Information about fuel

This page contains information for consumers about fuel makeup, quality and measurement.

Why vapour pressure is important

Vapour pressure is a measure of how easily a liquid vaporises, or changes into a gas. Petrol is supplied in liquid form, but at least part of it must be vapour to ignite in an engine's combustion chamber. This means that, on a cold day in a cold engine, enough petrol must vaporise to enable ignition. On the other hand, on a hot day in a completely warmed-up engine, it is also important that the petrol does not vaporise and expand so much that it does not allow any air to be mixed with it in the cylinder.

Oil companies balance these two extremes when blending petrol, taking into account the climate and the season where the petrol will be sold.

What diesel is, and how it differs from petrol

Like petrol, diesel fuel is blended from various petroleum components produced in a refinery. However, diesel contains components that have a boiling point range of approximately 200°C to 350°C. Vapour pressure is not an important consideration in the production of diesel, since the fuel does not need to change to a gas in a diesel engine.

Diesel engines inject liquid fuel into the cylinder, unlike petrol-powered engines with carburettors. However, temperature can have an effect on the operation of diesel fuel in an engine. At low temperatures, diesel fuel may thicken and restrict the flow of fuel to the cylinder. Oil companies produce diesel blends that are formulated for specific climates and times of the year to prevent problems in cold weather.

Does diesel have an octane number?

No. Diesel engines do not have spark plugs. They depend on the fuel igniting itself by compression, unlike petrol engines. The measure of a diesel fuel's ignition quality is called the cetane number, which indicates the readiness of a diesel fuel to ignite spontaneously under the temperature and pressure conditions in the combustion chamber of a diesel engine. The higher the cetane number, the shorter the delay between fuel injection and ignition.

What petrol is, and how it's made

Petrol is a blend of different components found in crude petroleum oil.

Crude oil is a mixture of many different substances. Most of these substances are hydrocarbons, which are molecules composed of carbon and hydrogen atoms. Generally, the more carbon atoms a hydrocarbon contains, the higher its boiling point.

A refinery uses the differences in boiling point of the hydrocarbons to separate them by distillation. This process yields products that range from very light components like propane, which has a boiling point of less than 20°C, to very heavy products with boiling point ranges over 400°C. A subset of these distillation products with relatively low boiling point ranges (20°C to 200°C) is used to blend petrol. Distillation products with higher boiling point ranges may undergo further heat or chemical processing to transform them into substances with lower boiling points that can be used as petrol components.

Oil companies blend different petrol components together depending on the octane number, vapour pressure, and other properties required for a particular petrol product.

Difference between 91, 95 and 98 octane

Currently, there are three grades of petrol sold in New Zealand with different research octane numbers: 91, 95 and 98. The octane rating of petrol is a measure of that fuel's anti-knock rating, that is the measure of a fuel's ability to burn stably, without detonation, at a selected compression ratio. The higher the compression of the engine, the higher the octane rating needs to be to prevent knocking. Knocking, otherwise known as pinking, is characterised by a banging noise coming from within the engine at high throttle settings at low engine revolutions and can lead to damage to engine components.

Engines designed to operate on high-octane fuels also have their timing adjusted for those fuels. When an engine is operated on a fuel with the wrong octane rating, it can either lead to knocking and resulting engine damage, or to lowered combustion efficiency - or both.

How the octane number of petrol is increased

The petrol that you buy at a service station is actually a mixture of different petroleum components. Each component has its own properties, including octane number. By blending carefully, the oil companies can come up with a formula that has the properties they require for a given type of fuel, including octane.

Lead in petrol

Adding certain lead compounds to petrol was an effective and cost-efficient way to increase the petrol's octane number. Lead is known to be toxic to humans, and its use in petrol is being phased out on a world-wide basis. New Zealand banned lead from petrol in 1996.

Why there's sulphur in petrol and diesel

Sulphur is present to a greater or lesser extent in all crude oils. When crude oil is distilled into petrol and diesel fuel blending components, some sulphur finds its way into those components.

The amount of sulphur allowed in petrol and diesel is limited because of the undesirable effects it causes during combustion, such as its odour, its corrosiveness, and its tendency to produce acidic by-products. In addition, advanced technologies that are used to meet stringent emissions limits generally require very low levels of sulphur in order to operate effectively.

How you know that you're getting an accurate measure from a service station

All petrol pumps must be tested and verified as correct under the Weights and Measures Act 1987. Trading Standards accredits private sector verifiers, known as Accredited Persons, to carry out this testing. The owner of the petrol pump can also request an Accredited Person to test the pumps regularly and, if the pumps are found to be accurate, a certificate of accuracy will be issued. This takes the form of a small adhesive label placed on the pump where consumers can see it.

Trading Standards carries out surveillance to ensure the system works and that accuracy is maintained.

If you think you've been delivered a short quantity of fuel, you can let us know using our complaint form.

Making a complaint

Why the amount of a fuel-fill may differ from your vehicle's fuel tank capacity

Car manufacturers' stated capacities for fuel tanks are not precise measurements, whereas the petrol pump is a calibrated measuring instrument. Also, the design of car fill pipes allows fuel or vapour to sometimes shut off the petrol pump nozzle at an early stage in the fill.

The nozzle is designed to shut supply off and prevent spills when used in the latched or automatic mode. This can give a false impression about the level of fill in the tank. It is safer not to fill the petrol tank right to the top, as petrol will expand - due to the temperature in the car being higher than that in the underground storage tank.

If you think you've been delivered a short quantity of fuel, you can let us know using our complaint form.

Making a complaint