Petrol as a Fuel

INTRODUCTION

Petrol consists mostly of alkanes and cycloalkanes obtained by the fractional distillation of petroleum, mixed with iso-octane (2,2,3 trimethyl pentane) or aromatics to increase its octane rating.

Small quantities of additives are common, to improve engine performance or reduce harmful exhaust emissions. Some mixtures also contain significant quantities of ethanol as a partial alternative fuel.

The blend varies with the season; in winter the fuel has to vapourise more easily and smaller molecules are needed.

In the heat of summer, volatile fuels may vapourise in the fuel lines. Compounds with a higher boiling point must be used.

BLENDING

Gasoline is produced at the refinery. Compounds separated from crude oil by fractional distillation (straight-run gasoline) does not meet the required specifications for modern engines, especially octane rating (see below). Blending is needed.

The main constituents of petrol are molecules with 4 to 12 carbon atoms. The ones which burn most smoothly, the aromatics, are classified as hazardous substances, and their levels in petrol are strictly regulated. Benzene is probably the worst.

An ideal petrol will burn smoothly, give minimal poisonous emissions, and will perform reliably if the fuel stands unused for a prolonged period of time.

CRACKING

Heavy oils unsuitable for use as vehicle fuels are usually 'cracked', or broken down into smaller molecules at the refinery.

Thermal cracking (heating to around 1200K) forms mainly straight chain compounds and alkenes.

Catalytic cracking (done at about 800K) uses a catalyst and forms cycloalkanes, branched alkanes and aromatics; all good vehicle fuels.

A third process, hydrocracking (heating with hydrogen) produces a fuel with low aromatics and low alkenes.

In this way modern refineries can 'drive' or 'steer' the process in the direction of the compounds they want, depending on the equipment they have and the type of crude oil they are working with.

OCTANE RATING

The octane rating of a petrol is important. This is to do with how smoothly a fuel burns.

Straight chain alkanes on their own tend to ignite too easily and cause harmful vibrations in the engine, known as 'knocking' or 'pinking'. You will hear it if you put two-star petrol in a five-star engine.

In earlier days 'knocking' was controlled by adding lead compounds to the blend. However the toxicity of lead has led to it being phased out, and leaded petrol is now rarely used in the UK.

Petrol with a larger fraction of branched chain alkanes, cycloalkanes and aromatics burns more smoothly. It is used in most petrol cars and is known as 'unleaded'.

PROBLEMS WITH 'UNLEADED'

Older car engines are damaged by 'unleaded', and lubricants have to be added to replace the lead compounds which are no longer there.

There are also health issues. 'Unleaded' is higher in harmful hydrocarbons, especially aromatics. These are hazardous to human health, and countries set strict limits, especially with benzene, a well-known carcinogen.

This has resulted in higher demand for the relatively harmless branched-chain alkanes.

Refineries are therefore having to install processing units to reduce the aromatics.

The exact meaning of 'octane rating' will be covered on another page.

N.D., habitat21

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