catalytic cracking, external fuel is not required.The necessary heat is obtained by burning off the coke deposited on the catalyst itself, during the regeneration process.
  • The gasoline formed contains much less gum and gum forming compounds.
  • Catalysts are selective in their action, and therefore, they permit cracking of only high boiling hydrocarbons.

 

Mechanism of Knocking:

  • In IC engines, the gasoline and air drawn into the cylinder is compressed by the piston and ignited by an electric spark.
  • As the flame front travels towards feed end of the combustion chamber, rapidly expanding combustion gases, compress the remaining un-burnt fuel ahead of flame front and raise its temperature.
  • If the flame front travels rapidly at optimum speed, the combustion of un-burnt fuel takes place but smoothly.
  • If the flame front travels too slowly, the entire last portion of the fuel-air mixture may get heated up beyond its ignition temp. and undergo instantaneous explosive combustion.

This produces thermal shock wave which hits cylinder walls and piston. This result in emitting of characteristic rattling sound called knocking or pinking.The tendency of knocking increases with CR.

 

Knocking:

  • The efficiency of power production in spark ignited internal combustion (IC) engines is related to the compression ratio   (CR).
  • The CR is the ratio of the cylinder volume (V1) at the end of the suction stroke to the volume (V2) at the end of the compression stroke of the piston.
  • This ratio is always greater than one, since V1 being greater than V2.
  • Theoretically, the power output and efficiency of an IC engine should increase continuously with increase in the CR.
  • H. R. Ricardo, with the help of a variable compression engine showed that in actual practice, the power increases to a maximum and then falls rapidly with further increase in the
  • The CR, corresponding to the maximum power output, iS known as highest useful compression ratio (HUCR).
  • The mechanism of the chemical reactions that lead to knocking is not clear.
  • It is believed that chemical reactions that are of importance are cracking and the oxidation of the hydrocarbons.
  • Probably the reactions proceed by a chin reaction.
  • It was recognized that the structures of the fuel hydrocarbons determines largely their knocking tendency.
  • The tendency to knock decreases as follows: n-alkanes> mono substituted alkanes > cycloalkanes > alkenes > poly substituted alkanes > aromatics.
  • The tendency to knock depends not only on the fuel used but also on the engine design, shape of head, location of plug, etc., and also upon the running conditions.

 

Octane Number:

  • Graham Edger proposed an arbitrary scale, octane rating in order to express the ant-knock properties of gasoline’s
  • Among alkanes, n-heptane knocks severely, while under identical conditions, 2,2,4-trimethyl pentane (iso-octane) has a high resistance to knocking.
  • For the scale proposed to indicate the anti-knock properties of gasoline, n-heptane was arbitrarily assigned an octane number of zero and iso-octane was arbitrarily assigned a value of 100.
  • By blending these two hydrocarbons in various proportions,primary reference fuels were prepared.
  • In the same engine under the same set of conditions and the same critical CR, various blends of the n-heptane and iso-octane are burnt and the percentage of iso-octane by volume in blend that knocks under these conditions is the octane number of the gasoline.

Thus octane number is defined as the percentage by volume of iso-octane in a mixture of iso-octane and n-heptane blend, which has the same knocking characteristics as the gasoline sample, under the same set of conditions.

  • Thus a gasoline with an octane number of 90, has the same knocking characteristics as a mixture of iso-octane and n- heptane containing 90% by volume of iso-octane. Since iso-Octane has good ant-knock properties, it is clear that greater the octane number, greater is the resistance to knocking.
  • Automobile gasoline’s have octane number ranging from 75 to 95. Aviation gasoline’s have a greater knock resistance and their octane numbers are greater than 100. In such cases the octane numbers are computed using the relationship,

Octane number = [ Power number –100 ]  + 100/3

where, power number is an arbitrary number proportional to the power being extracted by the engine.