The Solution: Change the fuel to prevent knock
Although engines can be designed to prevent knock, this reduces their efficiency and power output, so scientists developed ways to improve the anti-knock qualities of the fuel itself.
Figure 1: Octane and Iso-octane
Figure 1 shows two fuel molecules: octane (sometimes called n-octane) and iso-octane. Only the carbon atoms are shown, the hydrogen atoms are omitted for clarity. Both molecules have the chemical formula C8H18, meaning they both contain 8 carbon atoms and 18 hydrogen atoms but are different shapes, i.e. they are isomers. Iso-octane gets its name because it is an isomer of octane, though its true chemical name is 2,2,4 trimethylpentane (you can see why they gave it a shorter nickname!) Which of these two molecules would you think has better anti-knock properties?
The answer is iso-octane. Look at how small and compact the iso-octane molecule is, with 3 individual carbons branching off from the main 5-carbon backbone. Compare that to the long, flimsy octane molecule with its 8 carbon backbone and nothing branching off. When subjected to high temperatures and pressures, the flimsy octane molecule will tend to break apart into small, highly reactive chunks, while the tight, compact iso-octane will stay in one piece. This compact, branched shape is what gives iso-octane its excellent anti-knock properties.
So if you make a fuel with reasonably stable molecules, it will have good anti-knock properties. This is the basis for modern unleaded gasoline (petrol), which contains a mixture of compact, stubby, stable molecules which will wait patiently for the flame front to reach them despite the increasing temperatures and pressures in the engine's combustion chamber. Previously, gasoline used to contain small quantities of the heavy metal lead (Pb) in the form of tetra-ethyl lead, which stabilized the fuel to prevent it detonating. However, leaded fuel was banned because it causes brain damage in children and destroys the pollution-reducing catalytic converters in the exhaust systems of modern cars.
Although a long molecule may break down into small, reactive chunks, this does not mean that small molecules are prone to detonation. In fact, if a molecule starts out small in the first place, it tends to survive high temperatures and pressures very well. This means that fuels made of small molecules such as methane (natural gas) propane and butane (liquified petroleum gas) and ethanol are all highly resistant to detonation and make excellent fuels in a spark-ignition engine.
So now we know how to prevent engine knock, by carefully choosing fuel molecules that are resistant to high temperatures and pressures and therefore do not break apart and detonate. But there is one more problem. How do you measure and compare the anti-knock properties of different fuels? In the final page we will talk about the answer: octane ratings.