2 - Background

2.2 - Specific examples

2.2.1 - Thermodynamics of heat engines

2.2.1.6 - Fuel quality standards

There are three categories of engine fuels with different mandatory requirement

- fuels for use in uniform mixture and spark-ignition engines;

- fuels for use in non-uniform mixtures and auto-ignition engines.

- fuels for use in gas turbine engines requiring a continuous flame

Fuels for uniform mixtures include leaded petrol, unleaded petrol, liquefied petroleum gas, etc.

These fuels have to be extremely volatile so that the air-fuel mixture is as uniform as possible. This volatility, however, must also satisfy the following stringent specifications:

- a boiling curve giving the fuel spray percentage in relation to temperature;

- vapour pressure;

- the vapour-to-liquid ratio used to determine the tendency of the fuel to form vapour bubbles;

- the density used to determine certain fuel types and components;

- the maximum lead content;

- the octane rating which is a measure of the petrol’s detonation resistance.

And these are standard requirements!

The fuels used for non-uniform mixing include gas oil.

It is a mixture of different hydrocarbons with a boiling point between 180º and 360º C. approximately.

The main requirements for gas oil are

- density

- boiling curve

- kinematic viscosity;

- flash point

- cetane index.

These requirements are also standard!

The main requirements for gas turbine fuels are limited to:

- viscosity;

- All impurity amounts, and in particular, sulphur, sodium, vanadium and lead.

External combustion engines have the least demanding fuel requirements. This is because the motive fluid and the combustion gases are generally different. The fuel simply has to burn fast enough without leaving a residue.

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