Quality of gasoline, octane member and gasoline additive

The explosive nature of a hydrocarbon is determined by its volatility. The volatility of any liquid depends upon the temperature.

'The flash point of any liquid is the lowest temperature at which a liquid hydrocarbon gives off enough vapor to form an explosive mixture with air'.

The flash point of any liquid hydrocarbon is so adjusted that it remains safe under the conditions of its use. For example, the minimum flash point permitted in India is 44°C, while in France it is 33°C and in Britain it is 22°C.

Knocking of fuels

An internal combustion engine works with a system of pistons. A mixture of air and petrol vapor, is drawn from the carburetor into the cylinder in the down-stroke of the piston. In the upstroke phase, the mixture is compressed. The ratio of the initial volume to final volume is called the compression ratio. At the end of the upstroke of the piston, a spark ignites the compressed air-petrol (gasoline) mixture. As the gases burn, they expand and the flame front moves in a smooth manner and supplies power to the engine.

To achieve maximum efficiency of the engine, a high compression ratio of about seven to eight is required. However the increase in the compression ratio, results in the burning of petrol-air mixture in an explosive manner, that produces a metallic sound. This sound is called knocking, and indicates inefficient performance of the gasoline. High compression ratio engines also require less fuel, so petrol having less knocking tendencies are very valuable.

It has been found that the knocking tendency of the fuels falls off with the nature of the fuel as follows.

Straight chain alkanes > Branched chain alkanes > Alkenes

Knocking may also be prevented or minimized by adding compounds such as tetraethyl lead (TEL) to gasoline. Such compounds are called anti-knocking agents. To prevent the deposition of lead inside the cylinder, dibromoethane is added to the gasoline.

Quality of petrol (gasoline) - octane number

The controlled combustion of fuel in the presence of air, gives an internal combustion engine its power. A low quality fuel does not burn smoothly and causes an occasional explosive sound, which is known as knocking. This greatly reduces the power of the engine.

The quality of a fuel is indicated in terms of its octane number. Different hydrocarbons have different knocking tendencies. A fuel that produces minimum knocking is considered as a good fuel.

An arbitrary scale of octane number has been set up with n-heptane and 2,2,4-trimethylpentane (iso-octane) as the reference compounds. All fuels are graded in between these two limiting values by comparing with a suitable mixture of the above two compounds. 2,2,4-trimethylpentane (generally called iso-octane) has excellent anti-knocking properties and has been arbitrarily assigned an octane number of 100, whereas n-heptane, which is very prone to knocking is assigned an octane number of zero (0). Therefore, the antiknock property of a fuel increases with the increase in its octane number.

Thus, the octane number of any fuel is defined 'as the percentage of iso-octane in a mixture of iso-octane and n-heptane that has the same knocking as the fuel under examination'.

A fuel having an octane number of 80 behaves in a manner similar to a mixture having 80% of iso-butane, and 20% of n-heptane. Straight run gasolines may have octane values ranging from 20 to 73. Aviation fuel is rated as 100 octane.

The octane number of a hydrocarbon depends upon its structure.

• Branched-chain alkanes, alkenes and aromatic hydrocarbons have high octane numbers. Such compounds are added to straight-run gasoline to raise its octane number. These high octane compounds, viz., branched-chain alkanes, alkenes, and aromatic hydrocarbons are prepared by catalytic cracking and catalytic reforming.

• Straight chain alkanes have low octane numbers, the value of which decreases with the increase in the length of the chain.

• Cyclic alkanes have higher octane numbers than the corresponding straight chain alkanes.

• The unsaturated hydrocarbons have higher octane numbers than the corresponding straight chain hydrocarbons.

• Aromatic hydrocarbons have very high octane numbers.

Octane ratings of some compounds are:

n-Heptane	0
n-Pentane	62
tert-butyl alcohol	98
neo-octane Benzene	100
Ethanol	112
Methanol	116
Toluene	118

Quality of diesel - cetane number

Cetane number is defined as the percentage of cetane in a mixture of cetane and -methyl naphthalene, which has the same ignition quality as the fuel under examination at the same conditions.

Cetane is arbitrarily given a cetane number of 100 as it ignites rapidly while -methyl naphthalene ignites is given a cetane number of zero as it ignites slowly.

Gasoline additive

Gasoline additives are compounds added to gasoline, which improve the octane number of a fuel. Such substances are called antiknock compound that improve the fuels combustion in an internal combustion engine. Tetraethyl lead ((C₂H₅)₄Pb) is a main additive and is a common antiknock compound . A small quantity of tetraethyl lead (TEL) improves the anti-knock quality of the gasoline. Gasoline containing tetraethyl lead is called leaded petrol or ethyl gasoline.

A gasoline-engine (internal combustion engine) discharges various substances into the atmosphere. These are carbon dioxide, water, carbon monoxide, nitrogen oxides, unburnt hydrocarbons from leaded gasoline and certain compounds of lead. About 0.1 g of lead per litre of the gasoline burnt escapes into the atmosphere. Most of it finds its way into the soil.

Steps to reduce pollution due to automobile exhausts

While many compounds present in the exhaust of any automobile are harmful, lead is extremely toxic and poisonous. The increasing emission of such harmful substances into the atmosphere due to the increasing use of automobiles, is a serious pollution problem. Methods adopted to improve this pollution problem are:

• Removal of lead from gasoline is done by replacing tetraethyl lead in gasoline by certain aromatics and branched-chain hydrocarbons. The gasoline free from tetraethyl lead is called lead-free gasoline (or lead-free petrol).

• Certain devices such as, catalytic converters are now being used to oxidize the un-burnt hydrocarbons and carbon monoxide to carbon dioxide and to decompose nitrogen oxides into nitrogen and oxygen.