Gasoline is used as a fuel in the internal combustion engines of motor cars and aeroplanes. In such engines, a mixture of gasoline vapours and air is compressed by a piston within the cylinder of the engine and the compressed mixture is ignited by a park from the spark plug. As the compressed fuel-air mixture burns, considerable expansion is caused. This results in the movement of piston and the transmission of the force to the wheel of the motor car or the propeller of the aeroplane. The efficiency of the engine depends upon the extent to which the fuel-air mixture is compressed at the time of ignition. The greater the compression, the greater is the efficiency. But the increase in compression beyond a certain point result in the sudden and irregular burning of fuel mixture which causes jerk against the piston and gives rise to
violent metallic sound. This rattling metallic noise produced due to irregular burning of fuel mixture in the cylinder is known as knocking. Knocking reduces the power of the engine and also causes damage to the piston and the cylinder.
The maximum compression that can be attained in a given engine without any knocking taking place depends upon the composition of the gasoline used as fuel Therefore, it is a matter of importance to know the relative “anti-knock quality” of the fuel For this purpose, a scale involving the term octane number was introduced by Edger in 1929.
By carrying out tests on individual hydrocarbons, it has been found that n-heptane knocks maximum when used as a fuel Thus, due to its very poor anti-knock quality, it has been assigned octane number, zero. On the other hand, iso-octane (2,2, 4-trimethylpentane) was found to have the best anti-knock property, Therefore, it has been assigned octane number of 100.
| (octane number = 100)
All other fuels are graded between these two extreme values (0-100) by comparing with suitable mixture of above mentioned compounds taken as standard. For example, if a fuel vas octane number 85, it implies that it has same anti-knock quality as the mixture of 85 parts iso-octane and 15 parts n-heptane. Thus, octane rating of a fuel may be defined as the percentage by volume of iso-octane in the mixture of iso-octane and n-heptane which has the same anti-
-knock qualities as the fuel under test.
It may be noted that for aeroplanes the fuel used should have octane number 100, while for motor vehicles the fuel with octane number of 80 is rated as good fuel The ‘premium’
grade petrol used in Ghana has octane number of 91 or above.
Influence of Structure of Hydrocarbons on the Octane Number
A study of the octane numbers of different hydrocarbons has revealed that the value of the octane number depends upon the structure of the hydrocarbon.
Straight chain hydrocarbons have very low value of octane number. The value decreases further with increase in the length of the chain.
- Branching of the chain increase the value of the octane number.
- Cycloalkanes have higher value of octane number than the corresponding straight chain alkanes.
- Aromatic hydrocarbons have generally very high value of octane number.
Gasoline Additives or Anti-knocking Agents
In internal combustion engines, the problem of knocking can be checked either by using a fuel of higher octane number or by adding certain compounds to gasoline which decrease knocking. The compounds which ate added to gasoline to gasoline to improve its octane number are called anti-knocking agents.
Tetraethyl lead (TEL), (C2H5)4Pb is added to gasoline to the extent of 0.01 % to reduce knocking. Gasoline containing tetraethyl lead is called ethyl gasoline or leaded gasoline.
Use of lead anti-knock agents in the vehicles causes lead pollution of the environment due to release of volatile lead compounds. Lead compounds, especially organic lead compounds, are highly toxic. They affect the central nervous system of human beings and also retard the mental
development of their children. Many countries including Ghana have stopped using lead anti-knock agents because of lead pollution. The storage and use of leaded petrol has been banned in Ghana since January 2004. TOR started producing unleaded petrol in October 2003. To minimize knocking unleaded gasolines contain methyl tert-butylether(MTBE). It minimizes knocking and increases the oxygen content of gasoline making the fuel burn cleaner.
H5C2 – Pb – C2H5
Tetraethyl lead (TEL)
H3C – O – C – CH3
Methyl tert-butylether (MTBE)
Pollution problem. A gasoline-engine discharges various substances into the atmosphere. These are carbon (IV) oxide, water, carbon (II) oxide, nitrogen oxides, unburnt hydrocarbons, etc. Some of these substances are highly poisonous. Their emission into the atmosphere, poses a serious pollution problem to mankind. The leaded gasoline also produces some poisonous compounds of lead along with other above mentioned gases. The pollution problem is getting
aggravated with increasing use of automobiles.
Example 51.3. A sample of gasoline has the same knocking characteristics as a mixture of iso-octane and n-heptane in the ratio 3 : 1 by volume. What is the octane number of the gasoline sample?
Solution. Percentage of iso-octane (by volume) in the given sample of gasoline =
Hence, the. octane number of the given sample of gasoline is 75.