How is knocking reduced in vehicles using unleaded petrol?

An Internal combustion engine under load would develop a condition known as `pinging' or `knocking', where the fuel mixture would explode due to compression before the right time, causing rough running, stalling going up hills, and so on. Tetra Ethyl Lead (TEL) is one such component that could be added to petrol to reduce its tendency to `ping' under compression.

It breaks down to lead at upper cylinder temperatures. Lead atoms spread around and combining with the free radicals and slowing down the reaction. `Leaded' petrol was a grave danger to the environment, as lead is a poison when it is absorbed into the body. The reason for going completely unleaded is different — it is to reduce other pollutants, the unburned hydrocarbons and nitrogen oxides. To achieve this, catalytic exhausts have been adopted, and they cannot stand even the residual lead, which will affect the platinum catalyst. It was discovered that passing the exhaust gases through a filter of platinum caused a catalytic conversion of the oxides to other products, which could then be prevented from escaping into the greenhouses, used for food production.

There are three main groups of substances oil companies use instead of lead to improve octane number.
  • Aromatics: organic compounds based on the benzene ring, a 6-carbon ring with 3 delocalised double bonds,e.g., benzene, toluene, xylene,
  • Olefines: organic compounds, which have, double bonds. After combustion, one critical by-product is 1,3-butadiene.
  • Oxygenates: organic compounds containing oxygen molecules such as methane, ethane or MTBE (methyl-tertiary-butyl ether).

As benzene and 1,3-butadiene are the top toxic air pollutants. They are both highly carcinogenic substances, MTBE is the most popular additive in unleaded petrol.

The third group of alternative octane-raising substances mentioned above are the oxygenates. A major by-product of their combustion is acid aldehyde — the first substance the body produces in the alcohol-detoxifying process.

So the oxygenates are less toxic than benzene and 1, 3-butadiene. Another advantage of oxygenates is that, because they contain oxygen molecules, they cause the fuel to burn more efficiently-and thus lowering the levels of all pollutants from car emissions.

Source : The Hindu