5.6 Petrol 5.6.1 Petrol Engines LO: I understand how a petrol engine works.

5.6 Petrol

5.6.1 Petrol Engines

LO: I understand how a petrol engine works.

Burning petrol:• Petrol vapour and air are

mixed and compressed.

• The mixture is ignited by a spark.

Pollution:

• Inefficient burning produces carbon monoxide.

• The spark produces nitrogen oxides.

LO: I understand how a petrol engine works.

5.6.2 Octane Number

LO: I understand what is meant by the octane number of a fuel.

The standard for the quality of a petrol fuel is measured against iso-octane:

The higher the number the more smoothly the petrol burns.

2,2,4-trimethylpentane

Iso-octane burns smoothly and is given an octane number of 100.

H C C C C C H

H

H

C

C

H

H

C

H H

H

HHH

HH

H

HHH

LO: I understand what is meant by the octane number of a fuel.

The octane number can be increased by:

• decreasing the carbon chain

LO: I understand what is meant by the octane number of a fuel.

• increasing the branching

• using aromatics

If the octane number is too low the fuel will auto ignite at the wrong point in the engine cycle.

This can damage the engine.

5.6.3 Petrol Production

LO: I understand how petrol is made.

Petrol is made from 3 fractions of crude oil:

Reforming the naphtha fraction:

LO: I understand how petrol is made.

• changes the shape of the molecules without changing the number of carbon atoms

Hydrocracking the residue fraction:

Also, the more volatile a fuel is the better it burns.

LO: I understand how petrol is made.

• Splits up the molecules into smaller ones

Butane and pentane is added to fuel in winter months to make it vaporise more easily.