Octane Number

Question 1

What produces the power needed to move the wheels of a car?

Answer 1

In the internal combustion engine of a motor car the power to turn the wheels in produced in the cylinders by the explosive reaction between petrol and air

Question 2

What does the smooth running of a car engine depend on?

Answer 2

The explosion between petrol and air occurring in the cylinder at exactly the right moment

Question 3

What happens if the explosion between petrol and air occurs to quickly in an engine?

Answer 3

Knocking occurs which is where the pistons vibrate and a metallic noise is heard from the engine

Question 4

What causes knocking?

Answer 4

An early explosion due to the petrol-air mixture exploding as it is compressed instead of the explosion being caused by the spark

Question 5

What effect does knocking have on the engine?

Answer 5

The engine loses power and the inside of the cylinders can be damaged

Question 6

Define auto-ignition

Answer 6

The premature ignition of the petrol-air mixture before ignition of the mixture by a spark takes place

Question 7

What type of hydrocarbons cause autoignition frequently?

Answer 7

Straight-chained alkanes such as heptane, octane and nonane

Question 8

What type of hydrocarbons didn’t cause autoignition frequently?

Answer 8

Branched-chain alkanes such as 2,2,4-trimethylpentane

Question 9

Why does petrol contain large amount of branched-alkanes?

Answer 9

As they don’t auto-ignite and are therefore more efficient than those that contain straight-chained molecules

Question 10

Define octane number

Answer 10

The octane number of a fuel is a measure of the tendency of the fuel to resist knocking

Question 11

What octane number is 2,2,4 trimethylpentane assigned?

Answer 11

An octane number of 100

Question 12

What is the structure for 2,2,4 trimethylpentane?

4

Answer 12

A chain of five carbons bonded together with single bonds

A methyl group on either side of the second carbon counting from the left to right

A methyl group on the top side of the fourth carbon counting from the left to the right

Hydrogens bonded to each carbon where there is already a methyl group

Question 13

What is 2,2,4 trimethylpentane also called?

Answer 13

Iso-octane

Question 14

What octane number is heptane assigned?

Answer 14

0

Question 15

In relation to assigning octane numbers to other hydrocarbons, what is the role of 2,2,4 trimethylpentane and heptane?

Answer 15

These are reference hydrocarbons

Question 16

How is the octane number of a fuel measured?

Answer 16

By comparing its performance with known mixtures of heptane and 2,2,4 trimethylpentane in a test engine

Question 17

Explain what is meant by an octane number of 97

Answer 17

This means that the fuels works as well as a mixture of 97% 2,2,4 trimethylpentane and 3% heptane

Question 18

List the three factors which affect octane number

Answer 18

Length of alkane chain

Degree of branching

Straight or cyclic structure

Question 19

What effect do different lengths of alkane chains have on octane number?

Answer 19

The shorter the alkane chain, the higher the octane number

Question 20

What effect does the degree of branching have on octane number?

Answer 20

The more branched the chain, the higher the octane number which is why 2,2,4 trimethylpentane has such a high octane number as it has three methyl branches

Question 21

What effect does having a straight or cyclic structure have on octane number?

Answer 21

Cyclic structures have a higher octane number than straight chain compound e.g hexane has an octane number of 25 but cyclohexane has an octane number of 83