An oil refinery
Petrol engines and knocking

Knocking is a something we want to try to prevent in petrol engines. It occurs in four stroke engines when some hydrocarbons ignite too early. It causes a knocking sound and reduces the engines performance.

Photo of isomeriser
The isomerisation plant produces barnching chain alkanes which improve the octane number of fuels.

We measure the problem using the octane number. The higher the number, the better the fuel. A fuel with a low octane number is likely to cause knocking. One with an octane number over 100 should be fine.

There are two main ways of increasing the octane number of petrol (i.e. improving it).

  • use alkanes with branching chains rather than straight chains
  • use aromatic alkanes (with rings)
These alkanes are produced on the site using the isomeriser and the reformer. They are then blended into the petrol at the end. The blended petrol is constantly monitored for its ocatane number.

A car's engine is an example of a four stroke engine. It relies on a mixture of petrol and air being ignited by a spark plug. This should happen at just the right moment. If the explosion happens too early or too late, then it will be ineffective and might even slow the engine down. Let's see how a four stroke engine works. Roll over each stage to change the picture.

Stroke 1. The downward moving piston sucks a mixture of air and petrol vapour into the cylinder.

Stroke 2. The piston moves up, compressing the gas mixture. This is where a low octane fuel might ignite and cause knocking.

Stroke 3. Just before the piston reaches the top of the cylinder a spark from the spark plug explodes the gas mixture. This pushes the piston down and drives the crank shaft round.

Stroke 4. The piston moves up and pushes the gases out through the exhaust valve. As the piston moves down, it pulls more fuel/air mixture in to begin the cycle again.

4 stroke engine - interacts with text
The four stroke engine. Take a look at a Flash movie of the four strokes.