Unleaded petrol is petrol that had not had lead added to it as an anti-knock agent. When burnt in engines, this petrol does not give off toxic fumes, as leaded petrol does.
In the early days of motoring, in the first half of the twentieth century, motorists often experienced the phenomenon of their engine making a knocking sound after a few thousand miles. It was found that the knocking was caused because the petrol did not burn cleanly in the engine and this resulted in a build up of carbon residue in the cylinders. This carbon glowed red-hot and thus ignited the petrol prematurely - before the valve that admitted the petrol to the cylinder was properly closed.
It was this pre-ignition that caused the knocking. Chemists investigated the problem and found that adding very small amounts of lead to the petrol made the petrol burn cleanly with far less residue, thus curing the pre-ignition problem and eliminating the knocking noise. It also meant that cars produced far less smoke and fumes.
But over the next few decades evidence began to build up that the amount of lead in the atmosphere of cities was accumulating to a dangerously high level - a level high enough to cause damage to the nervous systems of newborn babies.
Governments around the world acted to ban the addition of lead to petrol as an anti-knock agent and instead oil companies began to sell unleaded petrol.
However, this did not mean a return to poor ignition. First, because they instead added substances called Aromatic hydrocarbons, and second, because car companies had been developing their engines and had found better and more fuel economical ways to burn the fuel that did not cause pre-ignition or knocking. At the same time, governments introduced new regulations regarding toxic emissions from cars and manufacturers were compelled to introduce cleaner exhaust systems so that the quality of air in cities was not compromised.
In the early days of motoring, in the first half of the twentieth century, motorists often experienced the phenomenon of their engine making a knocking sound after a few thousand miles. It was found that the knocking was caused because the petrol did not burn cleanly in the engine and this resulted in a build up of carbon residue in the cylinders. This carbon glowed red-hot and thus ignited the petrol prematurely - before the valve that admitted the petrol to the cylinder was properly closed.
It was this pre-ignition that caused the knocking. Chemists investigated the problem and found that adding very small amounts of lead to the petrol made the petrol burn cleanly with far less residue, thus curing the pre-ignition problem and eliminating the knocking noise. It also meant that cars produced far less smoke and fumes.
But over the next few decades evidence began to build up that the amount of lead in the atmosphere of cities was accumulating to a dangerously high level - a level high enough to cause damage to the nervous systems of newborn babies.
Governments around the world acted to ban the addition of lead to petrol as an anti-knock agent and instead oil companies began to sell unleaded petrol.
However, this did not mean a return to poor ignition. First, because they instead added substances called Aromatic hydrocarbons, and second, because car companies had been developing their engines and had found better and more fuel economical ways to burn the fuel that did not cause pre-ignition or knocking. At the same time, governments introduced new regulations regarding toxic emissions from cars and manufacturers were compelled to introduce cleaner exhaust systems so that the quality of air in cities was not compromised.
